[Kenwood] TS-850 PLL Lock instability

[Dave]

I am currently "resurrecting" a 10+ year old TS-850SAT.

One of the well documented problems of some older TS-850's is the unlock of 
the PLL because of the apparent instability of the small trimmer cap TC1 in 
VCO-2. I have reset TC1 a number of times ( for the required 5Volts at 
TP2). the radio will play fine for a few hours or even a few days but 
ultimately it goes out of lock again resetting TC-1 brings it right back. 
Sometimes just tapping on VCO-2 case will result in relocking of the VCO 
and getting something close to 5V at TP2. While monitoring TP2 and tapping 
on the case for VCO2, TP2 jumps around from 4.6 to 5.4 volts, other times 
rather sharp tapping causes no change at all.

So two questions.

1) Is what I'm seeing here the symptom of some other problem like to little 
loop gain as the result of some other degraded component and the TC1 is so 
critical as a result to appear to be the problem, but more loop gain would 
result in a less critical setting of TC1?

OR

2) If TC1 really is the culprit and needs to be replaced, has anyone ever 
experimented with using a better cap? In this case a ceramic piston trimmer 
looks like an ideal substitute, inherently more stable and easier to 
adjust. How about putting a fixed cap in place of TC1 and adding a slug to 
the coil next to it for tuning in  the VCO  instead?



Dave
NR1DX
nr1dx@arrl.net

"A man who picks a cat up by the tail learns a lesson he can learn no other 
way" .... Mark Twain