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.
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| 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. |
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.
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| 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. |
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.
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| 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. |
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.
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| Figure 4: The CAR board, reinstalled for testing. Click on the image for a larger version. |
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.
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| 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.
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| 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
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.
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| 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.
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This page stolen from ka7oei.blogspot.com
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Ham Radio With K0PIR
So you've seen all the hype about the New Kenwood TH-D75A. Here's my breakdown. After reading, do you think this is the ultimate portable transceiver? I researched the best videos on YouTube and made a playlist for your conveience. (See...
The post Exploring the Kenwood TH-D75A: The Ultimate Portable Handheld Transceiver appeared first on Ham Radio with K0PIR - Icom 7300 and 7610 SDR Transceivers and now Elecraft!.
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| The TS-570D's front panel |
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:
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
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