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j4976 30th June 2012 07:39 PM

AVT50 Repair
 
I have a Marshall AVT50 that keeps blowing fuses. I unplugged the leads to the TDA7293 board, and the fuse does not blow, but I removed that board and inspected it very carefully, and I can't find any sign of abuse, heat, or other distress. It looks new.

Should I replace the TDA7293 chip anyway, or can I test it first? Do they sell some that don't have the mute function, or will I be ok as long as the chip is a TDA7293?

Is there a cheap place to get these fuses?

Thanks.

geraldfryjr 30th June 2012 09:01 PM

Check the chip with an ohm meter.
Or any diodes or bridge rectifiers if there is any on the board.
Most likely it is shorted otherwise it wouldn't be blowing any fuses !!

Replace it with the required chip ,I.E. TDA7293


jer :)

j4976 30th June 2012 09:24 PM

So, is there a key somewhere that tells me which leads to check impedance across? Also, would I have to remove the chip from the board to get an accurate impedance reading?

geraldfryjr 1st July 2012 06:16 PM

The power supply pins to ground and the output pin as well.
if any of these combinations show a dead short then the chip is bad.

VS2000 AVT 150h, replacing TDA7293 w/ pics & schematics - Marshall Amp Forum

tda7293 marshall - Google Search


No,you don't have to remove the chip from the board to do this test.

jer :)

j4976 1st July 2012 06:23 PM

Ok. Yes, I have a pinout diagram.

So, its ok to test on the board? Just looking for dead shorts?

I'll get it done.

j4976 1st July 2012 07:05 PM

Ok. Pins 6, 7, 8, 9, 10, 11, 13, 14, and 15 all have less than 2 ohms resistance between them, and to the heatsink. That's the threshold for my continuity setting.

The other pins are all above that level.

geraldfryjr 1st July 2012 07:37 PM

Sounds like it is shorted to me !!

I have a friend that has one of those amps and he just had the power amp chips replaced.
And it is now working fine.
I didn't do the repair on it but I good friend of mine did as he does this stuff in my area.
We are not exactly sure of the cause but I guess it happens quite often as he keeps these chips in stock.

jer :)

j4976 1st July 2012 08:14 PM

Its good that they are so cheap.

geraldfryjr 1st July 2012 10:29 PM

Yes,They are amazing little chips for the money !!!

jer :)

j4976 2nd July 2012 03:04 AM

Hey, do you know what kind of stuff Marshall put between the chip and the metal plate? It seems to have been some kind of adhesive, and it doesn't look at all like the white conductive heat sink stuff I'm used to using.

Some thread said the chip needed to be insulated from the metal plate, but I think the plate helps cool the chip. Is the adhesive some kind of special thermally conductive, but non electrically conductive stuff?

Thanks. Don't want to overheat or short the new one.


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