Marshall JCM600 Head Drive channel low volume

[jvm]

I have a Marshall JCM600 head with low drive ch volume compared to clean ch. When I got the amp, it would not switch to the drive channel. I replaced the broken front panel selector switch and replaced both low voltage supply fuses that were blown. I now have channel switching function but the drive channel is low in volume compared to the clean channel. I have subbed a complete set of known good tubes with no change. The both channels sound fine but the drive ch has low output.
here is a link to the schematics:Marshall Amps Info & Schematics

[Jazz P Bass]

Check the plate resistors for the drive channel.

[Mick Bailey]

I've had similar symptoms - it was a cathode resistor fault in my case. Check the DC voltages around the drive channel tube - my resistor measured ok with the amp off.

Why were the fuses blown?

[jvm]

All plate and cathode resistors tested fine. I'm not absolutely sure what blew the low voltage supply fuses but the switch I replaced had been broken apart internally. Everything seems to be functioning fine other than the drive channel not having as much output as the clean channel.

[Mick Bailey]

If the DC voltages check out then there's a useful block diagram showing signal flow at the top of the preamp schematic. There are a few connectors in this amp - check to see that they're plugged into the correct sockets and correctly oriented. Also - an essential check - the pot solder joints on the PCB. The nuts loosen off and the pots wiggle until the joint gives.

If you have a scope and signal source then you can trace the signal through. If not, use a music source and audio probe (sometimes I think this is quicker as you're not watching a scope).

[g1]

Make sure there are no little metal bits from the old switch stuck in the amp. It's possible some bits floating around in there may have caused the fuses to blow and may also be responsible for the drive channel issue. Could have damaged some other part in the drive channel or still be stuck in there somewhere.

[jvm]

Finally got time to retrieve this Marshall from the "project" pile. Something odd I discovered by accident. I powered up the amp with preamp board on the bench( not grounded to chassis through pots) and both channels work great. Drive channel is loud and mean! I put the board back into the chassis and low volume on drive channel again. ????????

[jvm]

Found R47(10 ohm) shorted. replaced it and now neither channel signal making it to the MV. I suspect possibly a bad relay. Have to order.

[g1]

If the resistor is actually shorted this would be the first time I have ever heard of that. When resistors fail they increase in value. Did you check the bad resistor out of circuit? Did you check the new one in the circuit? I would guess there is something shorted in parallel with the resistor.
When you give a component number please tell us where it is in the circuit, there are hundreds of them and they can be very hard to find. The only R47 I see is a 1 meg resistor in area E4.

[jvm]

My bad, g-one. It was R7 not R47. 10 ohm tied to master volume. And it was open, not shorted. I need to stop having bourbon on my cheerios!

[g1]

R7 is 10Meg, not 10 ohm . So no doubt it reads open, and putting a 10 ohm in there would probably kill the signal.

[jvm]

Well, cut my legs off and call me short! You are correct, g-one. Maybe I should stay away from the bench until some of my distractions are dealt with. Wife totaled her car last week and is still having some issues with her neck. That, and trying to deal with insurance and replace the vehicle, while dealing with the weather, has me losing focus. I think I will leave this amp until the new relays get here and then start fresh. Thanks for spotting my goof!

[jvm]

Ok, here we go again. I replaced R7 with the proper 10M resistor and replaced both relays. No change. I removed the preamp board and connected the pots to ground with a clip wire so I can signal trace. At this point, drive channel is weak. If I lift my clip wire, drive channel works as it should. I started signal tracing with an audio amp and find that if I lift my chassis ground while listening at R127 (plate resistor V102B), my volume at R127 increases. R127 tests fine at 100k. Signal strength is not affected by grounding until the plate of V102B.

[g1]

Disconnect that connector at C121 and see if it still acts the same way regarding the chassis ground listening at R127.

[soundguruman]

Found R47(10 ohm) shorted. replaced it and now neither channel signal making it to the MV. I suspect possibly a bad relay. Have to order.

resistor does not short. Resistor will OPEN, NOT short.