Jerry's technique is valid, however for this particular problem you can
limit your search to a much smaller area than the entire radio.
As you have already said (sort of) D5 in the VCXO circuit on the
PASSBAND TUNING BOARD is the component which is supposed to shift the
frequency of the VCXO, and the voltage that biases D5 is likely
shifting. I would focus my attention on the PBT potentiometer, wiring
and connectors (connector 42) and perhaps monitor the +REG voltage
supplied to this board on connector 39. It is also possible that the
voltage control to D5 is not at fault, and some other component or
connection in or around the VCXO is flakey.
The fault is almost certainly on the passband tuning board. Since this
VCXO feeds nothing outside the passband tuning board, the only likely
off board causes would be wiring/connectors to the PBT potentiometer, or
the power supplies to the board.
DE N6KB
First cool the whole radio. Then warm just half of it. Then cool just a
quarter. You may to try both halves and both quarters to see which has
the effect. Then split that part in half and keep heating and cooling
smaller fractions until there's only one part being heated and cooled
and causing the effect. A binary chop works every time and is often
faster than any other search technique.
73, Jerry, K0CQ
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