audiobahn a2300hct has me stumped

[ppia600]

I got this amp and every power supply transistor was fried. Also, the three rectifiers on the negative side were shorted as well. I thought maybe the outputs may have been bad so I checked.. none were shorted, on either channel. I checked the large capacitors after removing the bad rectifiers and power supply mosfets and none were shorted.
After replacing all the power supply mosfets and the negative rectifiers (the positive side are ok) I attempted to power the amp up. It lights up and then clamps down on the voltage and the negative rectifiers short out again. Just in case one was bad new, I replaced the rectifiers again, and after trying to power up, it still has the same issue. The power supply mosfets all read ok, none shorted, and all mosfet input lead resistors are reading ok. I can't find any shorted transistors on the audio side of the pcb, so I am stumped.
The rectifiers were UF16C20A and the power supply mosfets were IRFZ48V. I replaced them with SR1603A and IRFZ46 because I couldn't find them for sale through any of the places I normally buy, and the specs seemed to be the same.
If anyone has a clue, I'd appreciate the help. I don't have an oscilloscope or any other expensive test gear. Only a few decent DMMs and a couple of DC power supplies (8A and 40A) I know the amp isn't great, but I'm trying to get it back up and running for a guy who claims it just "smoked". He probably just ran it at too low of a load, but I'll have to check that out later.
Thanks

[1moreamp]

Power transformer is shorted from primary to secondary Maybe? Shorted capacitors maybe?

[ppia600]

Well the transformers both look fine, no discoloration, and the none of the caps read a short. Thing is, the rectifiers are after the transformers, so I would think something upstream like maybe something in the lower audio stages causing the outputs to clamp or maybe even the power supply voltage jumping up to high when powering up and killing the rectifiers. I'm wondering if using the irfz46 instead of the irfz48v is causing it, but the rectifiers were shorted when I got the amp. Hey, don't you sell amps? I recognize your screen name from ebay. You sell some very nice amps. They don't make them like that anymore.

[1moreamp]

Nawh the Z-46's are not the problem, But you must have a dead short somewhere around that single diode that is being taken out everytime.

try running the amp without that single diode and see what happens.

[ppia600]

The amp has six to220 rectifiers, three on the positive side, and three on the negative side. For each side, all three are paralleled, and when I first got the amp I believe at least two of the negative ones were fried. After swapping them, I ended up toasting only one each time. I will double check, but I believe they are paralleled and that is part of the reason I am baffled. Thanks for checking back.

[audiobahnkid592]

look at the inducer coil......I have a similair problem in my audiobahn amp because the coil had broken and fired.

[ppia600]

The amp has two coils. They both read .9 ohms across any lead on either side. They are also both isolated from the primary to secondary side, so I don't see any short problems. I had a problem with a ppi pc2350 I used to own where the coil was faulty, but all I had to do was move the winding and epoxy it because one had rubbed through from the vibration of the coil. After that it was aok. This amp is different, it has two coils, both read exactly the same and neither are shorted from the primary to secondary side. Maybe one of the caps could be shorting when the voltage ramps up, but while they are in the circuit they all read fine. I hope I don't have to remove all of them and test them individually, they are all glued to the board.

[Perry Babin]

You need to power it up through a current limiter. Some people use an automotive headlamp. I use 2 ohm resistors. When powered up through a limiter, you can check a few things that you can't check when the amp is drawing excessive current. Even with the current limiter, you need to have the components clamped to the heatsink so they don't overheat and fail.

With a current limiter in series with the B+ power line, measure the DC voltage across EACH of the emitter resistors. At idle, there should less than 1mv across each of the emitter resistors. If you find one or more with significant voltage (more than 1mv) across them, you know the general area of the problem.

[ppia600]

The emitters of the audio output transistors? I could try that I guess. So maybe one of them is acting up only when voltage is applied? Its amazing the amp's "protection" circuits don't detect this problem, especially when the rectifier diodes are rated at about 8 amps per leg.

[Perry Babin]

Yes, the emitter resistors on the output transistors.

If one channel (or one transistor) is passing too much current (due to a defect in the driver circuit or a defective transistor), it will show up as a voltage drop across one or more emitter resistors.