During a routine customer WOPL upgrade on a PL400, I noticed two of the Nylon shouldered washers would not fit into the backplane of the amplifier, after I had chamfered all the holes and flattened the back and heatsinks with my calibrated flat bastard file. This is a well-documented problem and can be easily remedied with my cobalt, Letter "A" (.234"), drill bit (from www.CMLsupply.com).
After resizing the holes, the Nylon shouldered washers dropped right into place. I proceeded to continue with my assembly. When it came time to power up the amplifier on the DBT / Variac combination, things started to go downhill. First the DBT went very bright. So I backed off and started troubleshooting. Finding nothing wrong, I powered up again and blew the B- rail fuse. I pulled the Backplane Board fuses for the Left Channel, and promptly blew the B- rail fuse again. So I pulled the Backplane Board fuses for the Right Channel and the B- fuse held.
The problem was on the Right Channel Backplane but the Control Board test points were out of spec as well. What to do? The wiring checked fine and resistance checking for shorts with the power off showed no shorts. So I powered up the amp and applied a test signal to the Right Channel. The output showed distortion, so I increased the input signal to take a better look at the distortion (THD? IM? Crossover?). And then I heard a loud pop! The B- rail fuse blew and my distortion disappeared. I did not see a flash, so I wondered where an arc like that could have happened? Are you with me so far?
I pulled the Driver Transistors (bottom row) because they were the only outputs that were populated, and this is what I found:
I had forgotten to chamfer the holes I had drilled out / resized and there was a barely perceptible burr on the edge of both holes.
I had suffered the dreaded Sil-Pad cut-through. I studied the burnt Sil-Pad and the other one that was cut but had not burned through, and I filed the burnt ridge from the transistor case, and reinstalled it with a fresh Sil-Pad.
When I powered up the amplifier everything was fine. I installed the remaining outputs and the amp sounded great. I will never forget to de-burr the holes I re-size from now on.

After resizing the holes, the Nylon shouldered washers dropped right into place. I proceeded to continue with my assembly. When it came time to power up the amplifier on the DBT / Variac combination, things started to go downhill. First the DBT went very bright. So I backed off and started troubleshooting. Finding nothing wrong, I powered up again and blew the B- rail fuse. I pulled the Backplane Board fuses for the Left Channel, and promptly blew the B- rail fuse again. So I pulled the Backplane Board fuses for the Right Channel and the B- fuse held.
The problem was on the Right Channel Backplane but the Control Board test points were out of spec as well. What to do? The wiring checked fine and resistance checking for shorts with the power off showed no shorts. So I powered up the amp and applied a test signal to the Right Channel. The output showed distortion, so I increased the input signal to take a better look at the distortion (THD? IM? Crossover?). And then I heard a loud pop! The B- rail fuse blew and my distortion disappeared. I did not see a flash, so I wondered where an arc like that could have happened? Are you with me so far?
I pulled the Driver Transistors (bottom row) because they were the only outputs that were populated, and this is what I found:

I had forgotten to chamfer the holes I had drilled out / resized and there was a barely perceptible burr on the edge of both holes.
I had suffered the dreaded Sil-Pad cut-through. I studied the burnt Sil-Pad and the other one that was cut but had not burned through, and I filed the burnt ridge from the transistor case, and reinstalled it with a fresh Sil-Pad.
When I powered up the amplifier everything was fine. I installed the remaining outputs and the amp sounded great. I will never forget to de-burr the holes I re-size from now on.
Last edited: