Scytzoh

My punch amp is blowing fuses as soon as power goes on, no speakers plugged in.. A short maybe?? Could it be a FET and not smell or show it is blown? If so I've herd lots of contradicting statements on how to test them

kevmurray

Sounds like the output transistor(s) are bad. Use an ohm meter to check resistance from drain to source out of circuit. To find which one has failed you will have to remove them from the amp. I would just change them all anyway.

Scytzoh

ok sounds good ....which are the source and drain?? what is the meter supposed to read?? and ive herd it is not reliable to test while they are on the board....if so that sucks

kevmurray

With the amp disconnected from power you can check them while still in the amp. However, this will only tell you if any one of them is shorted since many are paralleled. To find the bad one(s) you would have to take each one out and check it. This is not really necessary though, since once you find the bad group you should just change them all. There should be more than a few thousand ohms between the center pin and the right most pin when looking at the label, pins pointing down. I'll post a photo if you like. What is the model number?

Scytzoh

P325.1 is the model how much are the FET's ? I guess I should take them out and bring them to the store. I've herd to ask for them all to be from the same lot because of slight differences

kevmurray

I'd guess $3-4 each.

For a switching (class D) amp it doesn't really matter how closely matched they are but its still best to change them all after a failure. In the pic attched, looking at the fets on the back wall, the pins are from left to right: Gate, Drain, Source. Resistance from drain to source should be very high. If you're in doubt measure between all the pins and post what you find.