Marshall JCM600 Head Drive channel low volume

[jvm]

g-one, unhooking C108 connector at C121 stops the grounding issue. No signal loss when grounding pots.

[g1]

Then it seems to be related to relay RL2. The pin of RL2C that connects to ground is probably not grounding till the pots are grounded. And it seems to be staying grounded when switching channels.
Check that RL2 is getting it's proper switching voltage by measuring across D6 in the footswitch circuit. Check in clean and drive, should switch between around 0 to around 20VDC. If the voltage is switching, the relay is probably bad. If the voltage is not switching, could be something else in the switching/footswitch circuit.

[jvm]

The voltage at D6 is .7V when in clean channel and 19.4V when in drive channel. I have already replaced both relays previously with no change in symptoms. I also replaced TR1, 2,and3.

[g1]

Sorry to have to ask such basic questions, but how are you setting up the channels? Clean has only a volume, drive has gain and volume. I would think the drive volume should be set full to get a comparison. With drive volume full, how high do you have to set the gain to get equal volume to the clean, with clean volume set to 5? (set eq pots same for both channels)
I'm a little worried about getting sidetracked with the pot grounding, it is still possible that what is happening is normal, and the problem is elsewhere.
Connecting CN108 adds a load around 200K to the signal at C121. With C108 disconnected you could add a 200K resistor between C121 and ground, if it causes the same signal drop, we are barking up the wrong tree. If the signal does not drop with the added resistor, there may be a short or low resistance somewhere in the CN108 wire or at the drive volume pot.

[jvm]

No problem, g-one. Ask anything, I appreciate your help. When I have the pots grounded, drive gain and level wide open, I have very little volume. Lifting the pot ground gives me the same or more level than clean channel depending on gain setting. I verified that both relays are actually getting proper voltage AT the relay. There has been a lot of soldering done on this board. I am looking for some previous damage that may be causing this problem but can't seem to find anything amiss. I have been doing continuity tests, visually examining with a HUGE magnifier. I will try subbing the 200k resistor as soon as I get a chance.

[jvm]

Ok, I subbed a 220k resistor at C121 to ground. Same loss of signal. I've also had several other "problems" creep up just from moving this board around. It has been badly damaged from previous soldering. I don't suppose anyone has a preamp board for a JCM600/JTM60 lying around? I'm about ready to scrap this one. It is seriously messed up.

[g1]

So the whole issue of the signal decreasing with the pots grounded appears to be normal. Probably should be a much larger signal there at C121.
What are your DC voltages at plates and cathodes of V101B, V102A, V102B.
Can you inject a signal at the input and measure the AC voltage at grids and plates of V101B, V102A, V102B.

[drewl]

Everyone of these I've worked on the clean channel is louder than the gain channel unless the gain channel is cranked.

[jvm]

To get any kind of audible level, I have to have the gain, volume, and master volume wide open. Obviously, something is not right. Here are my plate and cathode voltages:
V101B
Cathode-1.56VDC
Plate-195VDC

V102A
Cathode-1.6VDC
Plate-192VDC

V102B
Cathode-4.12VDC
Plate-321VDC
I'm on my backup meter and can't read AC mV

[g1]

V102B
Cathode-4.12VDC
Plate-321VDC

Seems that side of V102 is barely turned on, cathode voltage should be higher. Can you see whether the heater is lit on that side?

Edit: sorry, that cathode voltage is probably ok as that tube is biased very cold with the 22K cathode resistor. Check the other side of the plate resistor R127, should be about 340V.

[jvm]

R127 reads 321V at the plate side and 342V on the power supply side

[g1]

So the DC voltages for those preamp tubes seem to be ok.
You will need to go through the signal tracing thing again, and with the pots grounded. Ignore the issue with the boost when the pots are not grounded.
AC voltage readings would be helpful. You should see very large increases in signal from grid to plate of V101B and V102A.

[jvm]

I re-soldered the V102 socket and the problem is solved! I didn't see anything that LOOKED suspect but obviously it must have been a bad solder joint. Thanks for all your help in steering me in the right direction, g-one. I'm still worried about how hacked up this board is. It is one of my own amps so we'll see how it holds up. Thanks again everyone