Marshall AS80R preamp trouble

[Enzo]

There is no way for the switch to get backwards, so I'd believe the drawing was in error. But if both sides of the switch work, then assume it is all correct. This means your trouble lies elsewhere.

[Randall]

I have replaced the switch and re-worked many of the connections and tested signal paths in that circuit. Damn, this is kicking my butt!

[52 Bill]

I have replaced the switch and re-worked many of the connections and tested signal paths in that circuit. Damn, this is kicking my butt!

I'm late to the game here, what are you trying to fix?

Does the channel work and pass signal normally?

Is the perceived problem just that there is little gain change between the two switch positions?

[g1]

Whew.
Good that you traced out the circuit and verified the schematic is in error. However, I don't think you will like the result.

With contacts to the left:
* input resistor net adds up to 101k (R209+R209A)
* IC204a gain is 1+(15k/14k7)=2X
* output is attenuated 3k3/(3k3+10k+470)=0.24X
* so total gain is 2*0.24X=0.5X

with contacts to the right:
* input resistor net adds up to 10M+
* IC204a gain is 1+(15k/4k7)=4X
* output is attenuated (6k8+3k3)/(6k8+3k3+10k+470)=0.5X
* so total gain is 4*0.5X=2X

Since it turns out that the B side of the switch is reversed, we have to adjust what JM said, like this:
With contacts to the left:
* input resistor net adds up to 101k (R209+R209A)
* IC204a gain is 1+(15k/14k7)=2X
* output is attenuated (6k8+3k3)/(6k8+3k3+10k+470)=0.5X
* so total gain is 2*0.5=1X

with contacts to the right:
* input resistor net adds up to 10M+
* IC204a gain is 1+(15k/4k7)=4X
* output is attenuated 3k3/(3k3+10k+470)=0.24X
* so total gain is 4*0.24X=.96X

Therefore, we basically have unity gain for the stage with either switch setting.
My guess is that when you temporarily had the extra boost (post #14), one side of the switch or the other had cut out.

[Randall]

Oh my aching back! Now it makes sense. I found that there is a difference in tone when the volume is cranked up. Also, the tapers on the pots are strange in that nothing much happens until about halfway. I think I have been chasing a non existing problem for days. Customer said he thought it had "volume issues" in that one channel was weaker than the other. I think a cleaning was all this amp needed, not a long drawn out dissection!

As always fellows, I greatly appreciate the guidance.

[g1]

Often a complaint about amps (esp. Fender it seems) is that the volume comes 'all on' too suddenly at low settings.
Perhaps they over-compensated with a solution.