Marshall 2210 master volume not working

[65basser]

A troubleshooting week from hell! I wish this was the worst of my problems but sadly it is the tip of the iceburg. This latest torment comes in the form of a JCM800 series 2x12 combo. The chassis is a 2412 and the preamp ckt is the 2210. It came in with an intermittant channel switching problem that was resolved by replacing the 3046 IC. Now, the clean channel is fine but on the lead channel, the master volume doesn't work. It changes the tone(sounds like a phasing effect or sweep) but has little effect on the volume. I'v tried decoupling the signal at various stages but the only thing that eliminates the signal with the master down is pulling V1(of coarse) or disconnecting the coupling cap to the phase inverter. Even breaking the effect return contacts does not eliminate the signal. Again, this only happens with the boost channel. The master works fine for the clean channel.
Thanks in advance for any advice!

[Enzo]

Is the bottom leg of the control actually grounded? Check for continuity. Is the pot open? Does the wiper leg go to ground when at zero? Pull the pot and check all its properties.

[65basser]

The pot is grounded. I will test the wiper continuity at all points when I get back to it. I had to move on to other repairs for a while. Thanks.

[Chuck H]

If this is a new problem that popped up after chip replacement I might suspect the chip, the power supply for the chip, the traces and contacts for the chip or the chip installation. The schem makes it look like both channels run through the same master volume. So if one channel is attenuated, but the other isn't, the unattenuated channel's circuit is somehow bypassing (though affected by) the master volume. The phase affect is just that. Two signals of different phase are trying to combine at different levels when the master volume is operated. If the chip is doing what it should that shouldn't happen.

EDIT: Looking at the schem again it appears that there should be no way a chip malfunction could inject signal beyond the master volume. So if the master volume pot is working, as it seems to be, something must be hooked up cattywhompus in there. Is it possible the amp has had mod attempts or someone else messed with it before you got it?

[g1]

Looking at the schem again it appears that there should be no way a chip malfunction could inject signal beyond the master volume. So if the master volume pot is working, as it seems to be, something must be hooked up cattywhompus in there. Is it possible the amp has had mod attempts or someone else messed with it before you got it?

Like Chuck said, this sounds like a different problem than the chip issue. You said you still have signal getting into the power amp with the FX return switch opened. This should not happen, you need to find out if something is modded or miswired.

[65basser]

Nothing is modded or miswired. It appears bone stock. I don't think it has ever been serviced before. I tried subbing new filter caps in case the signal was being induced into the B+ somehow but it made no improvement. Wait a minute...it looks like R30 is connected to the plate of V3 not V4!
I'll have to check the grid and cathode wiring.

[drewl]

I had a 2210 head I got cheap because it had both of the problems you mentioned.
I had to replace the chip, and the master pot went bad.
So did the dual gain pot.
I ended replacing all the pots over the years, they're the only thing holding the pc board in place so they're under constant stress.
While you're there, reflow all the pot pc board connections, jacks, and any other suspicious looking solder joints.

After some work, I enjoyed many years of playing with the amp.

[g1]

Wait a minute...it looks like R30 is connected to the plate of V3 not V4!
I'll have to check the grid and cathode wiring.

R30 & R27 both go to the same supply rail, so if they are reversed it shouldn't make a difference. However, if point Y is connected to V3 instead of V4, it would explain the FX loop bypass. Maybe some sockets miswired?