Stumped on RF Punch 100 DSM amp - help

[Perry Babin]

Only half of those are the rail voltage. This amp uses all N-channel outputs so the positive rail is on the center leg of half of the outputs and the negative rail is on the third leg of the other half of the outputs. The output stage seems normal with only 8mv of offset.

[m109ner]

damn, just started popping fuses - 15, 20, 30 amp fuses. ????/

[Perry Babin]

Don't use anything larger than a 10 amp fuse when testing this amp.

[m109ner]

OK. BUt wouldn't a 10amp fuse blow if the others blew?

[m109ner]

No blown fuses until i started testing pins 8 & 12 of the LM339D chip. hmmmm???

[Perry Babin]

You need to find the fault, not use larger fuses. You likely have a shorted output transistor. After you find the fault and repair it, use a 10 amp fuse.

I'm assuming that you didn't turn the bias pots.

[Perry Babin]

The preamp module is the board behind the RCA jacks.

[m109ner]

nope. they've been the same as when I opened the amp. Each are at about 10 o"clock position clockwise.

On the amp, how do i test which one is bad with a MM?

[Perry Babin]

Read the basic amp repair page (link in sig file).

[m109ner]

Originally Posted by Perry Babin

The preamp module is the board behind the RCA jacks.

That preamp board has 2 chips LM833 with 8 pins. i measured both Pins 4 (-13.5V) Pins 8 (14.00V) on both chips before it starting blowing fuses. Darn! gotta find that bad output transistor.