Marshall MG100HDFX Blown Fuse

[DonJoe]

Dear Sirs:

Recently I bought a Marshall MG100HDFX. Previously the amp had problems with the TD7293 ship. I bought a new board from Marshall and replaced the old one. After doing the repair, I connected the amp to a home made cab that has four Celestion Rocket 50 8 ohms speakers wired in series paralell for a total of 8 ohms load. When I turned the amp on It blew the fuse. After that I replaced the fuse and connected the amp to a single 8 ohms speaker. This time I get no sound. When I test the amp with the headphones it gives me a good signal on both channels, but I get no sound from the speaker.
Was the load was too big for the amp? If it was, what could be the damage? Where should I start looking?

I'll appreciate very much any sugestions you may have.

[nickb]

This amp is OK down to 4 ohms so the load impedance isn't the issue. If you have a signal at the headphone jack and not on the speaker then you most likely have a power amp problem. Disconnect the speaker. Inspect the TDA7293 chip for signs of heat. Check PA power rails between pins 14 and chassis and 11 and chassis for around 45V ish. Check the voltage between pins 1 and 2 of the power amp connector (not the chip) for zero. Check the voltate across the speaker jack for zero.

[nickb]

Also, check the insulating washer under the chip. It has to be isolated from the chassis.