Marshall MF350

[the_castratos]

I wasnt thinking and started to unplug the speaker cable from the head while it was on, it shocked the crap out of me so i didn't get it all the way out and it sparked and blew the fuse......and more. if i put a fuse in it blows and smokes from the board that has what i thought was a transistor( but i guess its not what its called)and the fan. i was wondering how the test those(whatever theyre called) with a multimeter.....and what other problems i should look for.

[the_castratos]

nevermind for most of it... it was the ic's but i still dont know how to test them...

[Enzo]

Sorry you learned it the hard way, but like it says on the rear of the amp, always connect and remove speaker cords with power OFF. ANd always use the correct jack for your load impedance.

You got smoke and blown fuses? Then the power modules are bad. There are four of them, look close. ANy show black soot or smoke marks around the legs? That is a sign of death. If you already know there are bad power ICs, then what is there to test?

If one fails REPLACE THEM ALL. Mouser sells them for about $6. If you try to go cheap and replace only the ones you find shorted out, all that will happen is you will burn up your new ones. ALL FOUR.

The little boards are power amp modules. The actual integrated circuit on the board are type TDA7293V.

[the_castratos]

there are no signs of burns or anything like that. only little blown out holes on the IC's on one side that i didnt notice at first. i unplugged them and plug headphones into the emulated out and everything worked good. so you think i should replace the little boards?

[Enzo]

I have found that the boards themselves are usually OK if I replace the 7293s on them. I only replace the little board itself if it is too burnt for trace repair.

[the_castratos]

thanks alot now if my soldering iron wasnt such a piece of crap id be able to remove the IC's

[Enzo]

Try this: snip the IC off with cutters. Leave as much of the legs as possible. Now melt the solder and pull out each leg one at a time. Then go back and clean the old solder from the holes.

[Mars Amp Repair]

Also,
Be careful when you remove the cable plugs from the boards. Don't twist or pull them out hard if you can help it. I have an amp in from another servicer in which they removed & replugged those cables so many times that the cables became insidiously intermittent.

We went through 2 sets of IC's beore determining that was what was happening. On the first set, we actually replaced the boards at $20 ea...OUCH! Fortunately, those were claimed on the warranty. I get the IC's for $2.50 each from Line 6 (we are a warranty station).

Anyway, we thought it was the infamous 'last screw detector' syndrome, but low & behold the cables would need to be moved just right (usually after the amp was all back in its house) & then as Enzo put it...KAPOWW!!

Oh BTW, be certain to always wear eye protection when working on the MF-350, as IC shrapnell can be detrimental to the cornea. They must have had a premonition when they named this amp the 'MF' 350. It certainly applies ;-].

BTW, those cable are not available. We actually had to hard wire all of them...that's 30 connections times 4. What a job. glen