Ampeg PF350 dead PSU?

[Yetanothertweaker]

I have and Ampeg PF350. and it seems it´s power supply has died. The ´Fault´ light and blue front panel light both blink when power is on, and there are only pulses of + and - 15V supplying the op amps in synch with the blinking.

I must say this switch mode power supply is a bit foreign to me, and would greatly appreciate some assistance in the unit´s repair.

[52 Bill]

Welcome to the place.

I have and Ampeg PF350. and it seems it´s power supply has died.

Maybe. The good thing about switch mode power supplies is that they are designed to protect themselves when there is a problem detected in the amplifier circuit.

So your problem could be a bad power supply, but it is more likely that there is a plain old problem in the audio circuits causing the power supply to go into protect mode.

[Yetanothertweaker]

Thanks Bill! Unfortunately there does not seem to be anyway to isolate the PSU from the amp itself, everything is build onto the one PCB, so I guess I´m in for a long night. Removing the power fets seems to be a good place to start....

[Jazz P Bass]

PF350 schematic: Ampeg PF350.zip

[Yetanothertweaker]

Thanks!

[Enzo]

You don't have to remove the outputs to see if they are shorted or not, just check them in place, if they seem shorted, THEN pull them.

Also, check for shorted rectifiers on the secondary side of the SMPS.

[Mick Bailey]

SMPS startup cycling is usually either excessive load or a failed component on the secondary side. As well as shorted rectifiers, shorted caps are also a possibility. Sometimes these will bulge or ooze.

Bear in mind the safety issues of working on an SMPS. The primary side is all live and presents a high shock risk, hence most people use an isolation transformer when troubleshooting.

[Yetanothertweaker]

Good idea, appreciated