Marshall mf350 mode 4

[pontiacpete]

hi, I have one of these that is not powering up, blowing fuses. I have it on a light bulb limiter and it looks like there's a short, the light glows very brightly. I pulled the secondary connections of the PT off the board and measured across them(there appears to be no short when doing this). On the 32v winding I measured 36v. On the 19v winding I get 63v. Why is this? Is there a short of some other kind or is this normal and how should I proceed from here?

thanks for all help,
pete

[52 Bill]

I know we've discussed these amps here before. Isn't this the amp that Enzo will not take in for service anymore?

The usual suspects on these are the power output modules. The chip seems to short out just by looking at them funny.

Replace the transformer wiring and unplug the output modules instead, then see what happens when you power up.

Also search the forum for past discussions.

[pontiacpete]

Thanks 52 Bill, unplugged the modules and sure enough it starts up. Got the chip ordered.

[Enzo]

That is the one, I refuse to put my name on a repair on these things.

Meanwhile, you ordered "the" chip? DO a search and read through the existing threads on this model. Search separately for Mode 4 and for MF350.

My rule is if one of the FOUR power modules is bad, replace ALL of them.

[pontiacpete]

So it goes, I got the chips, all 4, put them in and it worked like a charm. THe owner took the amp back and on his first 'turn on', it blows the fuse and the chips are toast.
I get it back and put in 4 more and it works great(kinda like it actually). The owner, come to find out, started it without a load. I don't know here, but I thought many transistor amps could be ran without a load. Is this not true for the MF350?
Also, I was wondering, if a shorted speaker cable were used or a blown speaker, would that short these chips?
Or did this thing just plain break when it was moved from my place to the owner's?

I wonder if it's still working...

[Enzo]

Look at the amp cross-eyed, it will blow the chips. Listen to Glenn Beck, it will blow the chips. Play some song I don;t like, it will blow the chips. See why I refuse to work on them? The customer will of course blame me for not "fixing it right."

I talked to the main Marshall guy at Korg in New York and asked him if I was missing something, was there some way to prevent them from blowing up. He told me, that that was just the way they are, and that's why Marshall stopped making them.

[Gtr_tech]

Since it was a fairly successful amp, they should've revised the "design" and put in a *real* discrete output section in place of those stupid chip amps. 'Specially for the the price they get for them...

[52 Bill]

Normally a solid state amp can be plugged in without a speaker and in fact I think that these Marshalls can be as well, what these Marshalls don't like is having a speaker plugged in while it's on or while the power supply is still charged up.

There is one customer of mine that has one of these and he only had one problem so far. A chip fan died and the chip overheated. I replaced the outputs and the fans and it has been running fine for the last couple of years. This amp never moves from his music room, never gets unplugged, (AC or speakers) and it will probably continue to work until a new fan dies or there is some kind of power surge that takes it out.

I guess that this is what these amps were designed to be, nice sounding amps that never get gigged or treated to normal road wear.