Same outputs shorting each time

[Perry Babin]

Virtually all problems that I've seen with my (now retired) SC61 were bad connections on the wire connectors. I don't know if any were on the high voltage board.

 

[RJohns777]

I know this isn't the right place to discuss this. Mine is shorting the +42 VDC supply that feeds the high voltage transformer. Checked everything on the primary side. Isolated the secondary side. thought it might be the transformer. Replaced the transformer still has same problem. I guess I'll order the TL081 chip that controls the oscillation of the circuit. I won't comment anymore on this here. Thanks for the reply.

 

[RJohns777]

Just now getting back into this amp. Been battling equipment problems both test and shop AC. On a class D amp, this one, can the output filter choke and cap circuit be disconnected from the outputs without damaging the outputs? Just to see if it's causing the outputs to short.

 

[Perry Babin]

Do you have a working scope?

 

[RJohns777]

Yes. SC61

 

[Perry Babin]

Unless the FETs are failing immediately as the amp is powered up, you can look at the signal where the two inductors meet. It should be 1/2 of the rail-rail signal on the drains of the low-side FETs. While monitoring the signal, twist on both inductor to see if there is any significant change in the signal at the point where the inductors connect.

All FETs clamped when doing this.

 

[RJohns777]

Outputs are getting hot immediately. Doesn't take long to short an output. I've been through the driver board all is good with it. Pulled it and checked everything.

 

[RJohns777]

Went ahead and replaced the MPSA42 that are common to fail with W42s just to make sure. No over heating on the driver board.

 

[RJohns777]

Sorry, no evidence of cooking on the board. Left the leads long to get them closer to the fan.

 

[Perry Babin]

Do the FETs get hot that quickly when clamped tightly?

Are all of the output heating but only 2 (the same 2) constantly failing?

Have you desoldered the diodes at the outputs and carefully checked them as well as the resistors?

 

[RJohns777]

Okay, I must be dealing with a driver board problem. I have all the outputs out of the amp. On the positive rail voltage side I have a rail voltage of +89 volts on the source. +6 volts on the gate. On the negative side I have a rail voltage -89Volts on the source but -50 volts on the gate.

 

[Perry Babin]

Do you get the same spread when you measure directly across the gate and source?

 

[RJohns777]

Sorry, just got tour response. "Do the FETs get hot that quickly when clamped tightly?" Yes

"Are all of the output heating but only 2 (the same 2) constantly failing?" No, could be any of them. Originally repaired the amp. Tested. Customer brought it back with same two outputs shorted.

"Have you desoldered the diodes at the outputs and carefully checked them as well as the resistors?" No because any output can short now.

 

[RJohns777]

+ 82 on positive side. -50 on negative side.

 

[Perry Babin]

Black on source, red on gate? Recheck.

 

[RJohns777]

+6 Volts across the gate and speaker out side on positive outputs. +38 Volts across the gate and Source on negative outputs.

 

[RJohns777]

Black on source red on gate of positive outputs, +6 Volts, black on source red on gate negative side, +38 Volts. Center pin is drain. Correct?

 

[RJohns777]

This is what I'm getting on the gates of the high and low sides. No outputs installed.

 

[RJohns777]

By the way, I have a 100HZ signal into the amp. I know it's a pain in the rear but I do appreciate your help.

 

[Perry Babin]

Connecting a small capacitor (1000pf) across the high-side will clean up the waveform.

I need the low side with DC coupling at 20v/div.