Marshall 2205/4212 problem with master volume

[terrykvan]

I have a 1986 jcm 800 4212 split ch. 50w. The master volume has no effect on the boost ch. except that the eq doesn't work when it is turned all the way down. The clean ch. works ok as does the ch. switching. No bleed over but the amp as a whole seems to lack the gain it once had. However, if I plug a guitar into the effects return, it has plenty of gain and sounds good. Putting new tubes in has no effect. How could that signal get past the mas. volume? After checking the plate voltages on the preamp tubes, a couple of weird things I found were V1b had only 80vdc on it but after lifting the clipping diode on the cathode it went up to 212vdc. I don't know if that's normal or not. The diode is good. Also, V2a had 276vdc.Too high right? All cathode resistors and ground connections look good. The mas volume pot checks good. It works fine on the clean ch. Any help would be greatly appreciated.

[g1]

So we can get the proper schematic up, is this the version with transistor switching or the 3046 IC ?

[terrykvan]

3046 ic, thank you

[g1]

[g1]

Have you done resistance checks on the Master pot? Looks like it should kill any signal if it is working and turned down.
Does the master work on the clean channel?

[g1]

And just to be clear, are we talking about the boost channel volume or the amp master volume?

[terrykvan]

i put the ohm meter across the wiper and both sides of the pot and measured changes in resistance as I turned the master volume up and down. Yes it is the master volume.

[terrykvan]

And yes the MV works on the clean channel.

[g1]

Can you try it with V2 removed?
Is there still sound going through with master turned down?

[terrykvan]

Yes, no change. I made another observation. I disconnected the ground wire at the power socket inside the chassis and measured between earth and the chassis- 207vac. !! I then checked the wires from the OT secondary to see if any were damaged, moved them around a bit while observing the voltage. It did fluctuate down to about 155vac but that's it. So I will check all the wires some more. Not sure what else could cause that. Thanks for your help.

[g1]

i put the ohm meter across the wiper and both sides of the pot and measured changes in resistance as I turned the master volume up and down. Yes it is the master volume.

One side of the pot is supposed to be connected to the wiper and should read 0 ohms no matter how the pot is set. Are these pots mounted to the circuit board? If so, have you checked the solder connections? I don't see how it can work for the clean but not for boost ch. but maybe if one of the solder connections is bad?
(for anyone following along, MV pot is VR10 in lower left area of schematic, above point G)

[terrykvan]

One side of the pot is supposed to be connected to the wiper and should read 0 ohms no matter how the pot is set. Are these pots mounted to the circuit board? If so, have you checked the solder connections? I don't see how it can work for the clean but not for boost ch. but maybe if one of the solder connections is bad?
(for anyone following along, MV pot is VR10 in lower left area of schematic, above point G)

My mistake sorry, it measures from .4 ohms to 100k ohms from min. to max volume. It is a 1meg single pot.

[g1]

So does one end of the pot read 0 ohms to the wiper at all pot settings?
Have you looked at the solder connections?
Are the pots mounted directly to the circuit board?

[terrykvan]

So does one end of the pot read 0 ohms to the wiper at all pot settings?
Have you looked at the solder connections?
Are the pots mounted directly to the circuit board?

Yes, just like you said before. 0 ohms at all settings. then between wiper and grounded lug .4 to 100k ohms min to max. But my latest observation is that I inserted a plug into the effects return to break the connection to the entire preamp, and the boost channel still comes through. It is a pre eq signal (the eq knobs have no effect) but when the mas volume is turned up the eq knobs work. It's a blend of the 2 signals. I now suspect that something is amiss in the channel switching circuit. Although the channel switching does work. I'll check that again. Thanks for sticking with me here, I think we're close to diagnosis.

[terrykvan]

Even with V4 pulled out, It still gets through! Switching does work. when switched to clean, there's no boost signal.