Saturday, May 28, 2022

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

The TS-570D's front panel

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

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

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

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

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

The repair:

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

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

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

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

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

Going through the Service Adjustment Mode Menu:

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

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

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

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

  • Many of these menu items are partially self-calibration, which is to say that you establish the condition called out in the procedure and push the UP or DOWN button.  For example, on menu item #16 where the Squelch knob is calibrated, one merely sets it to the center of rotation, the voltage is shown on the screen in hexidecimal, and you press the button and the displayed value is stored temporarily in memory.
  • I'm a bit OCD when it comes to S-meter calibration, preferring my S-units to be 6 dB apart, S-9 to be represented by a -73dBm signal as noted by the IRU specifications, and for "20 over" to actually be "20 over S-9", or around -53 dBm.  The procedure in the manual - and the radio itself doesn't permit this, exactly.
    • To set the "S1" signal level (menu item #3) would require a signal level -121 dBm, but the receiver's AGC doesn't track a signal below around -113 dBm.  Instead, I noted the no-signal level on the display when menu #3 was selected and then set the signal level to an amplitude that just caused the hexidecimal number to increase and then pushed the button, setting "S1" to be equivalent to the lowest-possible signal level to which the AGC reacts.
    • To set the "S9" signal level (menu item #4) I set the signal generator to -73dBm and pressed the button.
    • To set the "Full scale" level (menu item #5) I set the signal generator to -23 dBm and pressed the button.  If you have followed the math, you'll note that "Full Scale" - which is represented as "60 over" should really be -13 dBm, but I observed that the AGC seemed to compress a bit at this signal level and the "20 over" and "40 over" readings came out wrong:  Using a level of -23 dBm got the desired results.
    • NOTE:  The service menu forces the pre-amp to be enabled when doing the S-meter calibration (e.g. you can't disable it when in the service menu) so the S-meter calibration only holds when the pre-amp is turned on.
  •  For setting menu item #1, "ALC Voltage" I was stumped for a bit.  It mentions measuring "TP1" - but this is not the "TP1" on the transmitter board, but rather the TX/RX unit (the board underneath the radio).
  • I noticed that if step #7 was followed to set the 100 watt power level, it was difficult to properly set menu items 23-28 (the "TGC" parameters).  These adjustments set to 100 watts, but if you have already set menu item #18 at 100 watts, you can't be sure that you've properly done it.
    • The work-around is that prior to step #6 in the procedure that you go to menu item #18 and adjust for higher than 100 watts - say, 125 watts.  If this is done, you can adjust menu items 23-28 (noting that menu #27 is adjusted out-of-order in procedure step #6) to 100 watts.
    • Once procedure steps 6, 7 and 8 are done (but skipping the adjustment for menu #18 in step 7) you can go back to menu #18 and adjust for 100 watts.
  • For procedure steps 16 and 17, I didn't have a 150 ohm dummy load, but I did have several 50 ohm loads, so I put three of them in parallel - which yields 16.67 ohms, which is also a 3:1 VSWR - and completed these steps.  It's worth noting that Yaesu uses 16.67 ohms for the equivalent step in its alignment procedures.  To set the "40 watts" called out in step 17 I used the front-panel power meter, which would have already been calibrated in the procedure.

The result:

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

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

This post stolen from