Hi Fred,
That VCO trimmer is a known problem: I replaced mine about 5 years ago for exactly that symptom. Anecdotal evidence says that the flux bath Yaesu used before wave soldering results in contamination of the ceramic, which eventually causes the value to go out of range of adjustment. Not sure if that is true or not, but it matches the symptoms for sure. I don't see that issue with other older ceramic trimmers ... so I suspect it was something yaesu did ... or maybe another component value going out of range (unlikely).
If you know the approximate cap range the VCO wants, I recommend getting a smaller trimmer value in that range ... makes it easier to find the sweet spot. The stock value requires a bit of patience and skill sometimes.
Just my thoughts ...
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-Mat Breton, N8TW
Mat Breton N8TW