Repairing a dead Kenwood TS-850S

Recently, a Kenwood TS-850S - a radio from the mid-early 1990s - crossed my workbench.  While I'm not in the "repair business", I do fix my own radios, those of close friends, and occasionally those of acquaintances:  I've known this person for many years and we have several mutual friends.

If you are familiar with the Kenwood TS-850S to any degree, you'll also know that they suffer from an ailment that has struck down many pieces of electronic gear from that same era:  Capacitor Plague.

Figure 1:
The ailing TS-850S.  The display is normal - except
for the frequency display showing only dots.  This error is
accompanied by "UL" in Morse.
Click on the image for a larger version.

This isn't the same "Capacitor Plague" of which you might be aware where - particularly in the early 2000s - many computer motherboards failed due to incorrectly formulated electrolytic capacitors, but rather early-era (late 80s to mid 90s) surface-mount electrolytic capacitors that began to leak soon after they were installed.

The underlying cause?

While "failure by leaking" is a common occurrence in electronics, this failure is somewhat different in many aspects.  At about this time, electronic manufacturers were switching over to surface-mount devices - but one of the later components to be surface-mounted were the electrolytic capacitors themselves:  Up to this point it was quite common to see a circuit board where most of the components were surface-mount except for larger devices such as diodes, transistors, large coils and transformers - and electrolytic capacitors - all of which would be mounted through-hole, requiring an extra manufacturing step.

Early surface-mount electrolytic capacitors, as it turned out, had serious flaws.  In looking at the history, it's difficult to tell what aspect of their use caused the problem - the design and materials of the capacitor itself or the method by which they were installed - but it seems that whatever the cause, subjecting the capacitors themselves to enough heat to solder their terminals to the circuit board - via hot air or infrared radiation - was enough to compromise their structural integrity.

Whatever the cause - and at this point it does not matter who is to blame - the result is that over time, these capacitors have leaked electrolyte onto their host circuit boards.  Since this boron-based liquid is somewhat conductive and mildly corrosive in its own right, it is not surprising that as surface tension wicks this material across the board, it causes devastation wherever it goes, particularly when voltages are involved.

There are some capacitors on the display/driver board on the front panel that should be replaced - but that's not where the majority of the problem lies.

The CAR board - the cause of "display dots"

In the TS-850S, the module most susceptible to leaking capacitors is the CAR board - a circuit that produces multiple, variable frequency signals that feeds the PLL synthesizer and several IF (Intermediate Frequency) mixers.  Needless to say, when this board fails, so does the radio.

They most obvious symptom of this failure is when damage to the board is so extensive that it can no longer produce the needed signals - and if one particularly synthesizer (out of four on the board) fails, you will see that the frequency display disappears - to be replaced with just dots - and the letters "UL" are sent in Morse Code to indicate the "Unlock" condition by the PLL.

Figure 2:
The damaged CAR board.  All but one of the surface-mount
electrolytic capacitors has leaked corrosive fluid and damaged
the board.  (It looked worse before being cleaned!)
Click on the image for a larger version.

Prior to this, the radio may have started going deaf and/or transmitter output was dropping as the other three synthesizers - while still working - are losing output, but this may be indicative of another problem as well - more on this later.

Figure 2 shows what the damaged board looks like.  Actually, it looked a bit worse than that when I first removed it from the radio - several pins of the large integrated circuits being stained black.  As you can see, there are black smudges around all (but one) of the electrolytic capacitors where the corrosive liquid leaked out, getting under the green solder mask and even making its way between power supply traces where the copper was literally being eaten away.

The first order of business was to remove this board and throw it in the ultrasonic cleaner.  Using a solution of hot water and dish soap, the board was first cleaned for six minutes - flipping the board over during the process - and then very carefully, paper towels and then compressed air was used to remove the water.

Figure 3:
The CAR board taking a hot bath in soapy water in an
ultrasonic cleaner.  This removes not only debris, but spilled
electrolyte - even that which has flowed under components.
Click on the image for a larger version.

At this point I needed to remove all of the electrolytic capacitors:  Based on online research, it was common for all of them to leak, but I was lucky that the one unit that had not failed (a 47uF, 16 volt unit) "seemed" OK while all of the others (10uF, 16 volt) had disgorged their contents.

If you look at advice online, you'll see that some people recommend simply twisting the capacitor off the board as the most expedient removal procedure, but I've found that doing so with electrolyte-damaged traces often results in ripping those same traces right off the board - possibly due to thinning of the copper itself and/or some sort of weakening of the adhesive:  While I was expecting chemically-weakened traces, already, there was no reason to add injury to insult.

My preferred method of removing already-leaking capacitors is to use a pair of desoldering tweezers, which are more or less a soldering iron with two prongs that will heat both pins of the part simultaneously, theoretically allowing its quick removal.  While many capacitors are easily removed with this tool, some are more stubborn:  During manufacture, drops of glue were used under the part to hold it in place prior to soldering and this sometimes does its job too well, making it difficult to remove it.  Other times, the capacitor will explode (usually just a "pop") as it is being heated, oozing out more corrosive electrolyte.

With the capacitors removed, I tossed it in the ultrasonic cleaner for other cycle in the same warm water/soap solution to remove any additional electrolyte that had come off - along with debris from the removal process.  It is imperative when repairing boards with leaking capacitors that all traces of electrolyte be completely removed or damage will continue even after the repair.

At this point one generally needs to don magnification and carefully inspect the board.  Using a dental pick and small-blade screwdriver, I scraped away loose board masking (the green overcoating on the traces) as well as bits of copper that had detached from the board:  Having taken photos of the board prior to capacitor removal - and with the use of the Service Manual for this radio, found online - I was confident that I could determine where, exactly, each capacitor was connected.

When I was done - and the extent of the damage was better-revealed - the board looked to be a bit of a mess, but that was the fault of the leaking capacitors.  Several traces and pads in the vicinity of the defunct capacitors had been eaten away or fallen off - but since these capacitors are pretty much placed across power supply rails, it was pretty easy to figure out where they were supposed to connect.

Figure 4:
The CAR board, reinstalled for testing.
Click on the image for a larger version.

As the mounting pads for most of these capacitors were damaged or missing, I saw no point in replacing them with more surface-mount capacitors - but rather I could install through-hole capacitors on the surface, laying them down as needed for clearance - and since these new capacitors included long leads, those same leads could be used to "rebuild" the traces that had been damaged.

The photo shows the final result.  Different-sized capacitors were used as necessary to accommodate the available space, but the result is electrically identical to the original.  It's worth noting that these electrolytic capacitors are in parallel with surface-mount ceramic capacitors (which seem to have survived the ordeal) so the extra lead length on these electrolytics is of no consequence - the ceramic capacitors doing their job at RF as before.  After (later) successful testing of the board, dabs of adhesive were used to hold the larger, through-hole capacitors to the board to reduce stress on the solder connections under mechanical vibration.

Following the installation of the new capacitors, the board was again given two baths in the ultrasonic cleaner - one using the soap and water solution, and the other just using plain tap water and again, the board was patted dry and then carefully blown dry with compressed air to remove all traces of water from the board and from under components and then allowed to air dry for several hours.

Testing the board

After using an ohmmeter to make sure that the capacitors all made their proper connections, I installed the board in the TS-850S and... it didn't work as I was again greeted with a "dot" display and a Morse "UL".

I suspected that one of the "vias" - a point where a circuit traces passes from one side to another through a plated hole - had been "eaten" by the errant electrolyte.  Wielding an oscilloscope, I quickly noted that only one of the synthesizers was working - the one closest to connector CN1 - and this told me that at least one control signal was missing from the rest of the chips.  Probing with the scope I soon found that a serial data signal ("PDA") used to program the synthesizers "stopped" beyond the first chip and a bit of testing with an ohmmeter showed that from one end of the board to the other, the signal had been interrupted - no doubt in a via that had been eaten away by electrolytic action.

Figure 5:
Having done some snooping with an oscilloscope, I noted
that the "PDA" signal did not make it past the first of the
(large) synthesizer chips.  The white piece of #30 Kynar
wire-wrap wire was used to jump over the bad board "via"
Click on the image for a larger  version.

The easiest fix for this was to use a piece of small wire - I used #30 Kynar-insulated wire-wrap wire (see Figure 5) - to jumper from where this control signal was known to be good to a point where it was not good (a length of about an inch/two cm) and was immediately rewarded with all four synthesizer outputs being on the correct frequencies, tuning as expected with the front-panel controls.

Low output

While all four signals were present and on their proper frequencies - indicating that the synthesizers were working correctly - I soon noticed, using a scope, that the second synthesizer output on about 8.3 MHz was outputting a signal that was about 10% of its expected value in amplitude.  A quick test of the transmitter indicated that the maximum RF output was only about 15 watts - far below that of the 100 watts expected.

Again using the 'scope, I probed the circuit - and comparing the results with the nearly identical third synthesizer (which was working correctly) and soon discovered that the amplitude dropped significantly through a pair of 8.3 MHz ceramic filters.

The way that synthesizers 2 and 3 work is that the large ICs synthesize outputs in the 1.2-1.7 MHz area and mix this with a 10 MHz source derived from the radio's reference to yield signals around 8.375 and 8.83 MHz, respectively - but this mix results in a very ugly signal, spectrally - full of harmonics and undesired products.  With the use of these ceramic bandpass filters - which are similar to the 10.7 MHz filters those found in analog AM and FM radios - and these signals are "cleaned up" to yield the desired output over a range of the several kiloHertz that they vary depending on the bandpass filter and the settings of the front panel "slope tune" control.

Figure 6:
The trace going between C75 and CF1 was cut and a bifilar-
wound transformer was installed to step up the impedance
from Q7 to that of the filter:  R24 was also changed to 22
ohms - providing the needed "IF-7-LO3" output level at J4.
Click on the image for a larger version.

The problem here seemed to be that the two ceramic 8.3 MHz filters  (CF1, CF2) were far more lossy than they should have been.  Suspecting a bad filter, I removed them both from the circuit board and tested them using a temporary fixture on a NanoVNA:  While their "shape" seemed OK, their losses were each around 10dB more than is typical of these devices indicating that they are slowly degrading.  A quick check online revealed that these particular frequency filters were not available anywhere (they were probably custom devices, anyway) so I had to figure out what to do.

Since the "shape" of the individual filter's passbands were still OK - a few hundred kHz wide - all I needed was to get more signal:  While I could have kludged another amplifier into the circuit to make up for the loss, I decided, instead, to reconfigure the filter matching.  Driving the pair of ceramic filters is an emitter-follower buffer amplifier (Q7) - the output of which is rather low impedance - well under 100 ohms - but these types of filters typically "want" around 300-400 ohms and in this circuit, this was done using series resistors - specifically R24.  This method of "matching" the impedance is effective, but very lossy, so changing this to a more efficient matching scheme would allow me to recover some of the signal.

Replacing the 330 ohm series resistor (R24) with a 22 ohm unit and installing a bifilar-wound transformer (5 turns on a BN43-2402 binocular core) wired as a 1:4 step-up transformer (the board trace between C75 and CF1 was cut and the transformer connected across it) brought the output well into the proper amplitude range and with this success, I used a few drops of "super glue" to hold it to the bottom of the board.  It is important to note that I "boosted" the amplitude of the signal prior to the filtering because to do so after the filtering - with its very low signal level - may have also amplified spurious signals as well - a problem avoided in this method.

Rather than using a transformer I could have also used a simple L/C impedance transformation network (a series 2.2uH inductor with a 130pF capacitor to ground on the "filter side" would have probably done the trick) but the 1:4 transformer was very quick and easy to do.

With the output level of synthesizer #2 (as seen on pin CN4) now up to spec (actually 25% higher than indicated on the diagram in the service manual) the radio was now easily capable of full transmit output power, and the receiver's sensitivity was also improved - not surprising considering that the low output would have starved mixers in the radios IF.

A weird problem

After all of this, the only thing that is not working properly is "half" of the "Slope Tune" control:  In USB the "Low Cut" works - as does the "High Cut" on LSB, but the "High Cut" does not work as expected on USB and the "Low Cut" does not work as expected on LSB.  What happens with the settings that do NOT work properly, I hear the effect of the filter being adjusted (e.g. the bandwidth narrows) but the radio's tuning does not track the adjustment as it should.  What's common to both of these "failures" is that they both relate to high frequency side of the filter IF filters in the radio - the effect being "inverted" on LSB.

I know that the problem is NOT the CAR board or the PLL/synthesizer itself as these are being properly set to frequency.  What seems to NOT be happening is that for the non-working adjustments, the radio's CPU is not adjusting the tuning of the radio to track the shift of the IF frequency to keep the received signal in the same place - which seems like more of a software problem than a hardware problem:  Using the main tuning knob or the RIT one can manually offset this problem and permit tuning of both the upper and lower slopes of of the filters, but that is obviously not how it's expected to work!

In searching the Internet, I see scattered mentions of this sort of behavior on the TS-850 and TS-950, but no suggestions as to what causes it or what to do about it:  I have done a CPU reset of the radio and disconnected the battery back-up to wipe the RAM contents, but to no avail.  Until/unless this can be figured out, I advised the owner to set the affected control to its "Normal" position.  If you have experienced this problem - and especially if you know of a solution - please let me know.

Figure 7:
The frequency display shows that the synthesizer is now
working properly - as did the fact that it outputs full power
and gets good on-the-air signal reports.
Click on the image for a larger version.

Final comments

Following the repair, I went through the alignment steps in the service manual and found that the radio was slightly out alignment - particularly with respect to settings in the transmit output signal path - possibly during previous servicing to accommodate the low output due to the dropping level from the CAR board.  Additionally, the ALC didn't seem to work properly - being out of adjustment - resulting in distortion on voice peaks with excessive output power.

With the alignment sorted, I made a few QSOs on the air, getting good reports - and using a WebSDR to record my transmissions, it sounded fine as well.

Aside from the odd behavior of the "Slope Tune" control, the radio seems to work perfectly.  I'm presently convinced that this must be a software - not a hardware - problem as all of the related circuits function as they should, but don't seem to be being "told" what to do.

* * * * *

This page stolen from ka7oei.blogspot.com

[END]

Fixing a TS-570G (The tuner couldn't find a match, timing out...)

The TS-570D's front panel

 A couple of months ago I happened to be at a swap meet in Northern Utah and talking to a gentlemen - with whom I had a passing acquaintance - as he was unloading his vehicles.  One of the things that he placed on his table was a Kenwood TS-570D, in its original box, with a price tag on it that seemed to be too good to be true.

Asking about it, he said that it worked fine, but that the "tuner wouldn't stop", so it had to be used with the antenna tuner bypassed.  Visually inspecting it, it looked to be in "good nick" (a 4 out of 5) so I shut up and gave him the money.

After digging out from underneath a few other projects, I finally took a look at it and sure enough, pressing the AT TUNE button started a bout of furious clicking that didn't stop for about 30 seconds with the radio beeping an error.  I couldn't help but notice, however, that there was no SWR or power output indication while the tuner was doing its thing - but if I bypassed the tuner, both of these were true.

Going into the menu (#11 - "Antenna tuner operation while receiving") I set that to "on" and noticed that the receiver went mostly dead - a sure sign that something was amiss with the signal path through the tuner.  Popping the covers, I whacked on the relays with the handle of a screwdriver while the radio was connected to an antenna and could hear signals come and go.  This attempt at "percussive repair" quickly narrowed the culprit to relay K1, the relay that switches the antenna tuner in and out of the signal path.

A few weeks later, after having ordered and receive a new relay, I cleared enough space on the workbench to accommodate the radio and commenced a repair.

The repair:

The antenna tuner is on the same, large circuit board as the final and low-pass filter, which meant that not only were there a zillion screws to take out, but I also had to remove the white thermal heat-sink compound from several devices, un-clip the back panel connectors and un-plug a few signal cables.  Using my trusty Hakko DFR-300 desoldering gun, I was able to cleanly remove both K1 and - because I had two relays, and they were identical - K3 as well, soldering in the replacement.

When I'd pulled the board, I also noticed that components "D10" - which is a glass discharge tube across Antenna connector #2 - had some internal discoloration, possibly indicating that it had seen some sort of stress, so I rummaged about and found two 350 volt Bourns gas discharge tubes and replaced both "D10" and "D11" - the unit on the Antenna #1 connector.  Unlike the originals - which are glass - these are metal and ceramic, requiring that I put a piece of polyamide (a.k.a. Kapton) tape on the board to insulate them from the traces underneath.  The leads of these new devices were also much heavier and would not fit through the board (drilling larger would remove through-plating!) so I soldered short lengths of #24 tinned wire through the holes and used these to attach the straight leads of the new discharge tubes.

After cleaning the board of flux with denatured alcohol and an old toothbrush, I put an appropriately sparse amount of heat sink compound on the required devices, loosely started all of the screws and with everything fitting, I snugged them all down, finishing with the RF output transistors - and then re-checking everything again to make sure that I didn't miss anything.

After plugging the connecting cables back in I noted that the receiver now worked through the tuner and pressed the AT Tune button and was greeted with lots of clicking and varying VSWR - but still, it continued and eventually errored out.

Figuring that the radio's computer may have been messed up, I did a complete CPU reset, but to no avail.  Because the SWR and power indication were working correctly, I knew that this wasn't likely to be a component failure like the reverse power detection circuit, so it had to be something amiss with the configuration, so I referred to the service manual's section about the "Service Adjustment Mode".

Going through the Service Adjustment Mode Menu:

Like most modern radios, this one has a "Service Menu" where electronic calibration and adjustments are performed and to get to it, I inserted a wire between pins 8 and 9 of the ACC2 jack and powered up the radio while holding the N.R. and LSB USB keys and having done this, a new menu appeared.  On a hunch, I quickly moved to menu #18 - the adjustment for the 100 watt power level.

What is supposed to happen is that if you key the radio, it will transmit a 100 watt carrier on 14.2 MHz, but instead, I got about 60 watts, and checking the related settings for 50, 25, 10 and 5 watts, I got very low power levels for each of those as well.  To rule out an amplifier failure, I went back to the 100 watt set-up and pressed the DOWN button, eventually getting over 135 watts of output power, indicating that there was nothing wrong with the finals, but rather that the entire "soft calibration" procedure would have to be followed.

Starting at the beginning of the procedure which begins with receiver calibration, I found everything to be "wrong" in the software calibration, indicating that either it was improperly done, or the original calibration had somehow been lost and replaced with default values.  I checked a few of the hardware adjustments, but found them to be spot on - the exception being the main reference oscillator, which was about 20 Hz off at 10 MHz, which I dialed back in, chalking this up with aging of the crystal.

During the procedure, I was reminded by a few peculiarities - and noticed some likely errors, and here they are in no particular order:

• Many of these menu items are partially self-calibration, which is to say that you establish the condition called out in the procedure and push the UP or DOWN button.  For example, on menu item #16 where the Squelch knob is calibrated, one merely sets it to the center of rotation, the voltage is shown on the screen in hexidecimal, and you press the button and the displayed value is stored temporarily in memory.
  

• I'm a bit OCD when it comes to S-meter calibration, preferring my S-units to be 6 dB apart, S-9 to be represented by a -73dBm signal as noted by the IRU specifications, and for "20 over" to actually be "20 over S-9", or around -53 dBm.  The procedure in the manual - and the radio itself doesn't permit this, exactly.
• To set the "S1" signal level (menu item #3) would require a signal level -121 dBm, but the receiver's AGC doesn't track a signal below around -113 dBm.  Instead, I noted the no-signal level on the display when menu #3 was selected and then set the signal level to an amplitude that just caused the hexidecimal number to increase and then pushed the button, setting "S1" to be equivalent to the lowest-possible signal level to which the AGC reacts.
• To set the "S9" signal level (menu item #4) I set the signal generator to -73dBm and pressed the button.
• To set the "Full scale" level (menu item #5) I set the signal generator to -23 dBm and pressed the button.  If you have followed the math, you'll note that "Full Scale" - which is represented as "60 over" should really be -13 dBm, but I observed that the AGC seemed to compress a bit at this signal level and the "20 over" and "40 over" readings came out wrong:  Using a level of -23 dBm got the desired results.
• NOTE:  The service menu forces the pre-amp to be enabled when doing the S-meter calibration (e.g. you can't disable it when in the service menu) so the S-meter calibration only holds when the pre-amp is turned on.
  

•  For setting menu item #1, "ALC Voltage" I was stumped for a bit.  It mentions measuring "TP1" - but this is not the "TP1" on the transmitter board, but rather the TX/RX unit (the board underneath the radio).
  

• I noticed that if step #7 was followed to set the 100 watt power level, it was difficult to properly set menu items 23-28 (the "TGC" parameters).  These adjustments set to 100 watts, but if you have already set menu item #18 at 100 watts, you can't be sure that you've properly done it.
• The work-around is that prior to step #6 in the procedure that you go to menu item #18 and adjust for higher than 100 watts - say, 125 watts.  If this is done, you can adjust menu items 23-28 (noting that menu #27 is adjusted out-of-order in procedure step #6) to 100 watts.
• Once procedure steps 6, 7 and 8 are done (but skipping the adjustment for menu #18 in step 7) you can go back to menu #18 and adjust for 100 watts.
  

• For procedure steps 16 and 17, I didn't have a 150 ohm dummy load, but I did have several 50 ohm loads, so I put three of them in parallel - which yields 16.67 ohms, which is also a 3:1 VSWR - and completed these steps.  It's worth noting that Yaesu uses 16.67 ohms for the equivalent step in its alignment procedures.  To set the "40 watts" called out in step 17 I used the front-panel power meter, which would have already been calibrated in the procedure.

The result:

As mentioned, the "hardware" calibration seemed to be fine and only the "soft" calibration was off and after following this procedure, the tuner worked exactly as it should.  What I suspect was occurring was a combination of the the output power being too low to calculate an SWR (e.g. setting the radio to "5 watts" yielded less then 2) and that the SWR meter calibration itself was incorrect and that this combination of factors prevented the tuner from being able to find a match.

Since the repair, the TS-570 has been used several times per week and it is working just as it should!

This post stolen from ka7oei.blogspot.com

[End]

Repairing the power switch on the Kenwood KA-8011 (a.k.a. KA-801) amplifier

Back around 1990 my brother mentioned to me that there was an amplifier, in a box, in pieces, in the back room of the TV/electronics store where he worked at the time and that if I made an offer I could probably get it for cheap.  Dropping by one day I saw that it was a Kenwood KA-8011 Integrated DC amplifier (apparently the same as the KA-801, except with a dark, gray front panel) laying in a box from which the covers were removed with a bunch of screws and knobs laying in the bottom.  I also noticed with some surprise that it had a world-wide voltage selector switch on the back and that the power cord had a Japanese 2-pin wall plug and U.S. adapter - and still does!  All of the parts seemed to be there so I offered some cash ($50, I seem to recall) and walked out with it and a receipt.

Figure 1:
 Spoiler alert:  This is the KA-8011 with the repaired power switch.
As noted in the text, the original, blue-painted panel meter lights were
replaced long ago with blue LEDs.

When I got home with the amplifier I knew that I had my work cut out for me particularly since, in those days before the widespread internet, I had no schematic for it and no-one that I contacted seemed to be able to find one.  Powering it up I noted that the speaker protection relay would never engage, indicating that there was a fault somewhere in the amplifier.

A visual inspection of the awkward-to-reach back panel's circuit board revealed several burned-looking leads sticking up from the circuit board where transistors had exploded near several burned resistors.  After a few hours of reverse-engineering a portion of the circuit I realized that the majority of the circuit at fault was one of four identical phono preamp input circuits (there are two separate stereo phono inputs) and associated low-level power supplies.  Between the intact amplifier sections and being able to divine the color bands on the smoked resistors - along with some educated guesses - I was able to determine the various components' values and effect a repair.

Figure 2:
The power switch, with a broken bat.
Click on the image for a larger version.

The amplifier now worked... sort of.  I then had to sort out a problem with the rear-panel input selector switch, operated by a flat, thin ribbon of stainless steel in a plastic jacket that was engaged from a front-panel selector.  I managed to cut off the portion at the front that had been damaged where it was pulled-on from the front panel having been loose in the box, punch some new holes in the ribbon, align the two (front and rear) portions of the switch mechanism and restore its operation.

Snap!

Having done the above, the amplifier was again operational and I have used it almost every day in the 25+ years since, needing only to replace the blue-colored incandescent meter lights with LEDs, powered from a simple DC filtered supply.  In the intervening years I also had to replace some of the smaller electrolytic capacitors on the main board that had gone bad, causing the speaker protection circuit to randomly trip on bassy audio content and with slight AC mains voltage fluctuations.

Figure 3:
Comparing the old (top) and new (bottom) switch components.  In order
to prevent it from interfering with the body of the switch some of the
metal on the new bat would have to be removed.
Click on the image for a larger version.

I was annoyed when one day, a few months ago, the power switch handle - which had been bent before I got the amplifier - and then "un-bent" during the repair - broke off in my hand when I turned it on.

Using the "bloody stump" of the power switch for a few months  I finally did a search on EvilBay to look for a new switch.  While I didn't find a power switch I did see a "tone control" switch for the same series of amplifier - so I got that, instead.  When it arrived I noted, as expected, that most of it did not mechanically resemble the power switch or look as though it would easily mount in the same location, but it did have essentially the same metal bat on the end as the original that I figured I could fit onto missing portion that had broken off the power switch.

Comment:

Even though the "new" switch was much too small - of insufficient current rating - to have been used to switch the mains (AC input) power, it would have sufficed to operate a relay.  To have done this would have required that new holes be drilled in the front sub-panel to match those of this new, smaller switch. While this would not be "original" circuitry, it would have looked the same from the front panel.  This is a possible option should the power switch itself become unreliable some time in the future due to the heavy capacitor-charging currents at turn-on.

Removing the original power switch I laid the two side by side and made notes of the differences between the metal bat of the new and original - which was narrower in some places to clear parts of the switch body - and taking a file to the new one I took off some metal to clear the possible obstructions.  I then noted on a crude drawing the length and orientation of the new bat based on the axis of the switch's pivot point.  Because the bat of the original switch was embedded in a block of molded Bakelite I knew that I would have to somehow attach a portion of the new switches' bat to the old, so I carefully disassembled with old power switch, cutting off and saving the original rivet on which the switch pivoted, noting where everything had gone and saving the small springs, contacts and some small Bakelite pins.

Figure 4:
The new bat, butt-soldered on the old switch.  Note that the bat from the
"new" switch has been filed to better-resemble the shape of the
original bat to clear the switch body.
Had I not been able to find a "similar" switch on EvilBay I could have
probably measured the original switch, found some scrap
steel of similar thickness and made a suitable replacement entirely
by hand with careful filing using the bat of another switch as a template.
Click on the image for a larger version.

Clamping the old part in a vise I cut off most of the original bat, leaving about 5mm of metal remaining.   Carefully comparing the old and new piece I then marked where, on the new bat, that I would have to cut to allow the repaired piece - consisting of the new and old butted and laid end-to-end - have the same length as the intact original.  Doing so, purposely cutting the "new" bat slightly long, I did some fine tuning with a file until the two pieces laid down precisely lined up as they should.

Attaching the new piece

Using some silver solder intended for stainless steel I applied some of its liquid flux - apparently a mixture of chloric and hydrochloric acid - and using a very hot soldering iron I "butt-soldered" the two pieces together in careful alignment and then filed the surfaces flat to remove excess.  While the bakelite switch body can handle a brief application of a soldering iron, I knew that it would not tolerate the heat from a proper, brazed joint.

This (weak!) solder joint was intended to be temporary, needing only to be good enough to allow a sleeve to be made by wrapping an appropriately cut piece of thin, tin-plated steel (from my junkbox) around the joint.  Once this sleeve was checked for proper fit and folded tightly, additional flux was applied and the entire joint - sleeve and all - was soldered, the result being a very strong repair with the restored bat being of the same length and at the same angle as the original.

Reassembly:

The trick was now to get every thing back together.

Figure 5:
The steel sleeve being installed over the butt solder joint,
before final folding and soldering.
Click on the image for a larger version.

Reinstalling the pivot and making a few clearance adjustments to the original switch's frame with a small needle file, the original rivet was then soldered into place and the entire assembly washed in an ultrasonic cleaner to remove the remnants of the corrosive flux from the bat and switch body.

In the base of the switch, the contacts, which were the same as those had it been an SPDT switch, were reinstalled - this time, rotated 180 degrees so that the previously unused contact portions would now be subject to electrical wear.  These contact were then "stuck" into place with a dab of dielectric grease so that they would not fall out when the switch body was inverted.

Figure 6:
The repaired switch, reassembled,  with the new bat spliced on.
Click on the image for a larger version.

After reinstalling the springs and pins, the rear part of the switch with the contacts was placed over the top of the moveable portion, held in the mechanical center, and the base was carefully pushed into place, compressing the internal springs and pins.  Holding everything together with one hand the proper operation of the switch was mechanically and electrically verified before bending the tabs to hold everything into place.

In reality the reassembly didn't go quite as smoothly as the above.  During one of the multiple attempts to get everything back together the smaller-diameter rear portion of the small, spring-loaded Bakelite pins used to push on the contacts snapped off.  To repair these pins the front, larger-diameter portions - that which pushed against the metal contacts - were placed in the collet of a rotary tool and a shallow hole was drilled into the rear portion where the broken pieces had attached to fit short pieces of 18 AWG wire:  By rotating the piece into which the hole was to be drilled, the exact center is automatically located.

The pieces of wire were then secured using a small amount of epoxy - a process accelerated by placing the pins in a 180F (80C) oven for an hour.  After the epoxy had set the wires were then trimmed to the length of the original sections that had broken off and the ends smoothed over with a small needle file to prevent their snagging on the spring.  The result was a repair that was stronger than the original pins that easily survived switch reassembly.

The results:

Figure 7:
After reassembly it was noted that the gray "skirt"
was hitting the front sub-panel frame, preventing it from
being set to the "off" position.  A bit of heat was applied to
set a permanent bend so that it would clear this panel.
Click on the image for a larger version.

The amplifier was then put back together - very carefully.  The only real issue that I noted was that the gray plastic skirt/escutcheon on the bat ended up about half a millimeter farther away from the switch body and closer to the sub panel than before, causing it to snag on the front sub-panel's cut-out when I attempted to move it to the "off" position.  Careful softening of the plastic with the rising heat of a soldering iron and bending it very slightly allowed it to clear.

Putting all of the knobs back on, tightening the bushing nuts and screws as necessary before doing so, I then tested the amplifier on the bench and was pleased to find that I'd not managed to break anything.

Finding that everything was working fine I put it back on the shelf where it belongs where I continue to use it often.

[End]

This page stolen from "ka7oei.blogspot.com".

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

Important:

If you own a Kenwood TS-450 - or perhaps any other radio - and the motors squeal when the auto-tuner activates, DO NOT ignore it:  If/when the motors seize, they will burn out!

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 (approx. 50F, 10C) 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 motor was going to have less effect on its bushings, and this noise indicated that the motor(s) in the tuner were in need of fresh 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 if suitable care is not taken:  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 where they are most exposed to the environment - 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 or may not 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 even been used successfully by the author to "un-stick" quite a few galled/damaged shafts and "fix" dried out fans 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 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, tissue or a cotton swab.

If it is not practical to access the end of the left-hand motor (the one marked with "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 try adding a bit more lubricant, but 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.

Comment:  Although I have not attempted such, it may be possible to drill a very small hole in the end of the plastic cap, once the metal cap has been removed, to allow lubricant to be directly put into the end of the shaft.  The risk is that small chips of plastic may foul the bushing, making it work badly or that the shaft itself may be damaged during the drilling.  If this is done, use only a drill press, a very small drill bit and hold the motor firmly in a vise.

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 no current flows the connection is open and 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 will need to make the radio go through a tuning cycle on every band.

Reassembling and reinstalling the tuner:

Figure 11:
The routing of the flat, white ribbon cable in the gap in the bracket.
Click on the image for a larger version.

Now, reassemble the tuner, first putting the tuner's 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!

* * *

Follow-up:

I recently came back from the 2018 Field Day where these radios were used again - and it was very cold on the morning (at 8100+ feet/approx. 2500 meter altitude) and the tuners in the radio did not squeal at all.  It's been only 2 years, but I'd still call it a good sign!


[End]

This page stolen from "ka7oei.blogspot.com".