Kenwood TS-790 repair

I never ran into a Kenwood TS-790 before, but I was surprised by its nice design and spacious PCB layout. The reported defect didn’t leave much to my imagination: the radio didn’t lock on 2 meters. All I probably had to do was to focus on the VCO.

With the service manual in one hand and a multimeter in the other I tried to set the VCO voltage at what it should be, but that didn’t work out very well. The voltage was either zero or off the scale, and changes in voltage while turning the variable capacitor inside the VCO block didn’t make any sense.

Variable capacitors (or trimmers as we call them here) are known for their reliability, even when they’re old. There’s no wear and tear either; you set them once to align something and leave them alone. I never ran into a defective one before, hence had no reason to suspect this particular component. In the end I couldn’t do much else than to reconsider TC1 being the culprit, for the simple reason that all the other components in and around the VCO block proved to be fine.

After disassembling the VCO block I removed TC1 and measured it. It was fully short circuited, with just a few gaps while turning it. After replacing the part and re-assembling the VCO block I had no trouble getting the radio back on its feet again. While I was at it anyway I did a few other checks, but apart from some minor adjustments the radio was fine. Even the backup battery (prone to leaking) was still OK.

I’m still wondering about possible causes for a trimmer to die this way. Dust maybe, moisture? I just don’t know.