Analysis of interference from a SolarEdge PV (solar) electric system.

Comment:

This article - while it centers about the investigation of a SolarEdge PV (PhotoVoltaic) system - the discussions of techniques and strategies should be generally useful when investigating interference from any make or model of PV system - or even interference from other sources.

* * *

Several months ago I got a call from a local amateur who was very concerned about a sudden rise in his noise floor across the HF spectrum (3-30 MHz).  This increase in noise seemed to be coincident with the installation and commission of a 5 kW PV (Photovoltaic, or "Solar") electrical system on the house of an adjacent neighbor.  I suggested that he talk to the manufacturer of the PV system to discuss the situation - and to request from them possible solutions.

A few weeks ago, he got back to me and he had, in fact, talked to the manufacturer and an online meeting was arranged in which they would remotely idle the neighbor's system while we were monitoring via the amateur's receive antenna.

Out of curiosity - and as sort of a practice run - I went over the weekend before the online meeting to get a better idea as to the spectral signature of this system - a SolarEdge series string system with optimizers - when it was operating normally.  The amateur had obtained permission from the neighbor to allow us to enter their yard to make very "close in" measurements (e.g. within a few inches/cm of the equipment, conductors) to obtain spectral "samples" of the system, thereby excluding external signals.

For these measurements, I used an amplified, shielded magnetic ("H") loop antenna (about 18"/50cm) in diameter as the "sense" antenna and an HP/Agilent/Keysight spectrum analyzer, recording the plots electronically - although a Tiny SA "Ultra" would likely have sufficed as well.  None of the readings were to represent "absolute" signal levels as all that we were really interested in were relative measurements, and as such all that we needed to do was keep our measurements consistent - that is, being able to precisely repeat the conditions between subsequent measurements.

Note:  For information about the H-field loop used for this testing - and using an inexpensive spectrum analyzer such as the "Tiny SA" or, better yet, the "Tiny SA Ultra", see a previous blog entry "RFI Sleuthing with the Tiny SA" - Link.

The nature of this QRM:

The interference observed by this amateur - now known for certain to be signature of this model of SolarEdge PV system - was evident as two general types of signals:

• Moderate to strong clusters of carriers every "even" 200 kHz.  At 200 kHz intervals (e.g. 7.0, 7.2, 7.4 and 14.0, 14.2 and 14.4 MHz) from below 3 MHz through at least 30 MHz could be heard a melange of closely-spaced carriers within about 500 Hz of each other on the lower bands.  While these carriers sounded like mostly CW (unmodulated) signals, there was also evidence of low rate data signalling buried in the cacophony as well as additional lower-level carriers.
  

• Background "white" noise amplitude-modulated at the mains frequency.  If you were just casually listening at this amateur's QTH on the HF bands - say 40 meters - you might be forgiven in the short term for presuming that nothing was wrong.  In reality the noise floor had been elevated several "S" units by the PV system - the result sounding superficially like plain, old white noise:  Switching from SSB to AM reveals the loud "hum" that is riding on the noise - modulation that is almost "invisible" if one is listening only using SSB.

While the appearance of the above interference coincided with the activation of the neighbor's system, that fact that it disappeared at night further pointed to a PV system as the source of the QRM.

"Close-in" measurements

Placing the sense antenna right at the main inverter on the back of the neighbor's house, we wanted to take a snapshot of the spectrum at that location:

Figure 1:  0-30 MHz sampled right at the main inverter

With each horizontal division representing 3 MHz, this 0-30 MHz plot shows a high concentration of noise in the 3-9 MHz area from a location right at the inverter.

Because Figure 1 represents the spectrum at the inverter, we wondered what it would look like at one of the solar panels so we placed the sense antenna right against one of the solar panels:

Figure 2:  0-30 MHz sweep with sense antenna placed next to a solar panel

Figure 2 is in the same frequency and amplitude scale as Figure 1 - but with the "reference level" adjusted by 20 dB to move the trace "up" a bit - and we can see that the spectrum next to the panel looks quite different from that sampled right at the inverter.  This isn't unexpected as Figure 2 would likely represent more of the noise that is emitted from the DC (input) side of the optimizer whereas the spectrum represented in Figure 1 would be more likely to show that of the DC output of the optimizer plus whatever noise was riding on the conductors carrying the DC input and AC output of the main string inverter.

Although it is difficult to be sure, the 0-30 MHz plots taken from a greater distance (10 meters or more) had the general appearance of the noise spectra shown in Figure 2 more than that of Figure 1 leading me to believe that a significant portion of the QRM may be being radiated from the panels themselves rather than just the conductors going from the optimizers  to the main inverter - but certainly, both are likely involved.

Note:  For both of these plots, the RF energy from the PV system was many 10s of dB above the typical background noise floor - in this case, 40-50dB for Figure 1 and at least 30dB for Figure 2 in the area around 7 MHz.

As the 0-30 MHz sweep does not have enough resolution to visualize the narrower 200 kHz signals, the analyzer was readjusted as depicted in Figure 3 - again with the sense antenna next to the panel:

Figure 3:  From near the panel, a "zoomed in" spectral sweep showing narrowband birdies.

In this spectrum plot we can see not only the "white" noise on the floor of the sweep representing the "hummy hiss", but also the much stronger signals every 200 kHz - plus a number of weaker signals in between:  It is these signals that are the most obvious to the casual operator and appear to be unique to a SolarEdge system of this model/type.

On this same day we waited until after sunset - monitoring the groups of carriers at 7.2 MHz and hearing them "flicker" out of existence as it got dark and we re-did the "next to the panel" measurements - this time the spectrum was devoid of the 200 kHz-spaced carriers (they were no longer audible on the amateur receiver, either) and the 0-30 MHz plots were 10s of dB lower than in the daylight. 

Important:  The 2 MHz sweeps in Figures 3-7 use a resolution bandwidth of 10 kHz which is almost exactly 4 times wider than the typical SSB bandwidth of an amateur receiver of about 2.5 kHz making their apparent level above the background noise appear lower than it is actually is.

What this means is the coherent signals - such as the 200 kHz carriers - appear to be another 6 dB farther above the noise floor in an SSB bandwidth than what the analyzer plots show.

Plots from a distance

Having captured some "close-in" plots, we now had an idea as to what the signals emitted by the PV system looked like.

A few days after we made the above plots we were in a virtual meeting with the manufacturer of the PV system (SolarEdge) from the ham's shack.  Having reconfigured the feed to his main radio, we could quickly switch the feedline from the antenna feeding the radio and the spectrum analyzer.

At this time we also learned that there was a second SolarEdge system south of this amateur's QTH - about 150 feet (45 meters) away across the cul de sac - and that the neighboring system and the one across the street would but remotely shut down, in that order, to determine how much QRM was emanating from each.

While we captured 0-30 MHz plots of each stage of system shutdown, for the purposes of this article we'll show just the "narrow" plots in 2 MHz sweeps as depicted in Figure 3 as the presence of the 200 kHz signal are generally representative of the presence of the broadband noise as well and these signals were easily identifiable and now known to be indicators of QRM from this type of PV system.

First, here's the plot from the amateur's 40 meter inverted Vee antenna with both systems on:

Figure 4:  6-8 MHz plot from the 40 meter antenna showing the 200 kHz peaks - and a bit of broadband noise as well.

 

The next plot shows the effects when the neighboring system was turned off, but the one across the street still on:

Figure 5:  The neighboring system off - but the one across the street still on.

As can be seen, the broadband noise floor around the 40 meter band (approximately one horizontal division below and above the marker) has dropped visibly - around 3-4 dB - and the amplitude of the carrier at 7.2 MHz has dropped about 6 dB - and the 200 kHz signals have disappeared almost entirely below about 6.5 MHz.  The system across the street was then shut off and the only remaining signals were those that happened to be on the 40 meter band.  (No trace is available for this configuration, unfortunately.)

As the 40 meter inverted Vee is oriented to favor east-west signals it was not necessarily the best candidate to test the effects of the PV system across the street, so we switched to a 20 meter antenna which was responsive in that direction and this trace shows the plot between 13 and 15 MHz:

Figure 6:  This plot of the 20 meter band and surrounding frequencies shows only propagated signals, with no sign of PV system QRM.

As both systems were off, the trace was quite clear - only showing signals that actually were on or near the 20 meter band, propagated from elsewhere in the world.  The folks at SolarEdge then turned on the system across the street with the following result:

Figure 7:  Same as Figure 6, but with only the PV system across the street activated.

The effect is very obvious:  In the vicinity of the 20 meter band, the appearance of rather strong signals every 200 kHz is obvious - and there is an obvious 2-4 dB increase in the noise floor indicating that this system, too, is causing harmful interference.

Readings on the radio:

It would seem that the folks at SolarEdge had worked with more than one amateur radio operator on similar issues and I was pleasantly surprised when they asked for some "S-Meter" reading comparisons with the neighbor's system on and off.  Using a calibrated signal generator, I'd already determined the signal level (in dBm) that correlated with the S-meter readings for the Icom radio - and here are the results for 40 meters:

Both systems off:

S1 (<= -84 dBm) - no carrier groups every 200 kHz.

Neighbor system on:

S4 (-78 dBm) - white noise

S9 (-67dBm) - carriers at 7.2 MHz

This shows that at 40 meters, the degradation to noise alone was on the order of 6 dB (most Japanese radios are calibrated for 3dB per S-unit) and that the cluster of carriers on 200 kHz intervals was far more destructive, rising a bit short of 20dB out of the noise floor.

As our time with the SolarEdge folks in the virtual meeting was limited, we were not able to do similar "S-meter" tests on 20 meters, but we can use the 40 meter results along with the relative strength of the 200 kHz-spacing carriers  and correlate them with the 40 and 20 meter spectrum analyzer traces and determine that the severity of QRM from the PV system on 20 meters on the receiver would have been roughly comparable to that on 40.

Analysis of these readings and implications:

As mentioned earlier, there are two types of interfering signals produced by these SolarEdge PV systems:

• Moderate to strong clusters of carriers every "even" 200 kHz.  These are very obvious, easy to identify, and quite strong compared to the noise with a few weaker signals in-between that were also clearly audible.
  

• Background "white" noise amplitude-modulated at the mains frequency.  This is also present, but it borders on insidious as the average amateur may not be able to quantify its existence - let alone its effects - as its effects may be obscured if one only listens for it using SSB modes.

Will my radio's DSP help?

The quick answer is "No".

While you might think that modern receivers' ability to "notch out" tones might help alleviate the effects of the signals every 200 kHz, you would be wrong.  It appears that each, individual optimizer module (there is one for every solar panel) produces one of these signals - and being based on individual oscillators, their frequencies will be slightly different from each other meaning that instead of needing to notch just one tone, your DSP would have to notch out dozens emanating from a single PV system - and it just cannot do that!  What's worse, these carriers are also modulated by the low-rate data used to communicate to/from each, individual module which broadens their spectrum as well.

As for the "white" noise, it is unlikely that noise reduction would help much, either:  The source of this appears to be an artifact of the actual voltage converters themselves and as it is random, it is as difficult to reduce in its effects as the normal background noise of the bands.

As each optimizer module contains is own switch-mode power converter to maximize panel efficiency, they, to - like any switch-mode supply - will produce harmonic energy.  It would appear that SolarEdge uses switch-mode controllers that employ "spread spectrum" clocking so that instead of having a myriad of harmonics and birdies all throughout the RF spectrum, that energy is "smeared" all over the place making it somewhat less obtrusive.

The use of spread-spectrum clocking is very widely used these days for the reasons noted above - and for the fact that it also enables the exploitation of a quirk when a device is subjected to testing for regulatory (FCC) compliance:  Aspects of that testing specify the maximum amount of signal energy that may be present in a given bandwidth - but by "spreading" it over a much wider bandwidth, that same amount of energy would be diluted and make the readings obtained during the testing appear lower.  This is perfectly legal and commonly done - but this technique does nothing to reduce the total amount of energy radiated - only filtering can do that!

It is apparent that in this particular case, both the neighboring system and the one across the street contribute a magnitude of interference that would be considered to be "harmful" in that it is perfectly capable of submerging weak-to-moderate signals into locally-generated noise - and if such signals happened to land near a 200 kHz harmonic rather than the elevated noise floor in between the effects are >10dB more destructive.

It is also apparent that the radiated noise extends - at the very least - from the 40 meter to 20 meter bands (7-14 MHz) but the 30 MHz plots imply a significant amount of RF energy above and below this:  The limited time permitted a semi-detailed analysis of only the interference around the 40 and 20 meter bands.

After the meeting:

At the conclusion of these tests, the analyzer readings that took were forwarded to the folks at SolarEdge for their analysis - and it is still too soon to know of any conclusions that would indicate what sort of actions that they might take.  We were, however, heartened to know that they seemed to understand and were sympathetic to the plights of amateurs affected by neighboring systems that might be adversely affect amateur radio operation.

The folks at SolarEdge themselves offered the best hope of resolution:  They noted that they have a special version of PV hardware (e.g. optimizers) that has additional filtering that could be retrofitted into an existing system to reduce the potential for interference.  As this retrofit would be done on their "dime" - and it would be rather expensive - they understandably want to be sure that they have identified only systems that are of their manufacture that are causing interference.

Is a system near you?  You can listen for yourself!

Somewhat ominously, I have since tuned to 14.2 and 14.4 MHz on my mobile HF station while driving around residential and interstate roads in my local area (Salt Lake City, Utah) during my normal commute/business:  I can, in many places, hear the characteristic "roar" of narrow carriers every 200 kHz - likely from SolarEdge PV - systems as these carriers seem to disappear during the hours of darkness.

I have heard this characteristic signal even in locations that appear to be several city blocks from any structure that might be equipped with a PV system.  They may also be heard on other bands - including 40 meters - but the signals emitted on the higher bands (e.g. 20 meters) seem to be emitted with greater efficiency.

It would seem that these 200 kHz-spaced groups of carriers really get out!

"I have interference from a PV system - what should I do?"

At this point I will not reiterate in detail remediation methods that might be undertaken by a radio amateur affected by this type of PV system:  The June, 2016 QST article (link) discusses attempted mitigation using ferrite devices in detail. (Note:  This article also refers to experiences with a SolarEdge system - but the spectra of the system described there is different from what I found on the systems described here likely due to it being a now-older system.)

 I will only mention in passing that there's the possibility that a degree of mitigation may be possible with the use of "noise cancelling" antennas of the sort offered by Timewave, MFJ and others - but their utility is also somewhat limited owing to practical concerns (e.g. such techniques work best on distant "point sources" of interference rather than very nearby, spread-out radiators in the near field).

If you have interference from a PV system, it is up to YOU to do your due diligence to determine that it is, in fact, a PV system that is causing the issues and NOT other devices in your house our those of your neighbors that is causing the problem.  If you own a PV system - or have one installed on your house - that you suspect is causing a problem, making detailed measurements with it on and off on various frequencies would be a suggested first step.

As this article relates only to the SolarEdge PV system that I investigated, I cannot possibly offer advice to another brand of system that uses other brands of equipment in regards to interference potential - but if you suspect that you or your neighbor(s) have this brand of PV system that is causing interference, I would suggest the following checks during daylight and hours of darkness as appropriate:

• Are there signals every 200 kHz?  Common frequencies where this would be observed include 3.6, 3.8, 4.0, 7.0, 7.2, 14.0, and 14.2 MHz.  This is definitely one of the hallmarks of a SolarEdge system of the same/similar model - but it similar artifacts may be produced by others.
  

• Does the "hiss" that is elevating your noise floor have an obvious "hum" to it when you switch to AM?  You can't easily hear this when you are in SSB mode.  Listen for this on frequencies in the vicinity of 60, 40, 30 and 20 meters on a frequency devoid of other signals.

• Does the "hummy hiss" greatly reduce when it gets very cloudy?  The "hummy hiss" - which appears to be a property of the voltage converters - seems to become more intense with increased output from the PV system.
  

• Do the 200 kHz signals and the "hummy hiss" go away after sunset and return only after sunrise?  Not unexpectedly, this is hallmark of many PV systems' noise generation.
• Be aware that some models/brands (although not the one discussed in this article) can produce RF interference if either solar illumination OR mains voltage is present and that it takes the removal of BOTH to silence them (e.g. turning of the mains breaker feeding the system at night.)

If you believe that you are being affected by a PV system, it is up to YOU to be prepared to take all appropriate measures to document the interference, do your own testing, and make repeated observations prior to reporting them to the manufacturer, a regulatory agency, or club or national organization.  A few things to consider:

• Treat this as if you were causing interference to someone else.  Just as if a neighbor complained that you were causing problems to their equipment, it is incumbent on YOU to determine if the problem is on your end.  There are likely many, many devices in your house that can cause similar types of interference so be sure that you have ruled those out - and DO NOT forget that you may have devices running on UPSs or battery back-up that may still make noise even if you shut off your power.  (Many UPSs are known to be noisy in their own right!)  In other words, be certain that your house is clean before involving them as this will not only make determining the magnitude/nature of interference from a PV system easier, but it shows good will and competence on your part.
  

• Document the issue over the period of days, weeks or even months.  Many sources of interference come and go - but if it's a PV system, it will be there day in and day out.  Noting over time the consistency of the noise may give you a clue if it's some other type of device - and if it, in fact, related to a PV system: GOOD documentation will only help your case.
  

• Once you have ruled out everything else, go ahead and contact the manufacturer - but be nice!  If you are confident that your own house is in order (e.g. you have ruled out other devices) then contact the manufacturer.
• If you have been following the above steps, you will already have some documentation which makes your specific case more solid.
• The manufacturer may schedule an online meeting to discuss the issue and run tests.  Be sure that you have the ability to use Zoom or Google Groups - or find someone who does.
• If the manufacturer runs tests, they will likely turn on/off suspected systems so YOU should be ready to document changes in noise floor - and of the signals every 200 kHz (in the case of a SolarEdge system of the type investigated here).  If you have already been taking notes/documenting, you should be already familiar with your local signal environment and be able to expedite the running of these tests - and have a basis of comparison as well.
• If the manufacturer decides that they wish to help remedy your situation, remember that they may be doing it at their own expense:  It is incumbent on YOU to be cooperative, competent, courteous, accurate and honest when you are dealing with them and their requests.
• If you feel the need to do so, you may wish to enlist the help of one or more friends to help you with these tasks that may be more experienced - and having a second or even third pair of eyes on the problem is always a good idea.  If you are not comfortable doing so, I would suggest have someone else - familiar with your problem - who can talk "nerd" be your spokesperson when dealing with the manufacturer!
• You should be clear to the manufacturer to define "interference" differently from "harmful interference".  If you can just hear weak birdies that don't really cause any issues, this could be considered just plain, old "interference" and you may not get as much sympathy or action as you like.  "Harmful interference" is that which - when present - obliterates even moderately strong signals that would otherwise be quite usable and thus, they should be taken more seriously.
  

While this article is rather specific to the SolarEdge PV system as described, this is likely be applicable to other manufacturers and models in more general ways.

Good luck!

* * * * *

P.S.  Overall, I was pleased with the knowledge and responsiveness of the SolarEdge folks with respect to interference to amateur radio stations.  After they have had time to digest the information supplied and executed their plan of action I hope to do a follow-up to ascertain the results of their mitigation efforts.

Myself and several other local amateur radio friends have PV (solar) at our own QTHs and experience ZERO interference.  As we had chosen to take an active part in our PV system design, we had installed SunnyBoy series-string systems which are known (and proven!) to have zero interference potential on any LF, MF or HF amateur band as described in the link(s) below.  Unfortunately, some installers will not entertain the use of this type of system if it is not in the suite of products that they offer.

Other local amateurs that I know have microinverter-based PV systems using Enphase IQ modules and have reported minimal or no interference.  As I have not (yet) had the opportunity to carefully analyze the spectral signature of this product, I can only go by their assertion that their own system has not caused them obvious problems.

Please post in your comments your experiences with PV systems - but please do so in the context of having fully read this article and at least perused the articles linked below.

  * * * * *

Other articles at this blog on related topics:

• It *IS* possible to have an RF-quiet home PV (solar) electric system - link
  
• The Solar Saga Part 1: Avoiding Interference - (Why I did not choose Microinverters) - link
  
• The Solar Saga Part 2:  Getting the system online - link
• Does the Tesla Powerwall 2 produce Radio Frequency Interference? - link
• Reducing RFI from the Tesla Powerwall 2 - link 
• Tesla Powerwall susceptibility to RF Transmissions - and how to deal with it - link 
  

Other articles related to this topic:

• Solaredge Found in Breach of Swedish Electromagnetic Rules

 

Stolen from ka7oei.blogspot.com

[END]

 

Using an external clock with the RX-888 (Mk2)

The RX-888 (Mk2) and external clocking

Figure 1:
The RX-888 with external clock input (right)
The enable/disable switch is barely
visible behind the USB connector.
Click on the image for a larger version.

Note: I have posted blog two previous entries related to the RX-888 (Mk2) that you may find relevant:

• Improving the thermal management of the RX-888 (Mk2) - link - This page talks about highly recommended modifications to the RX-888 to reduce internal heat to improve reliability.
  

• Measuring signal dynamics of the RX-888 (Mk2) - link - This is a discussion of how much (and little) signal is needed to stay within the dynamic range of the RX-888 and the effects of gain and attenuator settings.

Adding an external clock connection

While the internal 27 MHz TCXO in the RX-888 (Mk2) is pretty good, there may be instances where one wishes better accuracy and stability.  Fortunately, the RX-888 (Mk2) has provisions for doing so in the form of a jumper to disable the internal clock (when the jumper is removed) and a small connector (a tiny U.Fl) on board to accept that clock.

Unfortunately, it is up to the user to add the cable to feed an external clock - but short 4-6" (10-15cm) cables already fitted with a U.Fl male and SMA chassis-mount female connector are easily obtained from the likes of Amazon, EvilBay and others - just be sure that you do NOT get a "Reverse" (RP) SMA by mistake!

This leaves the jumper.  While many people simply remove the jumper and mount the external clock connector between the HF and VHF inputs - or sometimes to the right of the USB connector knowing - from then on - their RX-888 will be unusable unless there is an external clock input - I prefer to make use of the ability of the internal clock to be switched - using (ahem) a switch allowing for testing/use of the RX-888 in a "stand alone" configuration - but this is up to you.

If one is careful, it's possible to mount the external clock SMA connector and switch on the same panel as the USB connector, orienting so that its handle is toward the "Clock In" connector to indicate that an external clock is to be used - but labels or markings are always nice, too!

If one takes the route of mounting the external clock input between the HF and UHF inputs, the switch could be placed to the right of the USB connector - or, if as in the case of one of my RX-888s where I put a heat sink on the FX3 chip and there wasn't room there - I found a very small toggle switch that just fit between the case screw and left side of the USB connector and tip of this switch may be spotted just behind the USB connector in Figure 1, above.

IMPORTANT:  As the external clock input is simply wired in parallel with the internal 27 MHz clock.  What this means is that with the internal clock enabled, it will be present on the external clock input.  Similarly, if you supply a 27 MHz external clock without disabling the internal one, the two will "fight" each other and you'll get "garbage" results.

What type of signal to use as an external clock

• The best external clock source is a 27 MHz sine wave of between 1.25 and 3.3 volts peak-to-peak.
• A series coupling capacitance of between 100pF and 1000pF (470pF typ.) should be present on the "center pin" between the RX-888 to eliminate a DC path to ground on the signal line.
  

While a capacitively-coupled 27 MHz sine wave is recommended for reasons that will be mentioned later, a lot of devices offer square wave outputs - and getting these to work reliably requires at least a little bit of attention.

Using the Leo Bodnar Precision GPS Clock to drive an RX-888:

Because the RX-888 natively requires a 27 MHz clock this means that if you already have a 10 MHz standard (GPS, Rubidium, etc.) kicking around, you will not be able to use it directly.  While it's not too difficult to synthesize 27 MHz from 10 MHz (a number of Si5351-based devices can do this) it's most common for users of the RX-888 to use a device such as that sold by Leo Bodnar, which can be programmed for almost any frequency (from audio through UHF) with good precision and accuracy.

You can look at these products here:  https://www.leobodnar.com  (I have no stake in Bodnar, but I have used them and I and others have had good success.)

The most commonly-used device is the Bodnar "Mini" - which has one output - and this single output is often "daisy-chained" between RX-888s.  There is also the functionally similar LB-1420 with a single output and the "Precision GPS Reference Clock" which has two signal outputs - but there is very limited ability to set the "second" output to a specific frequency and it's mostly useful for outputting the same frequency on the two ports - or outputting a 1PPS signals on the "unused" port.

As the RX-888 (Mk2) external clock input is directly coupled to its Si5351 clock synthesizer, we have to act as if we are driving that chip directly.  While not directly specified in the Si5351 data sheets (at least the ones that I have found) testing done my myself indicates that a capacitively-coupled sine of about 750 millivolts peak-peak will trigger the '5351 reliably:  A bit of looking in online forums reveals the consensus that a 1 volt peak-peak sine wave is suggested so I would be comfortable with the suggestion of this amplitude being used a a guideline.

Testing with a square wave - such as that produced by the Leo Bodnar GPS reference revealed that the drive level was far more finicky - and this has to do with the fact that a "square" wave with a reasonably fast rise time does NOT remain a square wave for very long as it quickly turns into something rather spiky and distorted as depicted in the image below:

Figure 2:
A typical square wave output from a Bodnar GPS reference at the end of about 3 feet
(1 meter) of unterminated cable.  Ringing is evident!

This 27 MHz signal shows clear evidence of ringing:  This was measured right at the RX-888 with the signal passing through around 3 feet (1 meter) of 50 ohm coaxial cable.  As the '888 does not offer resistive termination, it presents a simple capacitance at the end of the cable and this tends to distort harmonic-rich waveforms like a square wave.

With multiple "spikes" that can occur on such waveforms due to distortion, it's possible - even likely - that certain combinations can result in multiple triggering peaks of the waveform.  In an extreme case, such distortion can cause the Si5351 to be triggered at twice the actual clock rate - but rather the result may be instability resulting in the RX-888 clocking which can be manifest as anything from no signals being "present" to those that are being off-frequency, varying, or just "noisy" - and this errant behavior may vary with temperature and slight changes in operating voltage.

It's important to realize that like the RX-888, the Bodnar is ALSO DC-coupled which explains why the above waveform in Figure 2 largely rests above the center line (zero volts) with the exception of some "ringing" which extends negative and is likely being clamped somewhat by the '888's internal diodes.

With a 3.3 volt waveform emanating from the Bodnar, we can reasonably expect that - if the signal isn't too "ringy" that a signal exceeding about 1 volt positive just once per cycle is likely to trigger the 888's Si-5351 correctly.

IMPORTANT:  If you try to directly drive an RX-888 with the output of a Bodnar, it will probably NOT work reliably!  I have observed this with my own Bodnar/RX-888s and many others have reported the same issue.

Remembering that the external clock input of the '888 goes directly to very sensitive logic devices, a simple resistive attenuator pad will do double duty:

• Rather than a very high impedance circuit that has a low resistance path from the outside world to a sensitive logic gate, resistance to ground offers a degree of protection by offering a relatively low resistance to ground and the series resistance provides at least some limit to input currents.
  

• While theoretically OK, the output of the Bodnar will not reliably drive the input of the Si5351 in the RX-888 directly, but being reduced to half or third of its original output seems to be pretty reliable and is less likely to cause clipping of diodes on the input circuit which can exacerbate ringing and other types of waveform distortion.
  

A 6 to 12 dB resistive pad - either 50 or 75 ohms - is a reasonable choice offering a bit of voltage reduction - but staying well above the 1 volt usability threshold - and such a pad, even if it is not connected to a 50 ohm load, will provide a bit of resistive termination, likely reducing the tenacity of reflections.  While a resistive pad does not offer DC decoupling between the center pin of the '888's external clock input, it works with the Bodnar as that device sources a square wave referenced to zero volts so the pad simply acts as a voltage divider for that square wave.

Testing has shown that the '888 seems a bit more forgiving of signal drive levels if there is a DC blocking capacitor on its signal input - something that could be provided by placing a "DC block" device (available in SMA, BNC or F-type connectors) between the '888 and the external clock source.

Caveats and warnings - and why the '888 is so finicky about its external clock

The external clock input of the RX-888 - as described in better detail in the next section of this blog post - is connected DIRECTLY to inputs within the '888 and as such, it has a few undesirable properties:

• There is a DC connection between the external clock, the oscillator output and the input to the 888's internal Si5351 synthesizer.  This exposes the clock input directly to extremely static and voltage-sensitive inputs.
• Because of this, it's very easy to damage the RX-888 when using and external clock, particularly if there are voltage potentials between different pieces of equipment.

• There is diode clamping between ground and the 3.3 volt input.  In the '888, this is primarily a BAT99 dual diode, but it also includes the protection diodes of the other devices in the circuit - namely the output of the onboard 27 MHz oscillator and the input of the Si5351 itself.  At first this might seem like a good thing - and it sort of is - but this means that any signal input to the RX-888 should be capacitively coupled - or directly to a 0-3.3 volt signal.  This is one aspect of the '888 that was definitely not well considered or implemented.
  
• What this means is that if you try to drive the RX-888's clock input with a source that is DC "grounded" - which includes devices that are transformer-coupled (e.g. a splitter to send the clock to multiple units) that the voltage output will be bipolar.
• For example: 
• If you were try to use a T1-1 isolation transformer to break a ground loop between the external clock input and the Bodnar - as well as other devices - the signal input may be 3.3 volts - but bipolar - that is, it will go above and below "ground" by about 1.65 volts - but since there is diode clamping, the negative-going signal will distort the waveform.
• The result of this can either be finessing required to find the precise drive level to make it work at all or - sometimes - you will find the signals at the wrong frequencies (sometimes at about half the expected frequencies) if the badly-distorted waveform triggers the input of the Si5351 synthesizer in the '888 twice on every clock cycle.

All of these factors often confound users of the RX-888 (Mk2) trying to feed an external clock - and things get more complicated if multiple devices are use.  For example:

• As with any sensitive piece of RF equipment, having multiple, disparate connections between pieces of equipment will usually end up with circulating currents - and since every conductor has resistance, this can cause noises to appear in the RF input.  A few examples:
• The RX-888 - or any SDR - will have multiple connections to it - typically the antenna and power input.  In the case of the RX-888 and many other SDRs, this means an antenna and USB connection.
• Isolating the RF signal lines from longitudinal currents (e.g. common mode) is a useful tool.
• Often, this can take the form of small coaxial cable (RG-142 or RG-174) wound with 8-12 turns on an FT-140 or FT-240 core of 31 or 43 material (the former being better for lower frequencies).  This is useful for HF (160-10 meters) but it loses efficacy below this and is not helpful if your interest extends into the AM broadcast bands and lower frequencies (e.g. longwave - including LF and VLF which includes the 2200 and 630 meter amateur bands.)
• Another tool can be an "voltage balun" - essentially an isolation transformer with no DC connection at all.  Often, these are built around the Mini-Circuits T1-1.  These lose their efficacy below a MHz or so so they may have excessive attenuation on LF and VLF frequencies.  At higher frequencies (above 10 MHz) their common-mode rejection also starts to drop meaning that in a very noisy environment, signals can "leak in" at high HF from the surrounding equipment - something that needs to be checked if you try it.
  
• Power supplies and computers (via a USB cable) are notoriously noisy, so you WILL get circulating currents flowing between the devices.  Having a choking USB cable (e.g. 6-12 turns on an FT-140 or FT-240 core of 31 or 43 material) can help significantly, as can doing similar on a DC supply line and also choosing a "known RF-quiet" power supply.
• Adding a "third" connection to the receiver - such as the external clock, in case of the RX-888 (Mk2) - can further complicate issues as it adds yet another  avenue of common-mode currents and noise.
• This connection, too, should be appropriately isolated - but doing so is complicated by the way the external clock input is implemented.
• The fact that the external clock device is connected to a potentially-noisy power supply and  a GPS antenna - which may or may not have its own grounding (which can further introduce circulating currents) is yet another thing about which you should be wary!
  

One issue that also arises is that output of devices like the Bodnar are square wave.  This, by itself, isn't a problem - and a direct connection between the Bodnar and '888  - since they both have 3.3 volt signal levels - works OK, at least with very short cables when using a 6-12 dB pad.

 

Conveying this square wave signal - particularly over greater distances and considering that the clock input to the RX-888 is high-impedance with a bit of capacitance means that long runs (anywhere near 1/4 wave at the clock frequency or longer) can result in reflections due to unterminated cables.  What one can do is put a 50-75 ohm termination at the far end of the cable. This, however, does not help with the issue of DC/galvanic isolation between individual receivers.

 

Testing the stability of your external clock mechanism:

 

As properties of solid-state devices change over temperature - and signal levels may vary depending on what other devices are connected to your clock source - it would be a very good idea to varying the clock signal to determine if you have enough margin to allow it to work if levels change, or if you are on the "ragged edge".

Reducing the signal level is the most obvious test:  The use of a step attenuator - or use a variety of fixed attenuator pads (be sure that they pass DC) and reducing the level by between 1 and 15 dB - and then observing when clocking becomes unreliable:  This will give you a good idea as to the margin between what you are feeding to the '888 and when it will quite - and it may prompt you to reduce your signal level slightly.

Using HDSDR under Windows

Determining when the clocking signal into the '888 becomes unreliable is a bit trickier in some cases.  By far the easiest is to use a program like HDSDR with the "SDDC" ExtIO driver on a fairly fast Windows computer with USB3 ports:  A higher-end Intel i5 or medium-high end Intel i7 will suffice.  Connecting the '888 to an external antenna and tuning in a reliable signal (like a shortwave broadcaster or a time station like WWV/H or CHU - or tuning it your own signal generator) while watching the waterfall will tell you immediately when the external clocking fails.

If you are using Linux with ka9q-radio, you can use the "Monitor" program to tune a signal with the audio being sent to the default audio device - but doing this is beyond the scope of the document.  If you are using a Mac, I don't have a suggestion unless someone speaks up.

Transformer-based signal isolation NOT recommended for the '888's clock input - sort of...

It is important for any receiver to minimize the amount of current circulating through the "ground" connections.  Such currents in an analog receiver can induce hum in unbalanced audio lines and if the receiver is actually a transceiver, those same signal paths can induce RF into seemingly unrelated equipment in the ham shack.

Sometimes overlooked is the fact that these same currents can induce RF currents on the cables interconnecting equipment and it is likely that these will find their way into the receiver's front end and degrade performance by raising the noise floor.  This is especially true when a computer-connect software-defined radio - like the RX-888 - is involved as we now have a connection (via the USB cable) to a device that is likely to be "noisy" at RF - namely the computer - but this also means that noise can come from other devices to which this computer is connected directly or indirectly, namely its power supply, other peripherals, its power supply - and noisy devices on the AC mains into which this power supply is plugged.

Current "balun"

For receiver RF connections one way to deal with this is to use a common-mode RF choke which is typically a dozen or so turns of coaxial cable wound on a T-140 or T-240 toroid - usually with 31 or 43 type material.  This will break up common-mode currents on the cable - at least at HF - and can reduce such issues and this works for both the signal (antenna) and external clocking lines.

At DC and mains frequencies such chokes offer little/no efficacy and at low frequencies (below a MHz or so) these chokes lose their effective series resistance owing to limited inductance.  What this means is that if you have strong circulating currents (e.g. current flowing between your antenna "ground" and house mains "ground") they will have little effect.

Voltage "balun"

A possible alternative is to use a transformer to couple between RF sources:  A reliable, low-cost, commonly-available device for this is the Mini-Circuits Labs T1-1 which provides complete galvanic isolation between the source and load with a reasonable degree of longitudinal isolation.

While the T1-1 works well for the RF input, it will not work so well for the RX-888's external clock input by itself and the reason for this is that the output from a transformer winding is, by definition, bipolar about the zero volt point.  In the case of an external clock signal of, say, 1 volt peak-peak, each half would be above and below zero volts and with a direct DC connection to the Si5351's input it is unlikely to properly drive/trigger it.

If the signal is of higher amplitude - such as our 3.3 volt square wave - half of this "ugly" waveform will lie below ground potential and that below the 0.6 volt diode conduction voltage will be clamped, potentially distorting the waveform even more.

If a transformer-based method of isolation is used it is strongly suggested that a capacitor be placed in series with the '888's signal input to allow the waveform and voltage to float above ground and avoid negative clamping.  As mentioned earlier, a "DC Block" device could be used if you choose not to build your own device.

Example homebrew devices:

Here are a few (relatively) simple devices that one could build on a piece of scrap PC board - or you could go through the effort of designing and building a board with these features.

Figure 3, below, shows a simple resistive coupler incorporating the features suggested above:

Figure 3: 
A simple 10-ish dB resistive pad with DC blocking to keep the external clock input of the RX-888 "happy" and to prevent clipping of negative-going voltage by built-in protection diodes.  The "small" capacitor value also minimized the amount of stored charge dumped into the '888 due handling/shorting of the input cable.

This diagram shows a resistive pad that offers about 10 dB of attenuation - the values being determined assuming a 50 ohm system - but since the '888's input impedance is almost exclusively capacitive (a few 10s of pF) it is operating more as a voltage divider presenting a resistive load that just happens to be around 50 ohms.  The coupling capacitor between the pad and the '888 offers DC blocking to make it more forgiving to varying signal levels.  While the capacitor blocks DC, the signal being input to the Si5351 will find its own level due to the clamping effects of the protection diodes in the '888.

Also shown is the optional inclusion of a 1000pF capacitor that can be inserted at point "X":  This will decouple DC and mains AC currents that might flow between the clock source and the RX-888 itself - but it is low enough impedance that it does not necessarily offer RF decoupling between devices.  With the circuit shown above, however, you can precede it with decoupling device - such as a common-mode choke (e.g. current balun - the type with a dozen or so turns on a toroid) or even a T1-1 transformer.

Figure 4, below, shows another possible approach:

Figure 4: 
This circuit provides both common-mode isolation and a degree of band-pass filtering of the 27 MHz clock signal:  Filtering to a sine-like waveform reduces glitching due to cabling issues (reflections, misterminations) as well as offers a degree of protection to the RX-888's input as the filter will limit the amount of energy that could be imparted.  It also provides a (small) degree of termination (<150 ohms).   The "optional" 1000pF capacitor shunts low level leakage of the 27 MHz signal due to transformer imbalance - but it is suggested that one use a common-mode choke to restore isolation at HF frequencies.

This device is slightly more complicated, but it offers several advantages:

• "L1" is a trifilar-wound toroidal transformer (that is, its turns consist of three wires gently twisted together before winding on the toroid).  Its intrinsic inductance is around 0.22uH and with the 150pF capacitor seen on the lower half of the diagram, it resonates broadly at 27 MHz - the external clock frequency for the '888.
• The resistors shown offer a bit of resistive termination to the signal source (a bit below 150 ohms) which can help to reduce reflections on the cable.
  
• These series 150 and 100 ohm resistors "decouple" the resonant circuit from the signal path somewhat and the values were chosen to allow sufficient "Q" to offer reasonable filtering of the input signal into a fairly good sine wave.

• 

  Figure 5: 
  The (nearly) sine wave output from the circuit depicted
  in Figure 4.
  Click on the image for a larger version.
  

  As this is a transformer-coupled circuit, there is no DC connection at all between the input and output.  Because it is resonant at 27 MHz, it will also offer a degree of rejection of other signals that might be present.  As the resonant circuit is wired to the "RX-888 side" of the circuit, it offers excellent protection to it.
  
• As with the previous circuit, an optional 1000pF capacitor is shown as well:  Including this will reduce the common-mode isolation between the input and output but it will suppress a bit of leakage of the 27 MHz clock signal that can occur owing to the fact that the transformer that is L1 is not perfectly balanced.

The disadvantage of this circuit is that it requires the winding of a toroidal transformer and tuning it to 27 MHz - something easily done with a NanoVNA or an oscilloscope and an oscillator.  

Figure 5 shows the resulting waveform that has passed through the circuit depicted in Figure 4:  It is nearly a sine wave and as such, it is much more resistant to causing false triggering on "ringing" edges as compared to a square wave.

Figure 6: 
The prototype transformer/filter circuit depicted in Figure 4
connected at the Bodnar, connected to the '888 with a
short BNC<>SMA jumper.
Click on the image for a larger version.

Figure 6 shows the circuit of Figure 4 in action, connected directly to the Bodnar's output and - via a very short BNC to SMA cable - to the RX-888 sitting atop it.

This prototype unit was built in a piece of copper-clad PC board material.  On the top side, the components were wired with flying leads to the connectors and "dead bug" on the copper itself:  Between the "Bodnar" and the "RX-888" side the copper was cut to provide the two separate signal "grounds" with only the transformer coupling between the two.

At some point, it may be worth designing a small PC board for this, but for the meantime a small number of these prototypes have been built and put into service very successfully.  As suggested earlier, the a step attenuator was inserted between the Bodnar and this circuit and the signal reduced until the '888 no longer reliable locked to the external clock and it was found that there was plenty of margin to assure stable operation under varying conditions.

Lots of other possibilities

Now that you know what the RX-888 "wants", you have a better idea of what you are likely to be able to "safely" use to drive the external clock input of the RX-888.

* * * * *

This page stolen from ka7oei.blogspot.com

[End]

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.

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]