Effects loop coupling capacitor blown again

[Timbo]

avt100-61-00-iss1.pdf

I had a Marshall AVT100X in the workshop this week with a dead effects loop. I found that a coupling capacitor in the return circuit (c253) was open circuit. I replaced it with another one of the same specs and all was well. For about 30 minutes! Now it’s gone again. Should I assume that the replacement was bad, or do you think there’s a problem with the rating of this component?

[pdf64]

That seems weird! It looks like neither side of the cap should be able to deviate outside the +/-15V supplies.

Front pcb?

[Timbo]

Yes, front PCB.

[Timbo]

Sorry, no - back PCB. Goes directly to pin 9 of cnb8 and then off to the front PCB via ribbon cable.

[Timbo]

Here’s the front PCB. The signal from c253 on the back PCB goes to the fx mix pot on the front PCB.

avt100-60-02-iss1.pdf

[Mars Amp Repair]

Hello Timbo,
Is it possible the customer is plugging the output of another amp into the return or send jacks? Seems like the only way enough voltage could be presented to that small coupling cap.
Just a thought, Glen

[Timbo]

No, just stomp boxes according to the customer.

[g1]

What do you mean by the cap being 'open circuit', and how did you check it?