Ampeg SVT 350H

[LeifB]

Hi everyone. i am new here.

i know my way inside and out of valve amps, but transistor amps are still somewhat of a mystery.

i have an ill SVT350H. Power amp isn't working.

FX loop is ok, preamp out is ok, it's when you get to power amp input that the signal goes to valhalla.

i know very little about transistors, i wonder if anyone knows of any issues with these amps, or if there's a 'money-shot' test point that will tell me something.

The 60V supply is stable, but something might be shorted, as all three terminals of the power transistors (if that's what you call them in this array) have the same potential... Something tells me this is not how trannies work!

Any help would be appreciated.

[Jazz P Bass]

Ampeg SVT 350H

Follow this link for the schematics:
http://music-electronics-forum.com/t13334/
There are test points that you should follow.
Did you test the speaker?

[LeifB]

have the schematic, followed, found faults, but with PCBs, i find it's a long drawn out affair trying to find faulty components.

[Jazz P Bass]

Ampeg SVT 4

The output section is MosFet transistors.
Watch how you test them.
They do not check the same as a BJT.

[LeifB]

Yeah, i'm afraid you'll have to dumb it down a little bit more. (i'll Google that first)

Only REALLY starting to read and understand transistors now.

[Jazz P Bass]

Ampeg SVT350H

The ouput MosFets cannot be at the same potential.
In or out of the circuit.
I would desolder & remove them & check the circuit board Drain, Source, Gate points where they came out of.
Out of circuit Drain & Source should read a high resistance when the MosFet is off. Reads pretty low when on. (Google "Testing A MosFet").