Peavey Deuce 240T Squealing Issue

[inapier]

I've got a squealing issue in a 76 deuce 240t. It only happens when plugged into the series input. When the gain is turned up past 7 or 8 on the normal channel the amp begins to squeal. It does this regardless of the gain setting on the effects channel. The gain can even be turned all the way off on the effects channel and as soon as you go past 7 or 8 on the gain on the normal channel, the squealing starts. Also, the guitar volume has to be on 10 for the squeal to happen. It also does this even when the amp is in standby mode because I can see the squeal on the scope when the amp is in standby which makes me think its isolated to the preamp of the normal channel. The squeal is also showing up in the effects channel with the gain all the way off for that channel. But if there was an issue with a component causing squeal in the normal channel preamp, why wouldn't the same problem show up when plugged into the normal input, as I stated it only squeals in the series input. I've checked voltages on all the transistors and everything is close to the schematic. Anyone have any ideas?

[inapier]

Issue has been resolved. Here's the deal with these and the 320t mace amps. From the inputs there is a dual conductor shielded cable carrying the guitar signal to the normal channel. This is the red wire coming from the shielded cable terminating in the circuit board. There is also a black wire in the same shielded cable which carries the output of the normal channel back to the input jacks where it is fed into the effects channel when plugged into the series input, thus creating the two channels in series scenario. The input wire and the output wire for the normal channel are running parallel and literally touching each other inside the shielded cable run for about 10 inches, this is a major no no in guitar amps. It doesn't matter that they're in a shielded cable because they're not shielded from each other, them being in a shielded cable together probably makes the feedback loop worse. I removed this dual conductor shielded cable and installed separate shielded cables for the input and output of the normal channel and every bit of the squeal is gone. The high end fidelity of the amp is also greatly improved. I would recommend this mod to any and all deuce and mace amps that run the input and output together in a single shielded cable.