Marshal 5213 Mosfet

[Colonel_Sanders]

Hello all!

A friend brought me this amp. Turned it on and a nasty buzz/hum comes through the speakers. I turned all the controls to 0 and disconnected the reverb and the buzz/hum was still coming through. The speakers seem to be fine.

http://www.schematicheaven.com/marsh..._100w_5213.pdf

This is the first solid state amp I've had to troubleshoot and was wondering if anyone can give me pointers on where to start? I'm guessing power amp, but what should I test?

Thanks!
C_S

[52 Bill]

I'd first check for a loose or dead filter cap.

If you give the amp a good Enzo whack with your fist can you make it stop?

Is there any dc on the speaker output?

[Colonel_Sanders]

Thanks Bill. Will try and check for all of this today.

Also, apparently the damage was done by putting bass guitar at full tilt through the amp.

[Colonel_Sanders]

Ok, whack is a no go haha.

Theres 2.1VDC on the output. One fiter cap was loose. Thats fixed now but theres still DC on the output.

The DC shows up on the speaker output, and headphone output, but not line output.

Amp shows + and - 50V and 15V. No problem there.

So, what can cause DC on speaker output???

Thanks heaps!

C_S

[52 Bill]

Boy, there really isn't much to that power amp is there.

Check the obvious transistors and diodes, etc.