Hi, I'm having a strange issue with this SMPS from a mixer that suffered an overvoltage. I've searched these forums and encountered useful threads, but I still can't figure out what's wrong with this thing.
The SMPS in question is from a Peavey PV 20 mixer, which is about 2 years old. Apparently it worked fine until it suffered an overvoltage a few months ago, started working erratically and then stopped completely. This is what I did:
- Initial check, 1.5 A fuse was blown. Replaced fuse and plugged the mixer again with no load, and fuse blew again;
- Traced a short circuit in the TOP244YN switch, so I ordered replacements. Replaced it with a new one, turned the SMPS on with no load... And it popped the switch almost immediately, without even blowing the 1 A fuse! Fortunately I had ordered more...!
- Replaced the 68 uF filter capacitor on the primary side, and 4x 470 uF capacitors on the secondary which were slightly bulged and with better voltage tolerances (25 V to 35 V)
- All the other electrolytic capacitors, resistors, diodes, all seem to check out. De-soldered most of those to test evertyhing on an open circuit.
At this point, the switch doesn't blow anymore, it just fails to start, ticking away every second or so. With every tick, the filter capacitor voltage drops from 340 VDC to 320 V, and then charges back up again. The outputs, with no load, oscillate between 22 and 25 V on the +-25V outputs. With the mixer connected, the LEDs blink for a fraction of a second every time the SMPS ticks.
I can't identify the source of the problem. I replaced the TOP 244 yet again to make sure it worked, and it behaves the same. I'm inclined to look at the feedback circuit but I have no idea how to measure it, I only know that there is feedback, when the switch ticks, there's a signal going through the optocoupler. The secondary doesn't seem to be shorting, because all the outputs are measuring "something" (+25V, GND, GND, -25V, 48V).
Pictures of the board tonight.
I'm running out of things to de-solder.