Mesa Nomad 55 Channels 2/3 Not Working/Weird Oscillations

[NeedHelpNomad]

Hi, first here's the schematic and I'll attach a video running through the problem.

Basically, Channel 1 seems to work normally so that should isolate the problem to the preamp section of channels 2 and 3. I've tried swapping tubes and that didn't seem to work. What's really weird to me is that changing the settings for channel 3 affects how channel 2 malfunctions, and I'm kind of at a loss here. I know about the issue with pots and this amp, but I checked and they actually have the plastic spacer washers and when I checked each pot with a multimeter they read normally when taken out of the circuit. Any ideas on what I could do to try and isolate the problem further?

[Pedro Vecino]

1:35: Gain control of channel 2 develops background noise when rising. Relay RY3 could be damaged.
One possibility of checking it may be to inject a signal into the gain potentiometer input of channel two to check if it sounds.

[NeedHelpNomad]

1:35: Gain control of channel 2 develops background noise when rising. Relay RY3 could be damaged.
One possibility of checking it may be to inject a signal into the gain potentiometer input of channel two to check if it sounds.

I don't have a signal generator (I should get one eventually), so could I test to see if RY3 is bad by bypassing it? As in, removing it and at RY3b putting a wire between the .02 cap and the 150K resistor/47pf cap, and at RY3a a wire between the Ch 2 gain and the grid of v4a?

[Enzo]

No signal generator? Other than for specific tests, in my shop I usually just use music for a test signal. I like it because it is more pleasant to listen to than a steady tone, plus it contains the entire audio spectrum, so I can easily hear if there are issues with the treble or bass or mids.

ANother option is a keyboard. DO you have a synth or other small keyboard with line outs? A steady patch like "organ" or "strings" should provide a steady sound. I used to wedge something between keys to hold one down. I had a small Yamaha with a demo tune I could trigger.

A sine wave generator is great for looking at waveform shape or for exploring clipping, but in general music from a CD player, tape deck, or my stereo receiver works well enough. To hot? Turn it down with a pot.

[Pedro Vecino]

This proposed system can also work but does not isolate the relay completely from the circuit if you don't disconnect one side of each component.
Injecting a signal into the potentiometer as Enzo says can be done in many ways. Itīs likely that simply touching it with your finger will generate a buzz when the gain potentiometer is raised (better with the input wire to it disconnected).

[NeedHelpNomad]

Ok I ran a line output from my recording interface into the input of the channel 2 gain control and the problem still persists. Here's a video:

[Pedro Vecino]

Well, there is already sound. You have used the central pin of the potentiometer (output), so the gain control has no effect.
In these circumstances you could disconnect from the board the pin of the 22n capacitor that is connected to the relay (RY3b) and connect it with the gain potentiometer input (left side looking at the front amplifier) by previously disconnecting the original wire that is connected.

[NeedHelpNomad]

Sorry for not getting back for a few months. I got busy and just left this on the bench, and now with the lockdowns I suddenly have a lot of time to figure out what's wrong with the amp. I tried what Pedro suggested with a spare .01 capacitor I had: one leg on the junction between the 3M3 and 150K resistors on the V1 plate, and the other leg on the input of the gain pot (while removing the previous connection from the 150K resistor to the input). The problem still persisted as usual, so it's not the relay. I also tried taking the tone stack out of the circuit to rule out faulty pots (and because it's really simple to check), and the problem persisted. I started looking at the block diagram to see any hints as to why channel 1 is fine but 2 and 3 aren't. I'm figuring it has to be something that's unique to channel 2 and 3 and not in channel 1. I looked at the preamp sections of the schematic and saw that channels 2 and 3 use J175 transistors for switching, while channel 1 doesn't, and upon further research these transistors are common failure points in Mesa amps, and one symptom of these transistors being bad is weak signal (which is something that's occurring for me, as well as the oscillations). I ordered a few J175 and will see if those do anything when they arrive. If that doesn't work, I want to try simplifying the circuit to remove as many possible factors to eliminate possible culprits. Is there a way of bypassing the entire reverb part of the circuit for channels 2 and 3? Would lifting the ground for the 100K reverb pots and lifting the junction of 120K resistors that go to "Rev In RY1b no" accomplish that?