marshall-avt150 problem-TDA7293 chips

[wyomingrocks]

Hi group, you really have a great forum here. Hope you can help me. I got a phone call last night from my nephew saying his new "used" marshall avt150 amp quit working after he plugged in to it. I went over there today and his fuse was blown, I replaced it and noticed a large spark behind the heat sink. I pulled it off and noticed the two TDA7293 chips were shot.

My question to you is when replacing these chips is there normally additionally circuitry like the caps on there boards that goes bad or does replacing both chips usually take care of the problem.

I do not have a schematic, so I am sort of shooting in the dark trying to help him out. We ordered a couple of chips, and I am keeping my fingers crossed that is all that is wrong. After I noticed what was shorting out, I removed both circuit boards the faulty chips were on to see if the fuse would blow and it didn't. With out a schematic it is difficult to know what is driving what and hopefully someone with some experience with these amps can help.

Also if possible, I have read about a service bulliten on here involving diodes to protect the input from frying these chips, if anybody has any info on that it would be great help also. Thanks for your help.

[Enzo]

Usually the handful of little caps on the tiny boards are fine. When the chips come off, inspect for burnt off traces. You can easily repair the burnt trace with a piece of bare wire, but you keed to know avout it first.

I always bring them up on a variac when I first powr up after replacing the 7293s. In any case, if the fuse holds you should be OK. if you get no sound, then teh mute circuit on the main board was probably damaged. The circuit is just a zener, a cap, and a transistor all in parallel more or less. One shorts and the amp stays muted. Usually it doesn;t happen, but often enough to check for it.

Send me a email with Marshall AVT150 schematic as the subject line

My adress is:
tmenzo at msn dot com

[gchiappe]

Hi group. I am new to this forum and just wanted to add my recent experience with a Marshall AVT150H. When I first tested it on the bench I learned that I needed a welding helmet to prevent retina burn as I observed the main fuse arc-flash.

Turns out that *BOTH* TDA7293 chips had failed in a dead short mode. The only insight I got as to why something like this might occur is that the owner told me he had a cell phone on top of the amp and when a call came in the amp stopped working.

I really don't know if the RF from the Cell Phone caused the failure and I am wondering if anyone else has observed any similar strangeness.

This is a great forum. Thanks

[Mars Amp Repair]

it's more likely that the tda7392's just decided to give up the ghost at that moment. they like to do that & do it a lot! g

[drewl]

Oh boy, I have an MG100 whatever with a blown fuse, and a single 72 yadda yadda chip. Installed new fuse and it blew right away....unplugged the header to the chip and it powers up, so the output module is likely blown!

Enzo, if you don't mind I'll email you tomorrow for the schem also since I'm off to bed now.
Who do you guys usually order these from?
I always buy a few of whatever I need, because if I have to order something once, chances are I'll have to order it again.

Thanks.

[Enzo]

I order 7293s from Mouser. DIgikey has them and charges a buck or two more than Mouser.

I been buying them 10 at a time lately. The Mode 4 sucks up 4 at a time, and the AVT amps have two often.

usually nothing else is damaged, except once in a while the mute.