Studer A779 half a channel is silent

[Retroperra]

Hi all, second post only but I read here from time to time.

I have a A779 that's broke, or atleast unusable on one channel.
You remember these small mixers have XLRs in and RCA stereo in with a switch.

Anyway, one of the channels has a the LINE XLR input silent, together with one channel of RCA.
I would be happy for some assistance to understand what could have given up.

I've switched the channels around and it's not the connectors or the connector board.

If I gain really high I get a low-low-signal plus some noise and hum - not 50Hz, but something else, on the broken channel of that input. The noise is affected by the EQs "in an equal way" as on the OK channel.  My guessing is that the error is probably before the EQ. See the attached block schematic, I've put a yellow square around the culprits ICs  - which are MC33078 op amps. But it could also be the switch itself which would be a bad thing.

What do you guys think?  Have any one seen a problem like this before. I understand that the 779 preamps are the same or similar as in other Studer mixers.

I don't have any extenders sadly, if they are even exist, so it'll be somewhat tricky to measure on the channel while it's powered up and working.

 

[Retroperra]

And here's a bad scan of the schematic

 

[zamproject]

Hello

Do you have issue whatever line input source is ? (bal XLR mono or stereo unbal cinch)

Best
Zam

 

[Retroperra]

Hi,

The line XLR is silent, the mic XLR works just fine.
I'll try both + and- seperatley on the line XLR tomorrow.
Since I can switch between all three sources I hope it's not the switch itself that's broken.

 

[Retroperra]

It was IC2, an opamp serving one of the input RCAs.