Friday, July 15, 2016

Fixing the squealing auto-tuner motors in the Kenwood TS-450

This year I was setting up one of the club's Kenwood TS-450s in preparation for ARRL Field Day:  We had already set up the tents the night before and I was now laying out the radios.  Since it was morning - at an altitude of over 8000 feet (approx. 2440 meters) ASL, it was quite chilly on this late-June morning.
Figure 1:
The TS-450 with the tuner in need of attention.

Having connected the radio to the tri-band Yagi assigned to it I turned it on and heard a brief squealing sound emanate from the radio.  Suspecting that I was hearing the sound of a dry motor in the TS-450's auto-tuner, I changed bands - which caused the radio to re-tune - and heard even more squealing.

At this point it is worth noting the importance of this observation.  Since it was rather cold, whatever sparse lubricant was present in the radio was going to have less effect on its bushings, but it did indicate that the motor(s) in the tuner were in need of lubrication.  If the motors in the tuner get too "dry", they will seize up and then burn out as the tuner's "smarts" have no real way of knowing if the motor is stuck, applying power to them until a time-out occurs and when this happens, the user will likely retry a couple times.  Normally, the motors draw something in the 40-60 mA range when operating, but stalled, this can increase to well over half an amp, explaining how the motor can be damaged rather quickly - either by burning up the brushes, overheating the rotor windings, or a combination of both.

The reason why I was familiar with this problem is because several years ago a friend brought in a TS-450SAT in which the auto-tuner had failed.  In disassembling it, we quickly determined that one of the motors driving the tuning capacitor had seized and now measured open-circuit.  Having nothing to lose the motor was disassembled, but it was clear that the damage was more extensive:  While the brushes were somewhat burned - and could have been likely been burnished - and at least one of the rotor windings was open.

We soon discovered that (at the time, at least) a spare motor was not available from the radio manufacturer and that there were no recommendations for alternates to be found on the GoogleWeb:  We think that we did, finally, find a substitute and if this turns out to be successful, I'll be sure to write a post about it.

Not wanting to have this same fate befall the motors in this radio's tuner, I retrieved the radios (the club actually owns two TS-450SATs) after Field Day and took them into the shop to be worked on.  The (approximate) procedure for re-lubricating the motors is as follows, but first, a few weasel words.

Figure 2:
Cover over the low-pass filter, accessible after removing
the top cover of the radio.
Click on the image for a larger version.
Warning:
  • This is sensitive and delicate electronic equipment:  DO NOT attempt to service it unless you have a familiarity with electronics and servicing techniques.
  • There is a real chance that - by accident or otherwise - the tuner/electronics/radio may be damaged/destroyed:  YOU are entirely responsible for determining if the procedure that follows is within your abilities.
  • Although there are low voltages involved, there is still some risk.
  • Remember that your situation may not be completely identical to this one and that some/all of the steps described may not apply to you.
  • You have been warned!
Removing the tuner module:

First, disconnect the radio from its power source, its accessories and the antenna.  Next, lay out a clean, well-lit work area and locate several small containers in which to place screws and various items.

Taking off the top and bottom covers of the radio - noting which color and type of screws go where - also remove the the internal cover that shields the low-pass filter compartment next to the antenna connector as depicted in Figure 2.
Figure 3:
 Coaxial cables connecting the tuner and low-pass.  Note
that the "front" cable has a piece of white heat shrink
tubing on it.
Click on the image for a larger version.

With this cover off, disconnect the two coaxial cables (see Figure 3) that connect from the tuner to the low-pass board.  Note that the cable to the front has a piece of white shrink tubing, marking it:  If it does not, mark that cable now.

 Also connecting the tuner and low-pass board, there is another multi-wire connector nearer the front:  Gently remove this connector, unplugging it from the low-pass board.  Also carefully unplug the flat ribbon cable that connects from the gap in the middle of the tuner module and goes to the front of the radio, observing how it is routed through the bracket on the front of the tuner module.

With the cables disconnected, the tuner unit may now be removed.

Figure 4:
 Showing the two screws along the bottom edge of the tuner.
There is one more screw near the front of the tuner which
would be in the upper-left corner of this picture, on the
top of the tuner.
Also note the white, flat ribbon cable along the top of the
tuner from the front and how it is routed under
the metal bracket with the potentiometers.
Click on the image for a larger version.
Along the right side of the radio (the front facing you) near the bottom of the main deck there are two screws (Figure 4) that should be removed and a third on the top of the tuner module, in the corner, near-ish the front as depicted in Figure 5.  Make sure that you note the style of screws that were removed and from where.

Carefully remove the tuner assembly, noting the orientation in which it was mounted.

With the tuner removed from the radio, carefully unplug the cable that goes the front of the tuner, plugging into a socket next to the flat ribbon cable mentioned above and shown in Figure 11.

Now remove its top and bottom covers, again noting the type of screws (probably the same as those holding the tuner in the radio) and setting them carefully aside.

Lubricating the motors' bushings:
Figure 5:
The location of the front screw holding the tuner in place.

There is a decision to be made at this point.  It is most likely that the "driest" motor bushings are those at the "front" (shaft end) of the motor - but it is also may be that both the front and rear bushings will need to be lubricated, in which case the motor will need to be removed and partially disassembled.

The first, safe assumption is that the front bushing is the culprit as it is not only the most exposed to the atmosphere, but it also gets exerted on it the most off-axis stress when coupling to the worm gear.  It is possible - with the aid of a hypodermic needle or using a small screwdriver or piece of wire suspending a drop of lubricant to work it into the bushing at the front of the motor without further disassembly, with the job of getting to the left-hand motor (the one marked with "L" on the green circuit board) being a bit more difficult.

Figure 6:
 A recommended, long-lasting lubricant to be used.  If this
is not available, use only a good-quality, light oil such as
sewing machine oil:  DO NOT use a generic "3-in-1",
motor, or a "household" of oil.
Click on the image for a larger version.
Now, unplug the multi-conductor cable - the one in the connector next to the one from which the flat ribbon cable was removed earlier:  This cable connects the two motors and potentiometers to the internals of the tuner board.

With the connector from the motors and position-sensing potentiometers now removed, it is also possible to power-up each motor, independently by applying voltage (6-12 volts - preferably from a power supply with a 1 amp current limit) to the soldered connections on circuit boards (marked "L" and "R") directly.  Doing so will cause the motor to operate - but observe carefully that when this is done, the motor can hit the hard stop of the potentiometers in an approximately half-meshed state in either direction:  In other words, the range of motion of the potentiometers goes from approximately half-meshed, continues clockwise (as viewed from the shaft end) through fully-meshed, un-meshed, and then half-meshed.  As soon as the motor hits the potentiometer stop, you must remove the power from the motor immediately.


Figure 7:
 The potentiometer board and 4 (removed) screws.  Note
that the potentiometers are connected to the board only by
their terminals:  Avoid bending/flexing them by
their leads.
IMPORTANT:  If you remove this board you will
have to recalibrate the potentiometers to the
proper capacitor position when reinstalling.  This
procedure is described later on in this posting.
Click on the image for a larger version.
When powering the motor(s) in this way, they may squeal - but note that how well what residual lubrication works will be somewhat dependent on temperature.  For re-lubricating the motor I would recommend "Super Lube", a PTFE (Teflon (tm)) based lubricant that is readily available from some auto parts stores or on Amazon:  This lubricant will not dry out and it attracts minimal dust and it has been used successfully by the author to "un-stick" quite a few galled/damaged shafts with good, long-lasting results.  If you do not have access to this lubricant or do not wish to get some, it is recommended that high-quality light sewing-machine oil be used:  Whatever you choose, DO NOT use everyday "3-in-1", motor or "Household" oil as this will fairly quickly dry out and get gummy!

Figure8:
Removing the motor assembly from the tuner.  This picture
shows two screws at the bottom edge of the plate holding
the motors.  Note that the screws are slightly
offset, under the circuit boards which means that the
screwdriver shaft will be at a slight angle which means that
you will need to take care when reinserting them to make
sure that they do not go in at an angle and get cross-
threaded.
There are two similar screws on the same plate along
its top edge that must also be removed.
Click on the image for a larger version.
To lubricate the motors in-situ (e.g. without further disassembly) set the tuner on end with the green circuit boards at the end of the motors facing down.  Now put a small drop of oil on the end of a small screwdriver or wire (such as a straightened paper clip) and touch the shaft between the plastic worm gear and the body of the motor:  Surface tension should cause the drop of oil to run down the shaft and into the motor.  Using the same screwdriver/wire, gently nudge the worm gear up and down within the limits of end-play to help work the drop of oil into the bushing at the end of the motor.  If the oil immediately disappears or you aren't absolutely certain that any has gone in, add another drop or two in the same manner as above, moving the shaft up and down to help disperse it.  Once you are satisfied that some lubricant has made its way into the motor, wick up the excess with a bit of paper towel or tissue.

If it is not practical to access the end of the left-hand motor (the one with the "L") you may need to remove the assembly from the end of the tuner.

Additional parts that might warrant lubrication:

It is possible that other parts within the capacitors' drive trains are also in need of lubricant.  If you suspect this to be the case, check the following:
  • The ends of the worm/reduction gears.  Where the ends of the plastic shafts protrude through holes punched in the metal one can put a small drop of lubricant.
  • The bushings of the variable capacitors.  These are rather tight, by nature, but it is possible that the lubrication within is drying out.  To work additional lubricant into these heat the (metal!) bushings with a soldering iron to get them fairly hot (e.g. boiling water temperature) and then while still at an elevated temperature, put some lubricant on the shaft at each end that it emerges from the bushing:  The heat will cause the lubricant to become less viscous and as it cools, some of it will be wicked into the bushing.
  • In each case, above, make sure that one wicks up excess lubricant with a paper towel or tissue.

Removing the motor and potentiometer assemblies:

IMPORTANT:  If you remove either potentiometer - even momentarily - you will have to recalibrate the potentiometer setting to the physical position of the tuning capacitor:  The procedure for doing this is described farther down this page.

First, remove the potentiometer bracket using the four screws along the bottom edge - two per potentiometer - just below the pots as depicted in Figure 7.

When this is done the motor and potentiometer assembly may be carefully pulled out.  Note that the potentiometer and motor assemblies are still connected via their wires, so be careful not to stress or break them.

Figure 9:
Lubricating the end of the motor shaft after the plate has
been removed:  After working the oil into the shaft by
spinning/powering it and moving it up and down,
wick up the excess oil with a paper towel or tissue.
Click on the image for a larger version.
Using a very small Philips (tm) type screwdriver, remove the two screws on the plate as indicated in Figure 8 along with two similar screws on the same plate on the other side of the motor:  Note the style of these four screws as you remove them and carefully set them aside.

Now, you have easy access to the worm gears and ends of the motor shafts which may be lubricated as depicted in Figure 7.  Again, the small puddle of oil can be worked into the end of the shaft by spinning it with one's fingers and gently moving pulling the shaft up and down, taking advantage of the small amount of end-play.  Once you are satisfied that a reasonable amount of oil has worked into the shaft - often evidenced by the fact that they "feel" smoother and to not squeal when spun by finger or when powered up - mop up excess using a tissue or paper towel.

Finally, do not forget to lubricate both motors - even if only one was making noise!

At this point again apply voltage (6-12 volts) to each motor, one-at-a-time and allow it to run - the shaft facing upwards - for a minute or two - to work the lubrication in.  When free-running, each motor should draw between 30 and 50 milliamps.  Also listen to the motor to determine if it sounds quiet and free of rattle or squealing:  If the motor makes excess noise and/or the current is significantly higher than 50 milliamps it may need to have its "other" bushing lubricated - or it may have already sustained damage.

Lubricating the bushing on the "brush" end of the motor:

Warning:  

It is recommended that you do this step only if the motor continues to make noise after working lubrication of known-good quality into the shaft end of the motor as noted above.
Disassembling and reassembling the motor is a bit tricky and requires attention to detail and it is possible that the motor can be damaged/destroyed by performing this procedure without due care!
If you wish to continue and disassemble the motor further, you are doing so with the presumption that you have good mechanical skills and some experience at doing this.  Furthermore, you undertake this task entirely at your own risk!

Unfortunately, the "other" end of the motor shaft is not accessible without partially disassembling the motor.  As noted above, doing this task involves care, observation and careful attention to detail!  It is also recommended that only one motor be worked on at a time.
Figure 10: 
Removing the metal end cap from the motor.  This is necessary
ONLY if you have determined that the motor bushing opposite
the shaft end is dry as well.
Warning:  Disassembling the motor requires good mechanical
skills to do so without damaging it and should be undertaken
ONLY if you feel confident in doing so.  If done improperly,
the motor can be damaged/destroyed!
Click on the image for a larger version.

For this task, the first thing to do is to mark the motor closest to the "L" or "R" indication on the circuit board:  Making a scratch on the motor case is recommended as an ink marking may be easily rubbed off in handling.  Now, unsolder the circuit board from the end of the motor and set it aside. Now remove the motor from the bracket using the two, short screws - which should be very carefully set aside -

Using a very thin blade - of a knife or screwdriver - work it between the metal end cap and the white tab on the side of the motor.  This cap snaps into a slight groove in the main motor housing and should pop off fairly easily.

With the metal cap removed, carefully work the blade under the white plastic tab and the body of the motor, working it up to form a slight gap.  Now, while spinning the shaft back-and-forth, carefully work the plastic motor end-cap up and off the end of the motor shaft.

At this point look for a small, white plastic cap with a hole in it the size of the motor shaft:  Usually it will still be on the shaft of the motor itself, next to the armature, but sometimes it will remain in the end-cap, under the brushes.  If the latter has occurred, carefully slide it out from underneath the brushes and put it on the motor shaft with the slight ridge oriented to the inside (toward the windings) of the motor, toward the armature:  This ridge helps space and insulate the armature.

Now, using a small screwdriver or wire as an aid, place a very small drop of oil inside the bushing in the plastic end-cap.  Since it is almost impossible not to get a bit of oil on the brushes, carefully use a bit of tissue or paper towel to wick away excess outside the bushing and on the brushes.

Look at the brushes very carefully:  They should be straight, overlapping and almost touching in the center where the shaft goes through.  If they are not and/or are slightly bent, using a small pair of tweezers, very carefully straighten them out:  The idea here is that when the motor is reassembled, the brushes should gently touch the armature.  Note that these brushes are split and have two "leaves".

Now comes the tricky part and where damage to the motor is most likely:  Reinstalling the end cap.

Look at the plastic end cap and note that there are two slots:  These are used to move the brushes away from the armature when it is being assembled, and to do this a small tool - made of, say, #22 wire or a bent paper clip, must be constructed.  First, bend a length of wire in a square-cornered "U" shape so that it can slip easily into both slots, protruding in only a few millimeters as to be able to move the brushes.  The idea here is to insert this tool into the slot as far clockwise as possible (viewed from the end of the cap) and then once it is inserted, rotate it counter-clockwise to move the brushes out of the way while simultaneously putting the plastic cover over the end of the motor.  Doing may be made easier by clamping the motor housing gently in a vise to hold it secure.

If all goes well the plastic cap should seat into its original position and the motor shaft should turn easily.  If the plastic cap does not want to easily go on straight and/or the motor shaft does not spin smoothly and easily once the cap is reinstalled and leveled, carefully remove it - inspect the brushes and, in necessary, use tweezers to very carefully straighten them out, and try again.

Once you have gotten the plastic end cap into place, apply voltage to the motor again:  It should run.  If it is open, a brush is either hung up or bent out of place, but if the power supply indicates a short circuit (a very good reason to use a power supply limited to just an amp!) it is likely that the brushes are bent/out of position and the cap will need to be removed and the brushes inspected/adjusted.

If the motor runs and draws its expected 50-ish mA of current, orient the metal cap carefully over the solder terminals - aligning the square protrusions with the solder terminals in the plastic with the square holes in the cap and snap it back into place.  Again, check the motor to verify that it runs.

Soldering the motor back to the circuit board, take note of the mark that you made when removing it.  If you didn't happen to note which polarity of the motor went where, look very carefully at the white plastic square protrusions at the solder terminals you will notice that one is marked with a plus (+) sign:  The positive (+) side should go nearest the terminal marked with "L" on the left-hand motor and should go farthest away from the terminal marked with "R" on the right-hand motor.

Once the motor has been reassembled, re-mount it on the metal plate using the small, short screws:  It is strongly recommended that one use blue or purple "thread locker" compound:  Do not use "red" locking compound as it may prove to be difficult to remove, if necessary.

Recalibrating the potentiometers to the tuning capacitors'
 positions:

Now, using your fingers, spin the worm gear on the tuner's gear assembly so that each capacitor is precisely fully-meshed.  Note:  If you wish, you can do this after the next step, applying voltage (only 5-7 volts to achieve slower motion) to each motor as necessary.

With the motor(s) re-mounted to the motor mounting plate, reinstall the motor assembly/plate back on the tuner with all four screws, carefully engaging the worm gear:  Verify that the capacitors are still fully-meshed, briefly applying power to "fine tune" their position if necessary.  Using 5-7 volts instead of 12 volts for this step will cause the motor to move more slowly, making it easier to precisely set the capacitors to the "fully meshed" position.

Again, note that the two screws along the bottom edge of the motor plate are partially blocked by the circuit boards on the motor, causing the screwdriver shaft to be offset slightly:  Carefully start the screws to assure that they are straight and not cross-threading before torquing them with the screwdriver.

WARNING:

If you had to remove the potentiometers for any reason, it is very likely that their position - which provides indication of the physical setting of the capacitors to the radio's computer - got disturbed.
Again, it is necessary to make sure that the potentiometers are set to a certain value with respect to each capacitor being fully meshed in order to assure proper tuner operation and to prevent breakage of the potentiometer and stalling of the motor and burning it out.

It is now time to re-mount the potentiometer assembly.  Referring again to Figure 7, above, orient the potentiometers as shown:  If the wires are on the "wrong" side of the motors due to handling, they can be carefully re-routed through the gap between the two motor circuit boards.

On each potentiometer locate the "top" (upper-most) of the three terminals - that is, the one farthest away from the mounting bracket with the two screws - and the center terminal.  Using an ohmmeter, adjust each potentiometer for 1.75-1.85k across the top and center terminals.  Now, barely start the four screws that hold the pair of potentiometer brackets in place and, pushing the gears on the potentiometers onto the gear assembly of the tuner, re-check the reading on each with an ohmmeter. 

If it is outside the range of 1.75-1.85k, there is enough room on the still-loose screws to move the potentiometer far enough away to disengage the gear:  Move the potentiometer one "gear tooth" at a time in this manner to get the reading as close to the 1.75-1.85k target as possible:  A value between 1.7 and 1.9k for a fully-meshed capacitor should be fine.  (One "tooth" of a potentiometer is equal to approximately 150-250 ohms of resistance.)

Once the two potentiometers are properly set with the above values, tighten the screws and re-check the potentiometer values before proceeding as they may shift slightly when maneuvering the screws.  It is worth noting that the holes on the potentiometer brackets are slotted, allowing each potentiometer to be moved slightly back-and-forth, individually, to "tweak" the values if desired.

It is not important that the potentiometers be set exactly for the above values as the total resistance values of these potentiometers can vary by 10% - it needs only be within the general range so that the radio's computer can read the analog voltage on the wiper leads of these potentiometers and then "pre-set" the capacitors' positions when one changes bands.  It should go without saying that once the radio is assembled, unless the potentiometers are exactly where they had been previously you may need to make the radio go through a tuning cycle.

Reassembling and reinstalling the tuner:
Figure 11:
The routing of the flat, white ribbon cable.

Now, reassemble the tuner, first putting the top and bottom covers on, noting that the cable connecting the potentiometers and motors goes outside of the top cover.  Now re-mount the tuner - avoiding trapping of any wires - into the radio using the three screws, plugging in the cable from the motors/potentiometer and the flat ribbon cable - routed as shown in Figure 11.  Now connect the two coaxial cables - the one with the piece of white shrink tubing on it going to the front - along with the multi-conductor cable that connected near the front corner of the low-pass board.


Final checkout:

The tuner/radio may now be tested:  Operate the tuner as normal and both capacitors should quietly adjust themselves as you change bands and a match be found when the button is pressed to cause it to tune.   After verifying that the tuner is operating normally, put the rest of the covers back on and enjoy the radio!

Sunday, June 12, 2016

A 1:1 balun was the best choice for feeding the horizontal loop...

Years ago I bought a Heathkit SA-2060 (non "A" version) 2kW-rated antenna tuner at a local swap meet for a good price.  While not as heavy-duty as some of the venerable Collins or Viking tuners, it had a nice-sized roller inductor and a pair of large, wide-spaced variable capacitors in a typical "T" ("High-pass") configuration.
Figure 1:
The Heathkit SA-2060 tuner and (now) 1:1 balun feeding the 450 ohm
window line.  It no longer sits, on edge, in the window, as it had previously -
a much more convenient arrangement!

I have used this antenna tuner for years, taking it to Field Day and other than having to tighten some screws and adding thread-locker as well as a bit of lubrication of the moving parts after I got it, it has served me well, (seemingly) capably matching the 200-something foot circumference horizontal "lazy loop" antenna at my home QTH that is fed with 450 ohm window line.

A month or so ago I was doing some rewiring after having my main electrical panel replaced in conjunction with the installation of a PV (Solar) inverter system and to do this work I had to "open up" some wall and ceiling spaces in the room containing my ham shack - but this also meant that I had to disassemble and relocate much of what was in the shack to accommodate that which had to be moved out of the way.  While the "radio area" wasn't particularly disassembled for this task, I ended up piling a lot of stuff in that part of the room, essentially making it practically inaccessible.

One of the things that I did during this work was to pull a brand, new 240 volt, 20 amp circuit for my Heathkit HL-2020 linear (really an SB-221 with a brown color scheme) and once I had the room more-or-less back together I reconfigured the amplifier for 240 volts (there were minor complications to this - perhaps another story) and I was ready to get back on the air.  From what I'd read, the combination of the higher mains voltage and the Peter Dahl transformer would provide a higher plate voltage under load along with higher output with slightly less drive - and testing with the dummy load, this appeared to be true.

For years my tuner had been sitting on edge in the window with the 450 ohm window line coming through an insulated gap, past the vinyl window frame, and connecting directly to the balanced wire connection on the back panel.  In the rearrangement I'd needed to take the tuner out of the window and in the process one of the wires of the window line popped off - something that I noticed as I was preparing to test the amplifier under load.  Happening to have the receiver on at the time, I reconnected the leg of the balanced line and...

There was no difference in the signal strength of the received signals.

Something was definitely wrong here!  I would have expected that with one leg of the balanced line disconnected that I'd get at least an "S" unit or two difference in signal strength, but there was no obvious difference at all.  Grabbing a screwdriver I shorted the balanced line and, again, could hear no difference, either, so I connected my antenna analyzer and noted that while there was a good match through the tuner, it did not change much if there was one or two wires connected, or if the "balanced" terminals were shorted together.

Hmmm...

Now, I was curious.  It would appear that I'd been actually running the "loop" in a "T" type configuration with the downlead being (more or less) end-fed and the remainder of the antenna being a sort of distributed top hat.  I've never really had trouble working other stations, nor had I really experienced any "RF in the shack" issues as I had a pretty decent, short ground with heavy decoupling of the HF coax feeding to the tuner via a large chunk of ferrite scavenged from an old computer monitor.  In other words, I'd had no reason to question the operation of the balun itself or how it functioned.

The tuner's cover was immediately off and I was comparing the balun connection with that of an SA-2060A manual that I'd found online and the results was inconclusive:  If the wires had been properly identified and taped at the time of initial construction, it looked correct, but if not the only way to verify this was to remove the balun and check it with an ohmmeter.   

I regret that I didn't make a note of how the balun core was wired, but I do know that it wasn't at all right so I made the necessary changes and then tested the balun on the bench with the antenna analyzer and the other end of the balun terminated with a 200 ohm resistor.  Unlike the original configuration of the balun, according to the analyzer it was now working as it should, having a reasonable match to 50 ohms and going to infinity when resistor was shorted or removed.

Putting the balun back in the tuner and reassembling it I had to readjust from my previously-noted tuner settings to find a proper match (a good sign that the settings weren't the same, actually!) and I then checked it out with 100 watts on 40 meters.  Everything appeared to be fine, although the tuner struck me as a bit more "touchy" in its adjustments as compared to before.

Firing up the amplifier I soon discovered that I couldn't tune it up without its "Plate" variable capacitor arcing over noisily.  Grabbing a "Cantenna" dummy load I verified that the amplifier itself was fine, but something else was wrong.  Turning the power all of the way down and then slowly up again I discovered that at around 200 watts of RF output the reflected power went up, suddenly equaling the forward power.  Popping the cover off the tuner again confirmed my suspicion:  The "output" capacitor in the tuner was arcing over.

What this meant was that the tuner was being asked to match something really awkward - but with my loop and given its length I thought it unlikely that the feedpoint impedance would be really high, but rather it was more likely that it was "low-ish" - probably below 100 ohms.

The problem with this is that I now had a properly-working balun that provided an upwards impedance transformation.  This meant that if the loop had, for example, a 50 ohm feedpoint resistance on my loop, the tuner would be "seeing" around 12.5 ohms.  This is bad news as making a transformation from 50 ohms to 12.5 ohms implies the likelihood of a high-Q configuration of the tuner itself which, in turn, implies high voltage and high current which further implies high losses!

Wielding my antenna analyzer I connected its BNC connector directly to the balanced line:  Since the analyzer was hand-held and I was checking at "only" 40 meters I didn't think that it would really matter much that it was properly "balanced" or not.  The readings indicated a resistive component of around 10 ohms with a reactance of around 180 ohms inductive, but in tuning around to other amateur bands I couldn't make much sense out of the readings and was particularly suspicious when none of the resistance values seemed to go much above 50-80 ohms.

Suspecting that without the "bandpass filter" effects of the tuner that I was the victim of an AM broadcast station a few miles away being detected by the reflectometer bridge in the analyzer and causing false readings, I dusted off my Heathkit HD-1422 RX noise bridge and connected it to my FT-817, running on battery - this combination being comparatively immune to stray, off frequency RF and like the analyzer configuration that I'd used, more-or-less "balanced" without any obvious ground reference.  With that configuration I got a more sensible resistance reading of around 35 ohms and a reactance measurement in the area of 130 ohms, inductive.  If I took the 35 ohm reading seriously that would mean that the antenna tuner was trying to match something under 10 ohms through the balun!

Figure 2:
The exterior of the Balun Designs Model 1171t 1:1 "ATU" balun.
This model is equipped with studs on the top of its weatherproof case
for connection to a balanced feedline.
This brought to mind a discussion that I'd had with another amateur some time earlier.  He pointed out that it seemed silly that most baluns with tuners offered only a 4-fold impedance up-conversion, but it was likely that a typical antenna fed with balanced line was more likely to see a lower impedance on most bands unless there was a configuration that was particularly prone to high impedance like a 1/2 wave end-fed wire (e.g. a "Zepp" antenna), a rhombic (don't most of us wish we could have one of those!) or a full-wave dipole.  What this meant was that in most cases that the average amateur would encounter, the tuner was going to be matching to substantially lower than 50 ohms resistive through the balun - something that is likely to cause problems like loss - which is invariably accompanied by heating - and high voltages, internally.  What had been a reasonable hypothetical scenario was manifesting itself as reality!

The clear solution in this case was to use a 1:1 balun, instead.  I had the choice of reconfiguring the existing 4:1 balun - which was now working properly - perhaps by rewinding it with some PTFE 50 ohm coax - but I decided, instead, to get another balun and keep the internal balun intact in the event that it would be needed (it's nice to have options!) as it could be easily inserted or removed from the circuit using a jumper on the rear panel.  Because I was intending that I be able to use the tuner/balun combination with my amplifier which was capable of the full 1500 watts output, I also knew that it needed to be both low loss and capable of handling very high voltages and high currents.

In perusing the various web sites and forums I looked at the possibility of making my own balun, but ultimately decided on the "1:1 ATU Balun" by a company called "Balun Designs." - link.  The products of this company not only had good reviews, but their web site was also impressive explaining in good, sensible detail why one balun was better than another for a particular application and also outlining situations where certain baluns that they sold should not be used and why.

Figure 3:
Inside the 1:1 balun.  It is wound with parallel, highly-insulated
enameled copper wire in the "Guanella" fashion - that is, the second
"half" of the windings cross over to minimize coupling
between the input and output to provide best isolation and to
minimize the "one turn" effect inherent with "normal" toroid
winding techniques.
The balun that I chose (Model #1171t) is a current balun which effectively operates in series with the signal path (unlike a more common "voltage" balun which typically resembles an autotransformer as in the case of a typical 4:1 balun) and is essentially a common-mode choke that isolates one side of the balun from the other by virtue of the bulk inductance of the core over which a transmission line is wound.  By suppressing the "common mode" aspects of the RF signals with a significant amount of inductance, the windings on the toroids effectively "choke" anything other than differential (balanced) currents and thus isolate one section of the feedline from the other - except for the equal-and-opposite RF that is supposed to be there!

While many of these current baluns are wound with PTFE coaxial cable to preserve the 50 ohm impedance, this particular balun was wound with what amounts to parallel-conductor transmission line consisting of enamel copper wire covered with PTFE spaghetti tubing.  What this means is that this "parallel transmission line" winding inside particular balun isn't particularly close to 50 ohms in its natural impedance (it's likely in the 70-100 ohm range) when terminated with a 50 ohm load the apparent match, when viewed with an antenna analyzer seems to degrade as frequency increases - likely a result of the "transmission line" comprising the balun's winding causing some impedance transformation.  This is of relatively little importance for this application since it sits on the output of an antenna tuner:  As long as it is low loss and can withstand the expected voltages and currents, it would have minimal effect on the overall system efficiency.

When this balun arrived I connected it to the output of the SA-2060 tuner with a short (approx. 18") RG-8 style jumper and was easily able to tune the antenna with settings radically different than before - another good sign!  Finding that everything looked good on the analyzer, I hit it with 100 watts - then 1500 watts and had no problems at all.  I did notice that the window line became warm to the touch and the balun core and windings also became perceptibly warm, but by no means "hot" as the thermal image in Figure 4 depicts.

Figure 4:
A thermal image showing the heating of the balun and transmission
line.  As can be seen, the closer to the "output" of the balun, the
warmer the windings got, but after approx 20 seconds at 1100 watts of
RF on 40 meters their temperature stabilized.  The image
above depicts a maximum temperature inside the balun of less
than 120 degrees F (49C) with the feedline at approximately
105 F (41C) both being warm, but not "hot."   (The "warm" UHF
connector at the bottom appears thus as it is reflecting heat from
elsewhere in the shack.)  Considering that over 1kW
of RF is flowing, this amount of heating represents negligible loss - likely
less than that occurring within the tuner itself.
Of course, the amount of heating will depend both on the power level
and the amount of current flowing through the balun, and this depends
on the matching/impedance conditions encountered.
I also observed that if I disconnected just one side of the balanced line the signals on the band dropped by several S-units and the relative floor relative to signals came up while shorting the two caused signals to all but disappear - exactly what I was expecting to happen in a circuit that properly rejected common-mode signals.  When checking across the band at different times of day I also observed that the noise floor was 1-2 S-units lower than before and that the previously S-5 noise from the switching supply on the nearby DSL modem was now barely detectable at the S-2 noise floor on 40 meters:  Thus are the benefits of common-mode rejection in the prevention of electrical noises from the shack and the house's electrical system from being conducted onto the feedline/antenna and into the receiver.

As far as the warming of the window line I did some calculation and determined that the feedline was likely seeing a VSWR somewhere in the range of 8:1 to 20:1 or so, which meant that it was losing as much as 0.5 dB along its 30 foot run - a worst-case loss of up to 11%, or in the area of 150 watts maximum at the test power of 1100 watts.  This is a small fraction (approximately 1/12th) of an "S" unit, but it would certainly explain the warmth!

A few days later I had the opportunity to check into a 40 meter round table with a group of friends across the western U.S. and conditions were abysmal, but not only could I hear all of the stations pretty well, one of the stations with the weaker signals reported that they could hear my just fine, with my signals being comparable to another station across town from me running about the same power - a reasonable indication that I wasn't burning up too much power in losses!

Final comment:
One of the first things that I do when I get gear at a swap meet - commercial, commercial kit or homebrew - is to check it out, making sure that all hardware is tight and electrical connections are solid, but I will admit that it never occurred to me to check to see that the balun was wired properly!
 

Tuesday, May 31, 2016

Audio breakup on the JBL 4315B speaker

Over the past several weeks I noticed a problem with one of my JBL 4315B 4-way speakers:  It sounded as though the woofer was "breaking up" - that is, there was occurring what sounded like some sort of distortion related to the travel of the cone in response to "heavier" bass content.

Figure 1:
The "business end" of the 4315B

Some background:

A year or so ago I had a similar problem with this same woofer:  The audio was breaking up very badly, so I put it on the workbench, connected it to the amplifier, and observed the same problem.  Having nothing to lose, I carefully removed the dust cover - a job made easier with the careful application of a heat gun set to "low" - and quickly saw that an aluminum stiffening ring (constructed with an open gap in its perimeter) just "outboard" of the voice coil (e.g. toward the "front" of the woofer) had broken loose from its original adhesive.

With this loose ring - or at least a portion of it being loose - the speaker's excursions via the force of the voice coil allowed a bit of physical distortion, causing the voice coil to rotate slightly away from its axis, permitting it to hit the magnet assembly.

To fix this, I laid the woofer on its face on the edge of a workbench, clamping it down to prevent it from falling off (and on to me!) and working from below to prevent debris from entering the magnet gap I removed the aluminum stiffening ring and scraped away the old, brittle adhesive from the inside of the voice coil assembly.  After this I righted the speaker and stuffed pieces of paper towel into the voice coil opening to prevent additional debris and uncured epoxy from getting into the coil-magnet gap.  To reinstall the metal ring, which I'd also cleaned of old adhesive, I used metal-filled epoxy ("J.B. Weld") making sure that the ring was straight and the adhesive uniformly distributed.  To assure that the ring would bond properly I jammed a small screwdriver in the gap in the ring to widen and wedge it tightly into place while the epoxy cured.

Figure 2:
More or less what the resistors looked like
when the cover was removed.
Click on the image for a larger version.
After allowing 48 hours for the epoxy to fully cure I again inverted the speaker, removing the piece of paper towel that I'd put in place to prevent debris and epoxy from fouling the voice coil.  Subsequent testing indicated that the woofer was, again, working normally so I replaced the dust cover with a new one, identical in size to the old, that I'd ordered previously, placing the speaker back into service.

Unfortunately it did not occur to me to take pictures of this repair until I was well into it, hence the "thousand words".

Back to present day:

Everything was working fine until recently and this recent 'bout of distortion initially let me to believe that the stiffening ring has broken loose, so I removed the woofer from the enclosure and the distortion suddenly went away.  This immediately indicated to me that it was unlikely that the stiffening ring had come lose as the speaker now in free air rather than in the tuned cabinet moved more freely that before outside its enclosure.


Suspecting that gravity may have caused the cone assembly to sag over time - a problem not too uncommon with larger drivers with foam surrounds - I rotated it 180 degrees, placed the woofer back into the speaker and the distortion reappeared immediately.  While the distortion was occurring I pressed gently at different places around the edge of the cone to see if I could cause it to get worse but I didn't find anything obvious:  Doing this - and finding one spot on which when pressed causes sudden, severe distortion - can be a helpful diagnostic to determine if the cone is off-center, either due to gravity-related sagging or some sort of damage to or alignment of the "spider", voice coil assembly and/or the surround.

Figure 3: 
This picture shows where the resistors' leads broke,
right at the body of the resistors.
At the top of the picture are some of the plastic
capacitors used in the crossover which seemed
not to be affected by the vibration.
Click on the image for a larger version.
Puzzling over this problem for a moment, with the woofer laying on the floor next to the speaker, I happened to notice that one of the plastic grommets emerging from the metal crossover box inside the speaker had popped out of place so I snapped it back into the hole - an action that was coincident with a sudden bout of distortion in the woofer.  On a hunch I started whacking the grille cover of the crossover with the handle of a screwdriver and observed that every time I did so I could hear a sort of "clicking" in the woofer.

Inside the '4315, just behind the woofer, a grille covers a series of power resistors and plastic capacitors comprising the crossover (additional components like the inductors are located in a separate compartment behind) and upon removing the cover I noticed that at least one of the 10 watt resistors behind was at an odd angle.  When I touched this resistor, the problem was obvious:  One of its leads had broken away from its body due to fatigue and the vibration of the woofer and was causing it to make intermittent contact.  Clearly these rather heavy 10 watt power resistors had been vibrating for years, eventually causing the connecting wire to break.

Inspecting the other resistors, the leads of two others broke away from the slightest touch meaning that I simply had to replace them all.

Figure 4: 
The new resistors in place.  The two 10
ohm resistors (brown, tubular) did not
have leads so heavy-gauge wire was
used to connect and support them.
Click on the image for a larger version.
Fortunately, these resistors were nothing special - just plain, ordinary 10 watt, 10% tolerance wirewound resistors of the values 2.4 ohms (1), 10 ohms (2), 20 ohms (2) and 50 ohms (1).  A quick rummage through my collection of power resistors indicated that while I could have "kludged" a repair that minute, I decided that I wanted to replace all similar resistors in both speakers and I made a shopping list for the next time I happened to visit the local electronics place.

A few days later I found myself at the local electronics store,  rummaging through their resistor collection.  I didn't find exactly what I wanted as they didn't have any of the rectangular "sand" 10 ohm, 10 watt units, but they had some of the same in ceramic tubular which would work with attached leads.  They also didn't seem to have any 47 or 50 ohm 10 watt units, but I found that they had plenty of 100 ohm, 6 watt resistors so I obtained twice as many of those so that I could parallel them to 50 ohm at 12 watts.  For the 2.4 ohm resistors I had my choice between 2 ohm and 2.7 ohm and chose the former since the lead length was better for mounting:  Since this resistor was simply in series with the upper midrange ("HF") driver, and there is "T" pad to adjust its level - I figured that this departure in value from the original 2.4 ohm 10% part to a 2 ohm 5% part was not of consequence.  (Various diagrams show slightly different factory values, anyway - none of which matched the factory-installed 2.4 ohm resistor, anyway!)

The repair job was pretty straightforward.  In order to get the slightly larger, ceramic-tubular 10 ohm resistors to work I soldered to them some #12 AWG connecting leads (to provide rigid support and prevent future vibration-related breakage) and folded over the tabs along with paralleling the two 100 ohm units such that the effective lead length was increased (see picture).

Figure 5:
The cover for the crossover in place.
Some "blobs" of black RTV were added
previously to suppress a buzzing caused by
a mechanical resonance of this cover at
certain musical note frequencies.
Click on the image for a larger version.
I quick test revealed that the repair was successful with all of the drivers on the speaker working as they should.

When I replaced the grille cover I was reminded another "fix" that I'd done many years ago:  Some blobs of RTV (silicone) sealant along the edges of the grille cover forming damping pads to eliminate an annoying buzzing noise that it would make when it resonated with particular bass notes.


With the repair of one speaker done I quickly moved on to its mate.  In it, I found no broken resistor leads, but one or two felt very "weak" indicating that metal fatigue had already been at work.

Finishing the resistor replacements I now had both speakers working properly, using "matched" components.

Final notes:

After a quick search I was able to find several different diagrams for the crossovers themselves, no doubt due to changes made during the production.  It was interesting to note that the precise values of some of the resistors in my speakers weren't exactly those on any of the versions of the diagrams which indicates that, perhaps, JBL didn't consider them to be extremely critical and used what was available from their suppliers.  (If one looks at the diagram one can tell that +/-10-15% isn't really likely to make much of a difference in the properties, anyway - at least not one that couldn't be compensated for by the adjustable pads...)

Monday, April 18, 2016

Combatting scintillation effects on optical voice links

One interesting aspect of the amateur radio hobby that is rarely discussed is the use of the "Above 275 GHz" bands.  While one might, at first, think that this might require some exotic "componentry" to use these wavelengths, to assume such would ignore the fact that this includes "optical" frequencies - which is to say, visible light.

Working with visible light has a tremendous advantage over other "frequencies" in that we have some built-in test equipment:  Our eyes.  While generally uncalibrated in terms of "frequency" and power (e.g. brightness) they are of great help in building, setting up and troubleshooting such equipment.

For years now lasers have been considered to be the primary source of optical transmitters - which makes sense for some of the following reasons:
  • Lasers are cool!
  • They may be easily modulated.
  • Lasers are cool!
  • "Out of the box" they produce nicely collimated beams.
  • Lasers are cool!
  • Low-power diode-based lasers are inexpensive and easy to use.
  • Lasers are cool!
While lasers are (almost) exclusively used for all types of fiber-optic based communications, one might ask oneself if they are equally useful/effective when the medium is the atmosphere rather than a stable, glass conduit?

The answer is:  It depends.

If one is going very short distances - perhaps up to a few hundred meters - the atmosphere can be largely ignored unless there is something that is causing severe attenuation of the signals (e.g. rain, snow or fog) but as the distances increase, even if there is not some sort of adverse condition causing such loss there are typically nonuniformities in the atmosphere caused by thermal discontinuities, wind, atmospheric particulates, etc. that causes additional disruption.

The fact that Lasers produce (generally) coherent beams in terms of frequency and phase - gas lasers usually more so than most semiconductor types - actually works against efforts in making a long-distance, viable communications link because the atmosphere causes phase disruptions along the path length resulting in rapid changes in amplitude due to both constructive and destructive interference of the wavefront.

In the past decade or so, high-power LEDs have become available with significant optical flux.  Unlike Lasers, LEDs do not produce a coherent wavefront and because of this they are generally less affected by such atmospheric phenomenon, as the video below demonstrates:

Figure 1:
Visual example of laser versus LED "lightbeam"
communications.

Admittedly, the example depicted in Figure 1 is somewhat unfair:  The transmit aperture of the laser used for this test was very small, a cross-sectional area of, perhaps, 3-10 square millimeters, while the aperture of the LED optical transmitter was on the order of 500 square centimeters.  Even if both light sources were of equal quality and type (e.g. both laser or both LED) that using the smaller-sized aperture would be at a disadvantage due to the lack of "aperture averaging" - that is, more subject to scintillation due to the small, angular size of the beam causing what is sometimes referred to as "local coherence" where even white light can, for brief, random intervals, take on the interference properties of coherent light:  It is this phenomenon that causes stars to twinkle - even briefly change color - while astronomical objects of larger apparent size such as planets usually do not twinkle.

Figure 2:
Adapter used for emission of laser light via the telescope.
Contained within is a laser diode modified to produce
a broad, fan pattern to illuminate the mirror of the
telescope.

For an interesting article on the subject of scintillation, see "The Sizes of Stars" by Calvert - LINK.

Based on this one might conclude that the larger the aperture for emitting will reduce the likelihood that the overall beam will be disrupted by atmospheric effects - and one would be correct.  The use of a large-area aperture tends to reduce the degree of "local coherence" described in the Calvert article (linked above) while also providing a degree of "aperture averaging".  As an aside, this effect is also useful for receiving as well as can be empirically demonstrated by comparing the amount of star twinkle between the naked and aided eye:  Binoculars are usually large enough to observe this effect.

For a fairer comparison with more equal aperture sizes the above test was re-done using an 8 inch (approx. 20cm) reflector telescope that would be used to emit both laser and LED light.  To accomplish this I constructed two light emitters to be compatible with a standard 1-1/4 inch eyepiece mount - one using a 3-watt red LED and another device (depicted in Figure 2) using a laser diode module that was modified to produce a "fan" beam to illuminate the bulk of the mirror.

Both light sources were modulated using the same PWM optical modulator described in the article "A Pulse Width Modulator for High Power LEDs" - link - a device that has built-in tone generation capabilities.  Since the same PWM circuit was used for both emitters the modulation depth (nearly 100%) was guaranteed to be the same.

To "set up" this link, a full-duplex optical communications link was first established using Fresnel lens-based optical transceivers using LEDs and the optical receiver described in the article "A Highly Sensitive Optical Receiver Optimized for Speech Bandwidth" - link.  With the optical transmitters and receivers at both ends in alignment, the telescope was used as an optical telescope to train it on the far end, using the bright LED of the distant transmitter as a reference.  With the telescope approximately aligned, the LED emitter was then substituted for the eyepiece and approximately refocused to the effective optical plane of the LED.  Modulating the LED with a 1 kHz tone, this was used with an "audible signal level meter" that transmitted a tone back to me, the pitch of this tone being logarithmically proportional to the signal level permitting careful and precise adjustment of both focus and pointing.

For an article that describes, in detail, the pointing and setting-up of an optical audio link, refer to to "Using Laser Pointers For Free-Space Optical Communications: - LINK.

Now substituting the laser diode module for the LED emitter the same steps were repeated, the results indicating that the two produced "approximately" equal signal levels (e.g. optical flux at the "receive" end.)  Already we could tell, by ear, that the audio conveyed by the laser sounded much "rougher" as the audio clip in Figure 3, below, depicts.

Figure 3:
Audio example of laser versus LED "lightbeam"
communications over a 15 mile (24km) free-
space optical path.
Music:  "Children" by Robert Miles, used in
accordance with U.S. Fair Use laws.

Figures 4 and 5, below, depict the rapid amplitude variations using a transmitted 4 kHz tone as an amplitude reference over a "Free Space Optical" path of approximately 15 miles (24km).  The horizontal axis is time and the vertical axis is linear amplitude.

Note the difference in horizontal time scales between the depictions, below:

Figure 4:
Scintillation of the laser-transmitted audio (4 kHz tone).
The time span of this particular graph is just over 250 milliseconds (1/4 second)
Click on the image for a larger version.


Figure 5:
Scintillation on the LED-transmitted audio (4 kHz tone).
In contrast to the image in Figure 4, the time span of this amplitude representation is nearly 10 times
greater - that is, approximately 2 seconds.  The rate and amplitude of the scintillation-caused
fading are dramatically reduced.
Click on the image for a larger version.

Laser scintillation:

As can be seen from Figure 4 there is significant scintillation that occurs at a very rapid rate.  The reference of this image is, like the others, based on a full-scale 16 bit sample.  Analysis of the original audio file reveals several things:
  • While the "primary" period of scintillation is approximately 10 milliseconds (100Hz) but there is evidence that there are harmonics of this rate to at least 2.5 milliseconds (400 Hz) - but the limited temporal resolution of the test tone makes it difficult to resolve these faster rates.
  • Other strong scintillatory periods evident in the audio sample occur at approximate subharmonics of the "primary" scintillatory rate, such as 75 and 150 milliseconds.
  • The rate-of-change of amplitude during the scintillation is quite rapid:  Amplitude changes of over 30 dB (a factor of 1000) can occur in just 20 milliseconds.
  • The overall depth of scintillation was noted to be over 40dB (a factor of 10000) with frequent excursions to this lower amplitude.  It was noted that this depth measurement was noise-limited owing to the finite signal-noise ratio of the received signal.
LED scintillation:

Figure 5 shows a typical example of scintillation from the LED using the same size emitter aperture as the laser.  Analysis of the original audio file shows several things:
  • The 10 millisecond "primary" scintillatory period observed in the Laser signal is pretty much nonexistent while the 20 millisecond subharmonic is just noticeable.
  • 150 and 300 millisecond periods seems to be dominant, with strong evidence of other periods in the 500 and 1000 millisecond period.
  • The rate-of-change of amplitude is far slower:  Changes of more than 10 dB (a factor of 10) did not usually occur over a shorter period than about 60 milliseconds.
  • The overall depth of scintillation was noted to be about 25 dB (a factor of about 300) peak, but was more typically in the 15-18dB (a factor of 32-63) area.
One of the more interesting results of this experiment was how minimally the severe amplitude distortion experienced with the laser actually degraded the overall intelligibility of human speech.  While the tones and brief music clips were clearly badly distorted, it could be argued that with the segment including speech, the degree of that distortion was not as apparent.  Clearly the voice content was being badly "chopped up" by the severe amplitude fluctuations, but with the redundant nature of speech and the fact that the drop-outs were quite brief in comparison to the duration of speech elements (sounds, syllables) it is quite reasonable to be able to expect the brain to fill in the gaps and make sense of it all.

A "Scintillation Compensator":

Despite the redundant nature of the speech maintaining reasonable intelligibility, it became quite "fatiguing" to listen to audio distorted in this manner, so another device was wielded as part of an experiment:  The "Scintillation Compensator", the block diagram being depicted in Figure 6, below.

Figure 6:
Block diagram of the "Scintillation Compensator" system.
Click on the image for a larger version.
This system is essentially a "Keyed AGC" system using a low-level 4 kHz tone from the transmitter as an amplitude reference for a tracking gain cell at the receiver:  If the amplitude of the 4 kHz tone being received from the distant transmitter goes down, the gain of the audio in the receiver is increased by the same amount and vice-versa.  The effect of this device is quite dramatic as the clip in Figure 7, below, demonstrates:

Figure 7:
Audio clip with a"Before" and "After" demonstration
of the "Scintillation Compensator" 
Music:  "Children" by Robert Miles, used
in accordance with U.S. Fair Use laws.

One of the more striking differences is that in the "before" portion, the background hum from city lights remained constant while in the "after" portion it varied tremendously, more clearly demonstrating the degree of the amplitude variation being experienced.  What is also interesting is that the latter portion of the clip is much "easier" (e.g. less fatiguing) to listen to:  Even though syllables are lost in the noise - being obliterated by hum rather than silence in the first part of the above clip - the fact that there is something present during those brief interruptions, even though it is hum, seems to appease the brain slightly and maintain "auditory continuity".

It should be pointed out that the "Scintillation Compensator" cannot possibly recover the portions of the signals that are too weak (e.g. lost in the thermal noise and/or interference from urban lighting) but only that it maintains the recovered signal at a constant amplitude.  In the first portion of the clip in Figure 7 it was the desired signal level that changed while in the second portion it was the background noise that changed.  In other words, in both examples given in Figure 7, the instantaneous signal-to-noise ratio was the same in each case.

Practical uses for all of this stuff:

The most important point of this exercise was to demonstrate that a larger aperture reduces scintillation - although that point might be a bit obscured in the above discussion.  What was arguably more dramatic - and also important - was that the noncoherent light source seemed to be less susceptible to the vagaries of atmospheric disturbance.  This observation bears out similar testing done over the past several decades by many others, including Bell Labs and the works of Dr. Olga Korotkova.

For a brief bibliography and a more in-depth explanation of these effects visit the page "Modulated Light DX" - LINK - particularly the portion near the end of that page.

The reduction of scintillation has interesting implications when it comes to the ability to convey high-speed digital information across large distances using free-space optical means under typical atmospheric conditions.  Clearly, one of the more important requirements is that the signal level be maintained such that it is possible to recover information:  Too low a signal, it will literally be "lost in the noise" and be unrecoverable.

As the demonstrations above indicate, the "average" level may be adequate to maintain some degree of communications, but the rapid and brief decreases in absolute amplitude would punch "holes" in data being conveyed, regardless of the means of generating or detecting the light.  Combating this would imply the liberal use of error-correction and recovery techniques such as Forward Error Correction (FEC) and interleaving of data over time - not to mention some interactive means by which "fills" for the re-sending of missing data could be requested.  The "'analog' analog" to these techniques is the aforementioned ability of the human brain to "fill in" and infer the missing bits of information.

While lasers are well-known to be "modulatable" at high rates, doing so for LEDs is a bit more problematic due to the much larger device (die) sizes and commensurate increase in device capacitance.  To rapidly modulate an LED at an ever-high frequency would also imply an increase of "dV/dT" (e.g. rate of voltage change over time) which, given the capacitance of a particular device would also imply higher instantaneous currents within it, effectively reducing the average current that could be safely applied to it.  What this means is that it is likely that specialized configurations would required (e.g. drivers with fast rise-times at high current; structurally-small, high current/optical density LEDs etc.) to permit direct modulation of very high (10's of megabits) data rates.

Using the aforementioned techniques has rather limited utility when the free-space optical links extend out to many 10's of miles/kilometers owing largely to the vagaries of the atmosphere and the practical limits of optical flux with respect to "link margin" (e.g. the need to use safe and sane amounts of optical power to achieve adequate signal to recover information - particularly as the rate of transmission is increased) but it may be useful for experimentation.

Additional information on (more or less) related topics:



Wednesday, March 2, 2016

The solar saga - part 1: Avoiding interference (Why I did not choose microinverters!)

Back in November I decided to get some solar (photovoltaic) "grid tie" power generation installed at my house. I decided that the best place to install this was on the roof of my detached garage because:
  • The roof area of the garage was comparable to that of the house.
  • Much less tree shading than on the house.
  • Because it was not an occupied structure with no finished attic space, it was exempt from certain requirements (e.g. walkways around the panel areas, etc.) that would have reduced the available area for the installation of the panels.
  • It already had an existing, high-current circuit that was capable of being used for both source and sink of electrical current.
The only thing that I really had to do in the garage was to replace the 70's vintage Zinsco breaker panel with a more modern "load center" as a sub panel:  Doing so was a straightforward job that took only a few hours and cost less than $125 for all of the parts.

Unfortunately there was a significant snag to the "electrical" side of getting it connected to the utility grid via "Net Metering" (it's not "online" yet...) but that will have to wait for a later installment.

What kind of solar system?

In residential, grid-tie installations, two types of solar systems are most commonly found:
  • Series string.  This is where the panels are tied together and go to one, large power converter.  Many of these inverters have inputs for at least two, separate strings for redundancy, to accommodate different illumination profiles (e.g. "east versus west") and also to (statistically) increase efficiency.
  • Microinverter.  In this approach each, individual panel has its own, "private" power converter.
The series string approach is a bit older technology and its popularity is being overtaken by the microinverter approach since the latter is touted with the ability to extract more energy from the entire solar plant since the output from each, individual panel is optimized rather than relying on the "weakest link" from the bank of panels comprising the series string. With modern panels that are intrinsically well-matched, the "weakest link" issue is not as significant as it once was, but that's a topic for a later discussion.

I will say right now that I chose the series string approach for a very practical reason:

Radio Frequency Interference (RFI).

Interference from microinverters:

Let me spin time back to mid 2013 when I saw on an email group a plea from a local amateur (Ham) radio operator for help to analyze a problem that he was having.

He'd had installed a sizable solar plant (approx. 3 dozen panels), each with an Enphase M190 microinverter and suddenly found that he faced a tremendously increased noise floor on both HF and VHF.  By the time that he and I "connected" he had come to some arrangement with the manufacturer and/or installer to install "ferrite beads" (at their expense) on the microinverters' leads in an attempt to mitigate the problem.

He asked me to come over to verify the nature of the interference and its approximate magnitude, prior to the installation of the ferrite devices, and I arranged to do so.

When I arrived, he demonstrated the problem:  When receiving on his HF dipole, which spanned over a portion of his roof and solar panel farm, he experienced 4-6 S-units (20-40dB) of additional noise from the microinverters, depending on frequency.  The noise was that of typical AC mains-coupled switching supplies, being grouped in spectral "bunches" every 10's or hundreds of kHz or so (I don't recall the spacing) on the lower bands (75, 40 meters) and by the time one got to 15 meters it was pretty much just an even "smear" of noise across the spectrum.  By switching to AM, it was apparent that the noise itself had an amplitude-modulated component related to the mains frequency that was not readily apparent when listening on SSB.

The problem was also apparent on 2 meters where low-level spurious signals emanated by these devices were intercepted by his rooftop antenna and would open the squelch and/or mask weaker signals - including those of some of the more distant repeaters.

Analyzing the problem:

For this visit I'd brought along with my FT-817 portable, all-band, all-mode transceiver with a small 2 meter Yagi antenna, a small shielded "H" field loop for localizing signal sources and a specialized 2-meter DF antenna/receiver, to be used with the Yagi, and in switching to 2 meter SSB mode using the rubber duck antenna on the FT-817 I could hear a myriad of low-level carriers as I tuned up and down the band.

Stepping out onto the roof we approached the solar system and I wielded my other gear:  The DF receiver/antenna combination showed the source of the signals - on any random 2 meter frequency - to be that of the solar array. Switching to the combination of the FT-817 and the small, shielded H-loop I was able to localize the conductors from which the energy was being radiated:  Not only did it seem to be coming from the AC power mains cables connecting everything together, but also the frames and the front surfaces of the solar panels themselves, indicating likely egress on both the AC and DC sides of the microinverters.

Part 15 compliance?

At this point one might ask how such a product appeared on the market if it caused interference:  Doesn't FCC Part 15 "protect" against that?

No!

First of all, it is worth re-reading a portion of the text from Part 15 that I'm sure that you have noted somewhere on a device or in a manual that you have laying around.  Quoting from FCC Part 15, section 105 subpart (b):

This equipment has been tested and found to comply with the limits for a Class B digital device, pursuant to part 15 of the FCC Rules. These limits are designed to provide reasonable protection against harmful interference in a residential installation.
This equipment generates, uses and can radiate radio frequency energy and, if not installed and used in accordance with the instructions, may cause harmful interference to radio communications.
However, there is no guarantee that interference will not occur in a particular installation.
(The emphasis is mine.)

The above speaks for itself!

It should be observed that while Part 15 limits the amount of incidental RF energy that can be emitted/radiated/conducted from electronic devices to a certain level, that level is NOT zero!  The fact is that a device may be perfectly legal in its amount of emission, but still be detectable, under the right circumstances, from a significant distance.  In this particular situation, there were at least three things going against our solar system owner:
  • He was in very close proximity to the microinverters and solar panels.  As noted previously, his antennas for HF and VHF were either on the roof, or crossed part of it.
  • HF operation, by its nature, involves rather weak, narrowband signals.  This makes it even more likely that similar low signals emanated from devices would be noticeable and obvious and that broadband noise could be quite apparent.
AND
  • His solar system comprised approximately three dozen panels.  What this means is that each of those microinverters is, by itself, radiating its own, set amount of interference.  If you take the number as 36, this means that as a system, the total amount of energy being radiated by all of those microinverters put together will be increased by nearly 16 dB - that's nearly 3 S-units!  Practically speaking those inverters nearest the antenna(s) will cause the most problem due to proximity, but you can certainly see that many devices in one location are likely to exacerbate the issue overall.
I had no way to accurately measure the emitted signals from the microinverters to determine if they were compliant with part 15 or not, but I'm willing to believe that a widely-sold product such as an Enphase M190 microinverter had been tested and found to be in compliance by reputable people.

Figure 1:
A look inside the newer, Enphase M250, a model newer than the M190's
described as causing interference problems.  At the moment the jury is still
out if the M250 (or M215) is much "cleaner" than the older M190 in terms
of radiated energy.  While some decoupling - possibly filtering - is visible
on the AC mains connection at the bottom, no inline chokes are
apparent from the top-of-board view on the DC (solar panel) side - only
some capacitors that appear to bypass it to ground (e.g. the case.)
This M250 was given to me by an installer after it had failed in the field.
Click on the image for a larger version.
We discussed what it would take to make this microinverters completely quiet and I knew a way:  Completely enclosing each microinverter in a metal box with L/C Pi filters on both the DC input and AC output leads.  Proper L/C filtering of the input and output along with appropriate capacitive bypassing so that not only does RF energy not escape from the unit, but it also offers little/no potential for RF currents generated within to appear differentially between the DC input and AC output leads.

I have discussed similar interference-elimination measures related to switching power supplies in my August 18, 2014 post, "Completely Containing Switching Power Supply RFI" - link.  This method can be completely effective in reducing the interference level of such devices to undetectable levels.

It would have been nice if if there was available a weathertight box into which each microinverter could be mounted, along with a separate set of filtered input and output power connections.  The design of such a device would be slightly complicated by the fact that the Enphase units communicate via their powerline connections, but it was likely that this could be accommodated in the filter design.

I was quite sure that such an after market product did not exist at the time and even if it did, it would be prohibitively expensive, particularly when multiplied several dozen times!

My host asked me if I thought that the installation of ferrites on the input and output leads would help:  I thought that it might help a little bit on VHF and UHF, but that I couldn't see it having any useful effect on HF - but I hoped that I was wrong!

As I left this ham's house I had my FT-817 connected to my vehicle's antenna, listening in SSB mode on 2 meters and I could hear the low-level signals from his solar array from a distance of nearly two blocks, line-of-sight.

Post ferrite installation:

A few weeks later I got an email from this same ham stating that the ferrites had been installed on the microinverters.  To do this, it was necessary to (practically!) un-install and re-install the entire system as very few could be reached from the roof, requiring a lift to access.

Did it help?

Not that he could tell.

Is his situation unique?

Apparently not.

There are many anecdotes of amateur radio operators facing terrible interference issues after they - or their neighbors - install a microinverter-type solar system.  Once such instance is documented in the following thread on Reddit:
Neighbors just got solar - They gifted me with S-9 RFI  - link

Another case was documented several years ago on the "Ham Nation" Web TV show (Episode #65) where the only way to reduce the problem to a tolerable level was to relocate the antenna some distance away from the house-mounted microinverter system, at the far end of the lot.

A link to the webcast of Ham Nation episode #65 may be found here:  Link  (The relevant portion starts at 16:40.)


Since the original posting of this article a write-up appeared in the April, 2016 QST magazine that details another ham's battles with RFI from a solar electric system.  While this system was not microinverter-based, it used devices called "optimizers" that work on similar principles to the microinverters in that high-frequency switching supplies are used to maximize the amount of power available from the array.

Why the ferrites didn't/won't work:


There is a misconception amongst some that loading wires with ferrites will stop the ingress/egress of RF signals.

This does not happen.

By putting a piece of ferrite on a conductor one increases the effective impedance at a given frequency, but that impedance is not infinite, and the effectiveness of the ferrite depends on several things:
  • The characteristic impedance (real, complex) of the conductor on which it is placed at specific frequencies (it varies all over the map!) 
  • The size of the ferrite (length, diameter, etc.)
  • The material type (permeability)
  • The frequency
  • How many "turns" of the conductor may be passed through the ferrite.
For retrofits, the answer to last one is generally easy:  One turn, as that is all that may be accommodated with a typical "split core" ferrite that is installed simply by placing it over a wire.  As was certainly the case with the Enphase units, the connecting wires were simply too short to allow additional turns of wire even if the ferrite device were sized to allow it.

In general, ferrites have greater efficacy with increasing frequency, but this is not surprising since their mechanism is generally that of adding a bit of inductive reactance to the conductor on which they are placed - but this also explains why "snap on" or split ferrites are part of a futile attempt when one attempts to solve HF-related noise issues:
They simply cannot provide enough reactance to attenuate by the needed 10-30dB to solve most severe interference situations at HF!
Figure 2:
The outside of the same Enphase M250 as shown in Figure 1,
above, showing connecting cables:  Not much room
to place large ferrites on these - much less multiple turns!
(The cables on the M190 are of similar length.)
Click on the image for a larger version.

The reason for this is immediately apparent if one studies the specifications of a typical snap-on ferrite such as the Amidon 2x31-4181p2 link.  Here are some typical specifications for this rather large piece of ferrite:
  • I.D:  0.514" (13mm);  O.D.:  1.22" (31mm) ;  Length:  1.55" (39mm)
  • Material type:  31 (1-300 MHz, typical)
  • Reactance of device, typical:  25 ohms at 1 MHz, 100 ohms at 10 MHz, 156 ohms at 25 MHz, 260 ohms as 100 and 250 MHz
As you can see, the impedance is stated as 100 ohms at 10 MHz.  Being generous, let us apply that figure to the 40 meter band where we can see that if this were applied to a line that had a 50 ohms characteristic impedance, we might (theoretically, simplistically) expect to see somewhere in the area of 8-16dB of additional attenuation caused by the loss induced by this device - but that is only 1-3 "S" units, and that represents only a "good case" scenario.  In the case of the aforementioned situation it would have taken several more "S" units to reduce the noise to the point where it was not highly disruptive.

What is more likely to happen is that the interconnecting wires will have wildly varying impedances at different frequencies - some higher, some lower - and the this will have a dramatic effect on the efficacy of this reduction.  In the case of the ham that I had visited I would not have been surprised that if a plot had been taken of the noise versus frequency, its "shape" would have been dramatically altered by the addition of the ferrite devices and, overall, the amount of radiated energy (interference) would have been measurably reduced.  The problem was that the level was so high to begin with that knocking it down by, say, 90% (10dB, or just under 2 S-units) still represented a terrible situation!

The Amidon device noted above is a rather large device and at least three of them would be required for each microinverter (one for each DC lead, one for the AC connection) and the expense of these devices - not to mention the installation (36 microinverters would require 108 ferrite devices!) - could really add up!

It should go without saying that a smaller ferrite - although less expensive - will have even less effect than a larger one!

Comment:
Ferrite devices such as described are often more useful for preventing RF from getting into devices:  Increasing the impedance on the connecting leads and wires may not only improve the efficacy of already-existing RFI protection devices such as bypass capacitors, but they can also break up loops through which high RF currents induced by a local transmitter might be passing "through" a device.  In these case the moderate effect of their added impedance may well be enough to adequately mitigate RF ingress issues.

Remember:  With RF ingress it is often the case that knocking down the RF energy by 6-12 dB will be enough to mitigate the issue whereas the amount of "hash" emitted by the microinverters would likely need to be reduced by more than 20 dB to make it undetectable.

"Grounding" won't help either:

Reading some of the correspondence in the Reddit posting (above) there is mention of "grounding" to eliminate/reduce RFI from these units:  To assume that "grounding" would likely solve or mitigate this problem would be to assume incorrectly!

The problem, again, is that RF energy appears to be conducted from the input and output (DC and AC, respectively) coupling wires which, themselves, can act as antennae:  "Grounding" the case - which would also "ground" the safety ground on the AC output - is not really going to help.

If the unit is installed according to code, there should already be a "ground" attached at the panels, anyway - but this wire connection, which is likely to be 10's of feet (several meters) between the roof and the Earth or grounding point is going to look like a "ground" only at DC and low frequencies - such as those found on the AC mains!

Any wire that is several feet long - grounded or not - is going to act as an antenna.

What this means is that it is entirely possible that at least some of the RF interference being radiated by the inverter is going to be conducted along the grounded metal structures (such as the solar panels and the frames) and wires in addition to the AC mains wiring.

Again, the proper way to contain such RF energy within the confines of the circuitry was discussed above:  Proper L/C filtering of the input and output along with appropriate capacitive bypassing so that not only does RF energy not escape from the unit, but it also offers little/no potential for RF currents generated within to appear differentially between the DC input and AC output leads.

The upshot:

If you are getting interference from a microinverter system - either your own, or your neighbors, is there anything you can do?

Since the installation of ferrites will have minimal effect on HF, the answer would seem to be "No, not really", aside from converting to a series-string system, or installing a series-string system, instead.

In the case of the ham operator that I visited, he mitigated the situation somewhat by moving his HF antenna as far away from his house as he could (which wasn't very far considering that he had limited space on his city lot) which helped slightly.  Nighttime was the only time during which he could completely quell the interference by turning off the breaker feeding the solar array, but during the day there was nothing he could do:  If either solar illumination or AC mains power was available to the microinverters they seemingly caused the same amount of interference, whether they were under load or not!

Are newer microinverters better/quieter?

It has been reported that the Enphase M190 microinverter has been obsoleted and has been replaced with newer models that are more reliable and more "RF Quiet".  On this second point, the jury seems to be out:  Anecdotally, there seem to be about as many reports of the newer models (from various manufacturers) causing interference as not, so the reports are rather confused.

I know at least two amateur operators with newer-model Enphase inverters (M215, M250) but they report other extenuating circumstances (e.g. their microinverter PV system is located some distance from their antennas and/or they already had notable interference from other sources before installing the solar power system) that they cannot say for certain whether or not there is a problem caused by their system.  At some point I hope to personally visit at least one of those installations in the coming months.


Series String inverters and interference:

While less efficient overall and somewhat less expensive up front, I decided to use a series-string inverter system.  From direct observation and reports by people that I know and trust I knew that units made by Sunnyboy and Fronius could be reasonably expected to cause little or no interference on their own.  Additionally, were an interference issue to arise, having a single point at which to filter (e.g. one large box with a relatively small number of input and output leads) I was quite confident that it would be possible to add additional filtering if necessary.

To be sure, one might (theoretically) lose up to 10-20% or so peak efficiency with a series-string system as opposed to a Microinverter that optimizes for each, individual panel, but considering the comparatively low cost of panels these days and the lower "up front" cost for a series-string inverter system, one can usually afford to "up size" the system slightly to compensate.

(Comment:  As noted previously, series string "optimizers" have been observed to cause significant RFI since their basic principle of operation would lend to them tendencies to produce unwanted "hash" unless well-designed.)

Maintaining the various systems:

Anecdotally, from both owners and maintainers of microinverter-based systems, it is not uncommon to experience the failure of several of the microinverters after a only few years, the rate-of-failure (apparently) following somewhat of a "bathtub" curve:  Several die early on, there is often a period of relative stability, and then they start to fail in greater numbers after several more years.

While these devices (microinverters) seem to have a good warranty, the issue comes about replacing the microinverter that is in the "middle of everything" on the roof.  On a roof with a moderate-to-steep pitch it may be necessary to use equipment such as a lift to be able to safely access the failed inverter - and it may be necessary to "de-install" several of the surrounding panels to gain access.  In other words, it will likely be many times the cost of the microinverter itself ($125-$300) in equipment rental, time and labor just to replace it.  For this reason it seems that many people simply allow several of them to fail before "calling out the troops":   Having several panels (effectively) offline at a time is something that detracts from the proclaimed efficiency benefit of the Microinverter scheme!

The large, series-string inverters appear to be extremely reliable, having excellent track records (at least for Fronius and Sunnyboy - the two brands with which I have any familiarity).  The obvious down side is if there were a failure with the converter, it would likely take a large portion - or all - of the production off line, but the replacement of the device is comparatively easy and would likely not be more than a couple times the total cost (parts plus equipment rental plus labor) of replacing a small handful of microinverters!

What about failures of solar panels?  Modern panels contain diodes that "wire around" sections that have failed or shaded, so unless a catastrophic failure occurs that completely removes it from the circuit, one will lose, at most, the capacity of the entire panel:  This is true with both microinverter and series-string configurations.)

Fortunately solar panels have been around for decades and have been proven to be quite reliable and rugged in terms of durability.  If a failure in a solar electric system is going to occur, the solar panel itself is less likely to be the problem unless the problem is actual, physical damage.

(Note:  There may be warranty coverages or service plans that mitigate the costs related to such maintenance, but since they vary wildly with installers and manufacturers, they are not covered here.)

Final comments:

Each system has its advantages and trade-offs:  In my case a primary concern was the avoidance of interference.  Since the advent of digital TV - and because fewer people listen to the radio or even have off-air TV these days - they likely wouldn't notice (or would care!) about interference issues that appear to be common with the microinverter approach.

One can always hope that newer microinverters will become increasingly quiet, but for now that seems not the case - if not in reality, certainly in perception.


In the next installment I'll talk a bit more about the installation of my system - trials and tribulations...