TDA7293 replacement in an AVT150

[hairydan]

Hi all my name is Dan. I've been lurking on this forum for some time now, but this is my first time posting. I have a few questions regarding the infamous TDA7293 chips in a marshall AVT150 amp I'm working on. The amp was blowing fuses at power up so I disconnected the wiring to the power amp boards and of course it stopped blowing fuses.

After a bit of reading up I got out the soldering iron and the solder sucker and replaced the two TDA7293's. I remounted the chips on the heat sink with fresh thermal paste, but accidentaly ripped one of the little plastic/film squares between the heat sink and the chip. I also neglected to discharge the power supply caps. So of course when I reconnected the boards, plugged in the amp, and fired it up it blew another fuse (and I'm assuming both of the 7293's).

Luckily i ordered 4 7293's from mouser "just in case" so the questions I have before i re-replace the chips are:

1.) What can I do to replace that little plastic/film square that I ripped in order to keep the 7293 from grounding out? The thought occurred to me that I could mount them to separate ungrounded CPU heat sinks (I have a few laying around) but I don't know how feasible that would be.

2.) Using my multimeter set to AC and grounded to the chassis I found about 100v on pin one of both CN1 and CN2 with the amp powered on, and nothing on pin 2 and 3. Is this right? For some reason it seems wonky to me. I also noticed some slight scorch marks on pins 1 and 2 of the CN1 connector that goes to the board, but CN2 looks clean.

3.) Will the amp work ok with just one of the modules connected? I figured it might be a good way to trouble shoot seeing as I wouldn't kill both chips at once if something went wrong.

[Enzo]

Insulating pads or washers can be ordered. DOn;t try to re-engineer the heat sinks. Got a dead stereo amp? Might find one in there. Otherwise search a parts house for transistor mounting hardware. An insulator for a TO247 looks like it might fit.

100v on the module? That doesn;t sound right. Doesn't really sound even possible. Where did you ground your meter? Look at the schematic. On the three pin power connector to each module, pin 1 is +V and pin 2 is -V. Pin 3 is the output bus. With meter to ground, you should find those +V and -V there. None of those three pins is ground.

[Prairie Dawg]

Insulating pads or washers can be ordered. DOn;t try to re-engineer the heat sinks. Got a dead stereo amp? Might find one in there. Otherwise search a parts house for transistor mounting hardware. An insulator for a TO247 looks like it might fit.

100v on the module? That doesn;t sound right. Doesn't really sound even possible. Where did you ground your meter? Look at the schematic. On the three pin power connector to each module, pin 1 is +V and pin 2 is -V. Pin 3 is the output bus. With meter to ground, you should find those +V and -V there. None of those three pins is ground.

I've got a Marshall MG100HDFX that I'm going to tackle pretty soon which has the same devise in the output.

The owner operated it with the speakers disconnected. Does this automatically mean the death of the TDA 7293? Is that where I should be looking?

Thanks

[Enzo]

The TDA 7293s are what blows up when it blows up. But operating without a speaker should not hurt it.

[Prairie Dawg]

The TDA 7293s are what blows up when it blows up. But operating without a speaker should not hurt it.

I did a little more looking on this Marshall MG100 etc etc. One of the speaker jacks was broken so I stole one from the effects loop down the road. It does produce some sound through an 8 ohm Celestion I had lying around here.

Would it produce a LOT more through a 4 ohm speaker? Is this supposed to be a pretty loud amp?

Shows you how much I know about solid state but you gotta start somewhere right?



Thanks

[Enzo]

A 4 ohm speaker would only be 3db louder, assuming equal efficiencies.

[Prairie Dawg]

A 4 ohm speaker would only be 3db louder, assuming equal efficiencies.

OK, didn't know. Solid state's weird science to me.

Enzo, you seem to have worked on your share of these things if you're buying TDA 7293s in the party pack.

What are some likely places to start looking for where the volume went? If I had to take a guess, if this is a 100w amp it's lost about 90w somewhere.

Thanks.

[hairydan]

Thanks Enzo! Glad to know that I can order a replacement insulating pad. I retested CN1 and CN2 and found about +50v on pin1 and about -50v on pin 2. Not really sure what I was doing wrong before but this seems to be more consistent with what you were saying. Will be rereplacing the chips once i get some fresh insulating pads.

Thanks for the help!