Need Help with Marshall Mosfet

[sasnak007]

I recently came to own a Marshall Mosfet Lead 100 head (model 3210) that doesn't really work, and I'm looking for any assistance I can get.

Basically, when I plug in the amp just hums. If I crank up the master volume, I can hear the guitar very faintly. Moreover, if I move the springs in the reverb unit, the sound comes through the speakers loud and clear. Also, if I plug into the effects loop return jack, I can hear the guitar fine (just can't use any of the controls and whatnot).

I've checked the solder points (went over a few just to be safe) and looked for any grounding issues. I thought it might have just been the input jack (grounding problem or something), but that doesn't appear to be the problem. I checked the voltages, and as far as I can tell, it all appears OK.

Sorry to rant, but I'm really in need of some help from someone with a little (or a lot) more experience with amp repair than me. Please, if you have any suggestions or might be able to help me at all, I would really appreciate it.

Thanks!

[J M Fahey]

Looks all the way like a defective Loop return Jack. Test it by hooking a known working good guitar cable from send to return jack, and your guitar up front, it should work.
If it does, just play it safe and replace both of them.
Good luck.