Hey guys,
I've got a single M-Audio BX8 Studio monitor that I'm repairing that has given up the magic smoke. The other one in the pair is still fine.
There is no schematic for it, but I have managed to find a similar schematic (attached). Its identical apart from the opamps and the voltage regulation transistors.
It is basically a plate amplifier with two amps in one, one for the woofer (TDA7294) and one for the tweeter (TDA2052) on one pcb.
I'm tearing my hair out as I can't seem to find the issue.
1. Its initial symptoms were as following:
When turned on it would be completely silent, but there would be noise if you turned up the volume.
2. After basic troubleshooting, I determined that the transformer is delivering its promised voltage, the bridge rectifier is still ok, filter caps were ok and most of the components look ok. I checked the pcb tracks for heating and only saw the D667 and B647 transistors had slight heating discoloration on the pcb.
3. I first thought it was the main chips - TDA7294 and TDA2052 as well as the D667 and B647 transistors that were bust. So replaced them.
Now the monitor makes a thump on the subwoofer and a high pitched whine when turned on and makes my computer monitor display wobble even though its an lcd. I immediately turned it off.
4. Next I ended up replacing the filter caps anyway, and replaced the bridge rectifier as well. I also replaced all diodes on the pcb.
I checked all components for the precise replacement.
5. It still makes the thump and high pitched whine.
6. I replaced the opamps on the board making sure I get the exact model.
7. Still the thump and whine. Does not output anything.
8. It ended up killing my Focusrite Scarlett 2i4 interface (won't turn on anymore) I think it leaked something back to the TS jacks and killed the USB host controller or perhaps the DSP too. There's a burnt smell inside it, but hopefully its only a surfacemount diode/cap/opamp that popped(lets hope) I've tested the monitors on another audio source and they do the same thing, so it isn't the interface. Getting the interface fixed in the meantime.
9. I've tested each track to make sure there are no breaks, tested each passive component for values, all seem fine. What can the issue be?
Is there anything I'm missing. What should I check next? How would you have troubleshooted it? What did I do wrong?
Please help.