Troubleshooting M-Audio BX8 powered monitor

[dissonance]

Just a quick update on this one: I went to check for poor thermal contact, and found that the screw holding the chip amp in question was broken in the heatsink! Visually, it looked fine; it wasn't until I put a screwdriver to it that I realized that it was snapped. I pulled the heatsink and will be drilling out the broken (self-tapping) screw and threading the sink for a proper machine screw in the next day or so. I'll then clean up the surfaces and apply some fresh silver thermal grease. Still not sure if this is cause or effect, however. It seems plausible that the screw may have snapped due to thermal stress, when I was cooling the chip during testing. I'll give an update later in the week.

[J M Fahey]

Fine, excellent !!!!
Just one little detail: if you drill it out, the hole will be too large for any threading.
1) Try to pull the self tapping screw with an extractor; you'll have to drill a small hole in its soft center and the extractor's reverse thread will bite into it. *or*
2) Drill it out of the heatsink, but use a mregular bolt/machine screw plus nut and, if needed, washers *or*
3) Grind the broken screw flush so as to have a smooth surface and drill a new hole just by it, maybe the new IC leg's length allows it to be mounted a little skewed *or*
4) grind flush as before, seat the Power Chip in the usual position, but press it against the heatsink with a small iron or thick aluminum bar, held by 2 screws in its ends, respectively to right and left of the chip body, as some power Mosfets are mounted in real big amps.

[dissonance]

Finally following up on this thread. I was able to drill out the broken screw and get the chip securely fastened with a machine screw (I actually did this to both chips while I was at it). I also cleaned the surfaces thoroughly and applied fresh thermal paste. Problem resolved! I burned the monitor in for a few hours at a fairly high volume with no further problems, and it's back in my studio seeing daily use. Yet another case for not overlooking the obvious.

Thanks to everyone for the suggestions and guidance! This was very educational for me, and I really appreciate all the assistance.