Dc offset pop sound on solo buttons and Console status buttons

[tlmaen]

Hi,
when i press any solo button on my 4ke console, I get an almost fullscales dc sounding pop.
Its really annoying, specially with clients attending.
When one solo button is pressed, pressing another solo doesnt make any additional sound.
Also when i change the status of the console it makes the same loud sound.
Is there anywhere to start looking?
(All of the center section cards are completely recapped).
I figured out that there is a cut bus, but that seems to be a logic thing.
Tried to identify the Pfl/Afl bus in the service manual, but that doesn’t shed any light in to it…

Thanks for your help!
Happy eastern 

[tlmaen]

Tried to narrow down the Problem, so I powered the console with one bucket at a time, but no luck.
Even when all buckets are switched off, it still makes this pop sound.
So I guess its somewhere in the center section…

The VCA Solos doesnt make that sound.
Just the channel solos ant the change of the console status.

Any Ideas where to look?
I find the service manual not very helpful so far…

Thanks,
Tilmann