DC on a preamp rising and falling.

[Hifibythekg]

DC offsrt on a preamp ranges from approx 10mV to 350mV, climbs and drops in a cycle that takes a minute or two to complete. (SM says it should be between 0 and 200mV.)

How should I go about finding the cause of this?
Freeze spray?

Trimpots have just been replaced by Bourns, so it's not them.
Power source voltage is rock steady at 30.2V
And which section of the amp does it likely stem from? Power supply?

Solder joins have been checked and reflowed but caps are still the originals, although the few that have been checked were all well within spec.

Any suggestions?
Amp is a Yamaha C-4 preamp.

 

[avionic]

And which section of the amp does it likely stem from? Power supply?

Again I ask.. What test points ???
Both channels ?

TP1 & 2 --- left channel
TP3 & 4 --- right channel

https://audiokarma.org/forums/index.php?threads/yamaha-c4-d-c-ranging-10mv-350mv.976495/

 

[avionic]

Power supply?

Could be if both channels are effected..

 

[avionic]

Any suggestions?

Stick with one thread per topic..

 

[avionic]

Select phono 1 or 2 and check your offset. This eliminates the MC pre-amplifier circuit.

 

[avionic]

It will fluctuate.. Key is try to keep it between +200mv and - 200mv..