Marshall DFX 250 no sound right channel

[Lineman622]

Today I've noticed that there is no output coming out of my right speaker in my twin amp. I've tried connecting both speakers one by one to the left channel and it turns out both speakers and their leads are working fine. Whenever I've tried plugging the speakers into the right channel though there only would be white noise. I've sprayed all the jacks with contact cleaner and the speaker contacts on the inside; after that there was no sound coming from the right channel at all.

Here's the schematic: http://www.ampix.org/albums/userpics...l_MG250dfx.pdf

Next troubleshooting steps?
Thanks in advance for the help.

[Enzo]

Plug a signal into the right FX return jack. Sound?

Also note the signal through the right FX return goes THROUGH the left.mono jack.

[Lineman622]

I've plugged my guitar into the right FX return jack but it's the same as before, sound only coming out of one speaker.
Thanks for helping out, what should I do next?

[Enzo]

Schematic. Page 3. FX return jacks. They go off the page to the left on pins 6 and 7 of CN1.

Page 1 lower right, mating connector. Pins 6 and 7. They feed signal path going left through IC4, all the way over to IC5 lower left. From that IC through the master volume control VR13. From there off page to left with signal names PAIN_L and PAIN_R

Page 2 upper right. Those two PA in signals appear and go through some shaping before feeding the power amp modules.

VERY IMPORTANT. "No sound" is not the same as "no signal". If the speaker makes any little background sound even without any hint of the music coming through , that is not "no sound". SO: is there ZERO sound in the right speaker? Or just no music?

In any case, we work back from the speaker. You tested that. Next the power amps. There is a small module for each channel that is its power amp. They are the same. If the cables will reach, you could swap the modules in place. Otherwise, you could carefully swap places. If the dead channel moves with the module, you need a new one. If you dismount a module make sure not to lose the little insulating mica piece.

You can work from the front too. APply a signal to the input. I just use a CD player myself. Get out your scope or a signal tracer and follow. Remember pins 6 and 7? Is there signal there? How about over at the mater volume, signal there?