Adam Audio Sub 7

The Sub 7 is a nice, small recording studio subwoofer. But its power supply is hot garbage, and is prone to blowing up for no reason. Adding insult to injury, the PCB is full of a hard, black gunk that engulfs many components and is a pain to remove. I have repaired at least 5 of these units with similar issues with its power supply. The most common symptom of a PSU issue is that the sub doesn't turn on, or if it does, it turns off when mildly turning up the volume.

WARNING: Switch-Mode Power Supplies employ dangerous, potentially deadly high voltages in their primary side, and their capacitors may hold a dangerous charge even when the device is not plugged in. Do not attempt this repair if you're not familiar with the necessary safety precautions, and even so, proceed with extreme caution.

Suspect components

 * D6 and D8: These secondary output dual Schottky diodes are the prime suspects. They've blown in all the units I've repaired. Check for shorts with a multimeter. Model number is F12C20C. I've not been able to find this exact part, but a suitable replacement with even better specs is the STPS20200CFP from ST, which is available at Mouser and Digikey.
 * D5 and D9: One or both of these diodes from the primary Snubber network might be blown. Check with multimeter for shorts. Note that these may be covered in black gunk. D5 is FR107 and D9 is 1.5KE200A.
 * R4: This 15Ω SMD resistor is in series with the auxiliary winding of the transformer, which provides power to the main switching IC. This one tends to fail open (check with multimeter). This in turn causes the PSU to turn on, but then to fail when presented with a very mild load, as the switching chip can't get enough current from the primary dropper resistor (R19).
 * U5: This is the main switching chip, model KA5Q1565RF. I've had to replace this chip only once, and it was very evidently burnt and showed cracks in the package.