Marshall MF350 repair

[phantombox]

Hi.
I'm new to this forum.
Anyway, I read a lot of threads about this MF350 amps on this forum, and I have a few questions of my own:
I was checking the IC's on the motherboard. Since the amp that I'm repairing does sound on the pre-amp section including the mute-switch, I only concentrated on the power-amp.
BTW, is it common that these IC's do fail on the MF350? There are no signs of deterioration visible anywhere on the motherboard (and yes, I know that IC's can fail anyway). With all 4 TDA7293 IC's still disconnected, there are 2 other IC's that are suspicious: IC5a and IC9a don't give a typical reading with a DVM (digital volt meter) set to diode reading.

I would rather not solder these IC's out of the motherboard because usually they doesn't work anymore when I do that. (I ALWAYS prefer DIP sockets instead of hard-soldering...)

I checked the 15-pin connectors that go from the motherboard to the TDA7293 PCB assembly for continuity and so far they all check OK.

Any ideas or comments on IC5a and IC9a?

Phantombox

[Steve Conner]

Can you explain what exactly is the problem? Your description isn't clear. Does the preamp work or doesn't it?

If the preamp does work, don't worry about troubleshooting it. If it doesn't, use signal tracing to find the fault.

[phantombox]

The preamp does work.

[phantombox]

Across R81 and R82 I measure +47VDC and -47VDC, with no TDA7293 connected. Is this value acceptable?

[Jazz P Bass]

Sounds right.
If & when you replace the TDA ic's, you should replace all four.
Also, the power supply must be drained when you reinsert the connectors or you have a good chance of blowing them again.

[phantombox]

Finally, I got the M(other) F(uXXer) 350 running all right.
350W is a lot of Ws...
Anyways, I suggested to the owner to get rid of it asap and get a JCM2000

[Steve Conner]

Excellent advice