Marshall 4211 channel switching problem

[idontknowyou]

Im currently working on an 80s Marshall JCM800 4211 split channel reverb. It is having a weird issue that is driving me crazy. When the footswitch is not plugged into the amp, both channels should be active, but currently the normal channel is at about 25% volume and very dark. The boost channel works fine. However, when the footswitch is plugged in, the normal channel comes back to life and everything works as it should.

I have read on other forum posts here how problematic the switching functions are on this amp, and I have tried replacing the IC. Ive tried 4 different chips, all have the exact same effect. I have also replaced the two 1N914 switching diodes. No change.

I don't fully understand how the channel switching is working. Is the chip effectively shunting the 2 signals to ground? Is the chip responsible for the channel mixing or is that happening externally? I don't totally know what I should be looking for. Still learning the solid state stuff. Any insights would be greatly appreciated.

I should also mention that all of the electrolytics have been replaced, and I have thorougly cleaned the footswitch jack and fx loop jacks.

[Latole]

If everything works normally with the FS, IMO, the problem wouldn't come from the parts you replaced, otherwise even with the FS it wouldn't work.
What's the difference between connecting the FS and not connecting the FS?

Answer: the normally closed contact on the FS plug would be defective.

Could this be where the problem lies?

[idontknowyou]

Ok upon further testing I have found out that even when the footswitch is inserted, there is bleed from the normal channel. This is the same state the amp is in when the footswitch is not inserted. Basically the boost channel is engaged and the normal channel is bleeding in at about 30% volume. The footswitch does manage to switch to the normal channel, and in that state there is no bleed from the boost channel, but that state can only be achieved via the footswitch. At no point can I make the amp blend both channels together 100%

Can anyone clue me in on where the channel mixing occurs? I'm missing something major.

I did also find that the input jack is not normalling when there is no plug inserted. Pushing the first contacts closed effectively puts the amp into normal channel only mode, but this is kind of moot because as soon as a plug is inserted that connection would break anyway.
The ground scheme on this thing is confusing.

[idontknowyou]

Might also be worth noting that the amp I have deviates from the available schematic slightly. Specifically, the numbered capacitors stop at 36. So C41-44 in the switching section are absent from this circuit.

[Latole]

There a few JCM800. Are you sure about 4211 number ?

[idontknowyou]

That's what is designated on the chassis. Its a 2x12 100 watt combo. In the archive of JCM800 schematics here: (https://www.drtube.com/marshall-jcm800/) the 4211 is said to be the combo version of the 2210 head, and that is the schematic provided.