PPI PC2300.2

[RiLoWa]

When I got this amp it had three of the IRFZ44N's shorted out and two of the three outputs 2n6491's shorted. One of the three outputs tested OK. Work I have done so far is replace the three FETS with IRFZ44PBF's, kept the one original output in place for further testing and I replaced one of the 10 Ohm differential input resistors that was out of tolerance.

My problem is when the amp is off it draws excessive current and the transformer squeals. I have a 2 Ohm resistor & 5A fuse in line with the B+. I took the resistor out of the circuit and it immediately blew the 5A fuse without trying to turn the amp on. When 12V is applied to turn the amp on, it does so and plays fine at ~.8A draw.

I have tried twisting the tranformer but it doesn't seem to be shorted. resistance between B+ and ground starts out at ~70 Ohms and climbs as the caps charge. Should it be so low and is this a sign the filter caps need replaced?

I have read elsewhere the SG3525 is usually suspect when the FETS short out, but since it turns on and plays fine what should I check next?

[Perry Babin]

Check the gate voltage on all of the power supply FETs when it's drawing current with no remote input. Do any go above about 1v?

Does checking the voltage on any FET (or group of FETs) make the current draw decrease?

[RiLoWa]

I have ~2V on the gates of the three FETS I replaced and ~0V on the other three. It draws ~2A and it makes no difference while checking the gates.

[Perry Babin]

Follow the circuit back. Does the power supply drive the FETs directly or does this amp use MPSA06/56 drivers?

[RiLoWa]

Pin 14 of the SG3525 drives the problem side gates.

[Perry Babin]

Assuming that you don't have any leaking FETs, it would appear that the 3525 is defective.

[RiLoWa]

Today I received my new outputs and a new SG3525 and have installed them. I still have the same problem. Without remote lead, speakers and RCA's connected, the amp will draw ~.715A and the protect light is on (red) as well as there is a slight squeal on the transformer.

With everything connected and turned on it draws ~1.250A. It plays clean audio. I'm not detecting anything getting hot, but when I turn the volume up a bit, it seems to strain the transformer and at that point it draws ~1.350A.

Thus far I have not removed and re-checked the new FETS I installed prior to original posting.

[RiLoWa]

Amp is behaving much better now that I replaced the other bank of three FETS which were in fact leaking. Measuring the gate voltage on those three were originally under a volt, but tonight after replacing the 3525 I was getting ~8V on Pin 11 and a completely different signal than what appeared on Pin 14. Now both Pins 11 & 14 are at ~4.35V with a nice square wave. All six original FETS were damaged and my assumption is the original 3525 was probably damaged also.

I'll stress it tomorrow and see if it is good to go, thank you again Perry.

[RiLoWa]

For those following the thread, it is good to go.