74' Marshall JMP 50w broke

[bmd]

This amp is 34 yrs old.Sat unused for 10 yrs at friends house. When I got it back I had tech check it out{I guess he brought it up slow} and started to use it. Had some noise problems(excessive hum) and used something else for 6 months. I made up 2 bias-rite sockets and hooked them up , when I turned amp on I hear a pop like something arced in one of the bias -rite sockets. Turned amp off immediately. Now when I flip standby on I get loud humming & crackling with\without volume up or zeroed. I get this with/ without any tubes. I've only been turning standby on momentarily since this happened.Almost sounds like someone holding the end of an input cord ,a short somewhere. Speaker is hooked up ,ohms are correct,plugging input cord in,same thing. Nothing visibly out of order,burnt components,arced sockets,bare wires etc.

[Chuck H]

Hum with no tubes in is a bad thing. The speaker should be isolated without tubes in. So nothing should get to the speaker unless there is a short in the output transformer, a problem with the chassis ground or voltage backing up through the feedback loop from a bad decoupling cap.

Short story is that because you don't have much electronic experience this will be a hard problem for anyone to dianose from a remote location. I think you should take it to a tech. A different one than you did last time. The way your story reads, that amp was in need of new filter capacitors when you took it to the last guy. And yet he sent it home anyway. The initial symptoms you describe sound like filter cap failure. Then the whole thing $h!t the bed. He should never have sent that amp home. Now you likely have new and more expensive problems to deal with.

Sorry

Chuck

[Dave Curtis, dB AudioTech]

This amp is 34 yrs old. Sat unused for 10 yrs at friends house. When I got it back I had tech check it out {I guess he brought it up slow} and started to use it. Had some noise problems (excessive hum) and used something else for 6 months.

Should have brought it back to the tech. He forgot to change the 'lytics.

I made up 2 bias-rite sockets and hooked them up, when I turned amp on I hear a pop like something arced in one of the bias-rite sockets.

Uh, oh. Are you sure the probes were wired correctly?

Turned amp off immediately. Now when I flip standby on I get loud humming & crackling with/without volume up or zeroed. I get this with/without any tubes. I've only been turning standby on momentarily since this happened. Almost sounds like someone holding the end of an input cord, a short somewhere. Speaker is hooked up, ohms are correct, plugging input cord in, same thing. Nothing visibly out of order, burnt components, arced sockets, bare wires etc.

What's it look like inside your bias probes? You may have fried your OP tranny.