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Long standing bug in Kenwood TS-430s finally fixed!

Discussion in 'Radio Circuits, Repair & Performance' started by W0RIO, Apr 18, 2019.

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  1. W0RIO

    W0RIO Ham Member QRZ Page

    It sure feels good to fix a bug that you've been chasing for a long time.

    I have an old Kenwood TS-430s that started acting flaky a few years ago.
    The symptom was that the rig would start up fine, but after a random amount of time
    passed, the display would go blank and the sidetone would beep constantly.

    I attempted to find the problem several times, but it always eluded me.
    I found that if I whacked the top of the rig, most of the time, it would start working normally. *
    I decided to try one more time and set the rig up on the repair bench. I gently poked around the top
    board with a nylon screwdriver to no avail. The rig popped out on me a few times and I started to notice
    that if I whacked above the shielded low-pass filter select board it would sometimes pop back to working.

    The rig is built so that all of the boards can all be un-screwed, folded out and powered up for calibration purposes.
    I tried the nylon screwdriver on the lower top board to no avail. Then, I tried the bottom board, no results.
    I also looked for visual clues such as cold solder joints but found nothing too bad.
    I started digging around the most inner part of the radio, the filter board, this was near the point that seemed
    to be vibration sensitive. Tapping the board did not yield any changes. Finally, I un-screwed the filter board
    and inspected the bottom. There were two tiny ceramic capacitors soldered to the bottom of the board and
    one of them had a bare lead that was touching an adjacent trace. I removed both caps, shortened their leads
    and added a tiny piece of teflon tubing around the leads where they crossed over other traces before resoldering.

    That fixed the intermittant bug and the rig is back to operating the way it should.

    Persistence finally paid off.

    The rig is far from virgin, when I bought it, it had obviously been worked on several times based on the
    chewed up and missing case screws. I also made several modifications to the rig including lowering the fan-on
    temperature and adding a filter swap modification so that the CW filter may be used in SSB mode, this is
    critical for use with PSK31 and other digital modes. You can see some of my mods here:

    * Whacking and poking a rig may cause de-calibration, alignment may be necessary. When poking and
    prodding, always avoid anything resembling a tuned circuit such as trimmer caps and and coils with fine wire.
    K9ASE and KA0HCP like this.
  2. K8ERV

    K8ERV Ham Member QRZ Page

    Luvit when a plan comes together

    TOM K8ERV Montrose Colo
  3. N5CM

    N5CM Ham Member QRZ Page

    Good detective work!
  4. KF4ZGZ

    KF4ZGZ Ham Member QRZ Page

    I love the precision calibrated "whacking"!
    K9ASE likes this.
  5. W0RIO

    W0RIO Ham Member QRZ Page

    That's how our grandparents used to (temporarily) fix their TVs and radios.

    Seriously though, a lot of bad electrical connections can be ferreted out by the focused application of
    mechanical impulses. It's surprising how many bugs can be found by whacking, prodding and
    even flexing printed circuit boards while they are powered up. A lot of caution is required when doing this...
  6. WR2E

    WR2E Ham Member QRZ Page

    Speak for yourself young fella! ;)
  7. W0RIO

    W0RIO Ham Member QRZ Page

    I was WRONG! I decided that I should spray some de-oxit on the relay contacts on the LPF board for
    preventative maintenance and the problem came back. The frequency display went blank and the sidetone went BEEEP!

    But... I was close. It turns out the 5V regulator on the top board was the real culprit, it was very close to the area on the
    rig that was sensitive to tapping with the plastic screwdriver. I un-screwed the top board and noticed a small crack around
    each of the regulator pins. Shutting down the 5V supply caused the problem to occur at will.

    The fix was easy, I just re-soldered the 5V regulator and also touched up the solder on the 8V regulator and the audio amp IC.
    Hot components are often associated with PCB solder failures, and the leads on these parts were clipped off close to the board,
    which makes them more likely to fail this way since they hold very little solder. The previous owner used the rig in a mobile setting,
    so it also had been subject to a lot of mechanical vibration.

    I'm not sure what effect the shorted capacitor wire I previously repaired would have caused, but at least that's no longer a concern.

    So, even more persistence paid off, I think I really nailed it this time.
    WB2GCR and K7TRF like this.
  8. W0RIO

    W0RIO Ham Member QRZ Page

    K8AI likes this.
  9. W9BRD

    W9BRD Ham Member QRZ Page

    The Kenwood R-1000 receiver can suffer flaky-display problems that arise from exactly this cause: Solder joints between a runs-hot regulator pass transistor -- fastened to an off-board heatsink with a screw and insulating pad -- and the radio's small power-supply subboard go bad as a result of thermal cycling. Resoldering the affected connections cures it for another decade or two. (Also replacing the pass transistor with the same part, but equipped with springy bends in its leads to handle the cycling differential, would be a more complete fix.)

    In your case vibration resulting from mobile operation may have sped up the disconnections, but quite likely thermal cycling played a strong role. Imagine speeding up the movie: If the regulator runs warmer than ambient as a result of internal dissipation, it and its leads will expand from "cold" every time the rig is turned on, and shrink back that little bit on cooling off. Especially if the regulator -- like the R-1000's regulator pass transistor -- is fastened to an external object such that it can't move with its circuit board across thermal cycles, the scene is set for degradation of the weakest link -- which would be the relatively thin deposit of solder between the regulator leads and its circuit board.

    Good for you! Rigs have been sold or left forever in a corner or garage for problems just as simple in fact but similarly hard to find and fix.
    Last edited: Jun 8, 2019

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