Marshall Silver Jubilee reissue JCM25/50 - Quiet Lead Channel

[earache]

Hello

I have on my bench a Marshall JCM25/50 Silver Jubilee reissue Model 2555.

The Lead Channel as chosen by the pull switch is almost completely silent.

Everything on the pc board looks ok, nothing burnt or detached. I tried replacing the preamp tubes one-by-one with a known good tube, with no change.

Thoughts? Here’s the schematic:

https://schematichell.com/marshall/2...5_50w_2550.pdf

thanks

earache

[The Dude]

Do you have a scope? If so, follow the signal path and see where it goes low. If not, you can do the same with a DVM set to ACV.

[g1]

Try tapping on the relay and see if it makes any difference. You could also jumper across the appropriate pins to test (short LED3 anode to D2 cathode).

[earache]

This one has a number of challenges it seems, although it could be me…

I can’t find certain components per this circuit board schematic (see my initial post).

For instance, g1 suggested I “short LED3 anode to D2 cathode”.

I can’t locate LED3 anywhere, though there are LEDs 1 and 2 evident. I also was looking for components that seem to be related to the relay “RL1” and I was unable to locate R30 and C23

While doing some fact finding I took a generic TRS foot switch and tried to operate the boost function with that. All it did was make both channels quiet.

I can hear the relay RL1 clicking when I operate the pull switch on pot VR3

Could it be that this amp is a PCB reissue of an earlier model named “Model 2555” and so has different reference designations for the components? Or some entirely different circuit components/names?

Another question - would an LED used merely as a diode in a circuit, light up when in operation? Neither LED1 or LED2 light up at all when any of the channels or boost functions are in operation.

[earache]

…….additionally, tapping on the relay made no difference in operation…and there is some challenges to trying to follow the signal path thru the amp because it has a PCB for the main board and many of the caps are box style, with no visible leads

[g1]

Could it be that this amp is a PCB reissue of an earlier model named “Model 2555” and so has different reference designations for the components? Or some entirely different circuit components/names?

Yes, the component designations will likely be different.
Post a request in the schematics section and specify re-issue. Maybe one of the miracle workers here will have it.
Where LED's are used for clipping function, they may not light up under normal operation.

[earache]

Problem solved. I found that the “Lead Master” pot had taken an impact from the front at some time in its life, enough to crack the phenolic wafer right above the legs.

I had a very similar pot in my stash and was able to replace the broken one. There was a great deal of difficulty in removing the old one, what with no-lead solder and terribly fragile pads on the circuit board. The plug in connectors that I had to wrestle with to take out the small board that the pots lived on were a bear to unclip. You would think they didn’t want anyone to ever service this thing.

The topper was that when I finally got the pot legs off the board, it lifted a trace off the board which I had to replace with some buss wire! Fun times.

I answered my previous question about LEDs in circuit and if they ever light up. They do when you hit a note and they glow brighter the harder you play!

Thanks for your kind help everyone!

earache