Precision Power PC650

I have a PC650 that has developed slight turn on thump and a more significant turn off thump on all channels that I need some help solving.

The current configuration on the test bench has one pair of speakers attached to the rear set of outputs and a single woofer attached to the subwoofer outputs in bridged mode. I have the input board removed, but it makes no difference in or out and gain controls do not make any differences.

The turn on thump is about equal timing on the three speakers. The turn off thump happens twice and isn't always equal on all three speakers. DC offset measurements on each channel can be observed at startup and shutdown. While the amp is operating the DC offset on each channel is at acceptable levels.

This amp does not use the ceramic driver boards and it also uses 7915/7815 as regulators. It does have the 4n25 optocoupler in it, but I don't measure anything wrong there.

Any tips are appreciated.
 
The gate leg is connected to the secondary ground. The thump happens just prior to seeing the drain climb to ~ .150V and ~ -13.90V on the source legs.

It is thumping before muting is released or right when it being released.
 
Yes, the thump is heard just before voltage is seen on the gate and the drain yes.

My mistake, it is the source that is on the secondary ground, the data sheet I was looking at was for 2sj108.

I measure .9mV between primary ground and secondary ground.
 
Noise is on all channels just before G-S voltage on each j108 yes.

Checking each channel one at a time, all channels are able to drive a 4 ohm dummy load to clipping at the same point.

To check all channels simultaneously to clipping, I will have to do that later, but I didn't have any perceived issues in that regard when it was in the car.
 
Do you see any oscillation on the 'bridging' speaker terminals as the amp powers up/down?

If you ground the scope to the secondary ground (close to the muting transistor) and probe the drain of the muting transistor, do you see a pulse that corresponds to the noise you're hearing?

Do you hear the same noise bridged, or not?
 
No oscillations on any of the bridging terminals powering up or down.

The noise is in front of any scope changes at all the j108's drains.

Yes, it is present bridged and unbridged. Actually louder bridged.

I see the dc offset at the bridging terminals at the same time as the noise.
 
I have not, measuring them in circuit didn't indicate anything unusual to me, they all measured between 6 and 7 ohms source to drain. Plus, I thought it unusual for all six to fail.

When I pull one, it measures 6.7 ohms both directions drain to source.
With red probe on gate, 460K ohms gate to either drain or source, open to either drain or source with black on gate.
 
That looks normal. You might want to check them in the future on diode-check as well from gate to source. They should read as a diode would.

At this point, I don't know what else to check. I'll post if I think of something.

How loud is the noise? Very clearly audible from the seating position in a vehicle or only audible with your ear on the speaker?