Acoustic 120 SQUEALING on Power Up

[MikeBloomfield]

I am repairing an Acoustic 120 Bass Head that is squealing when it's turned on and would appreciate any help with possible scenarios.

The amp doesn't squeal when both speaker jacks are used, nor when either the send or return has a jack inserted.
I had thought the problem was in the preamp/EQ, but the fact that there is no squeal when both speaker jacks are used makes me uncertain that's the case.

Thanks for any help offered!

PS: the amp works fine once it's turned on.

[gbono]

Zobel network on the output board/jack has damaged component? Post schematic.

[MikeBloomfield]

Zobel network on the output board/jack has damaged component? Post schematic.

Thanks for your reply. I sent you a PM.