Marshall MG100HDFX blown TD7293

[DonJoe]

Dear friends:

I purchased a Marshall MG100HDFX on an e-Bay auction. It came in with a blown 7293 chip.

The fan and the power board came in disasembled. I purchased a new 7293 chip and replaced the old one. I cleaned the fan and attached it to the heat sink then I put a dab of thermal compound on the heath sink and attached the board to the heat sink with a screw. I used a metal screw to attach the
7293 to the heat sink, I really don't know if this the correct way to do it. I do not have a photos or diagrams of the power board installation , so I did this trying to figure out where goes what.

The problem is when I powerd On, the new chip started smoking and the amp fuse blew up.

I purchased a new power board, but I am afraid to test it before I know that it won't blow again. My question is, what would be the cause for the chip to blow on first place?

I don't know if the way I installed the power board and the heat sink is correct. So if anybody could post a photo it will be appreciated.

I will apreciate very much any suggestions on where to start.

[52 Bill]

...I put a dab of thermal compound on the heath sink and attached the board to the heat sink with a screw. I used a metal screw to attach the 7293 to the heat sink,...

Did you use an isolation washer for the screw and a mica insulator for the chip? The chip must be electronically isolated from the metal heat sink.

If you don't have one, I'd suggest putting together a light bulb limiter for use while working on this amp. It may save you from additional chip damages.

[DonJoe]

Marshall MG100HDFX blown TD7293

Thakl you for you help. I did have a hard time finding mica isolators. The lightbulb limiter I am working on today. Do you think it would be safe to test the new board after I make the changes you suggested?

[52 Bill]

I suggest installing the new power output board/chip. Be sure that the chip is correctly isolated from the heat sink. Use heat sink grease on both sides of the mica insulator. Also be sure that you use a plastic insulator on the mounting screw. Check with your ohmmeter to be sure that there is no connection between the metal heat sink and chip.

Check the speaker and speaker wiring, as a short in either will cause problems. For the first test of the amp, leave the speaker disconnected.

Then power up the amp using the light bulb limiter. If the bulb lights up brightly, then you'll know that there is still a problem in the amp.

If it lights up very dimly or not at all, you're probably on the right track. Turn off the amp and let the power caps drain for a few minutes before hooking up the speaker for the next test.

[DonJoe]

Muchas gracias

Thank you Bill, you have been very helpfull. I'll follow your suggestions, when I'm done I'll post the results.