Ampeg PF350 dead PSU?

[Yetanothertweaker]

I have and Ampeg PF350. and it seems itīs power supply has died. The īFaultī 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 unitīs repair.

Schematic here:

AMPEG PF-350 SCH Service Manual free download, schematics, eeprom, repair info for electronics

[mikeydee77]

I had one of these in recently that would occasionally flash and bang. Turned out screw threads and burrs from the cabinet were lying on the pcb. Also the little slot at the front is great place for young fingers to slide coins in.

So just wondering if you have checked for physical defects?

[J M Fahey]

Well, it IS a very complex switching power supply, and not for the faint of heart.
Only a few specialized Techs will deal with them; most will not (me included) , even otherwise quite experienced ones, let alone a regular Musician.

Just to suggest something to do:
a) those are 2 semi independent supplies, each with its own controllers, switchers, and low voltage rectification, *maybe* the lowest voltage one (+/-15V) has or sees a problem which causes it to overload and turn off while the higher voltage one (+/-55V) works fine, so check that.

b) IF the +/-15V supply is on a separate board and joined to preamp with some connector/ribbon cable you might try unplugging the preamp.
If now you have +/- 15V *at the supply end* it means some short on the preamp is loading it down, you must find it.
If you have +/-55V but +/- 15V still self shits off, then the supply itself is damages and must be troubleshooted .... and so on and on.

There's not a "generic" answer such as "oh, check C14, it always shorts, replace it and all will be fine" but a complex chain of tests and decisions based on what you find along the path.

[jmaf]

I've been staring at the schematic and I can't find the fault LED. What alien aircraft is this from?

[Jazz P Bass]

The "Fault" LED is on the Preamp schematic.

Coordinates A-4.