Wow! What a roller coaster ride of a day...
I woke up early this morning to test the amp boards. Plugged them into the PSU, turned on the power, and all LEDs lit up. I thought I was golden... For about 2 seconds before there was a bright spark accompanied by a loud "POP" from one of the IXTP01s on one of the amp boards. The first pop was quickly followed by a second pop before I could yank the power cord from the wall. I was initially too embarrassed to post this in this forum: The sparks resulted in scorch marks on the IXTPs on the one board.
Spritzer confirmed to me that I should have used aluminum oxide isolators instead of just plain heatsink paste between those high-power mosfets and their heatsinks.
Fortunately, the damage was on only one of the boards. I created some space between the IXTPs and heatsinks and cleaned off the heatsink paste on the second board and plugged it in. The LEDs came on and stayed on with no sparking from the board, and everything was stable enough for me to adjust the balance and offset pots on the board. By the end of the day, I was able to hook up a phone as a source and my beat up pair of Stax 303s. The result was loud, distortion-free music from the one board. I screamed so loudly with joy, my wife got pissed off at me... Back in the dog house again.
Hopefully, the only damage on the first amp board is only in the scorched IXTP01s and nothing else... I have some replacement IXTPs along with proper aluminum oxide isolators on order from Mouser... Damn these long waiting spells for replacement parts. At least the first board is working.
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