Fender Performer 1000 popping on turn-off

[rf7]

This guy put a hardware store plug on his AC cable and ended up with 120Vac on the chassis, because the green earth wire came loose inside and ended up shorted to the line voltage. He plugged into the effects loop and toasted a bunch of stuff. I have fixed all of that stuff, but now it's got a nasty pop on turn-off.

I've shorted the FX return line to ground, but I still get the popping. The 16V rails are not popping, but I do see the 40V rails popping on the scope.

Any ideas?

[Enzo]

Any loud noise will probably appear on the 40v rails on many amps.

If you had loose/misplaced mains in there, look CLOSELY at the copper traces on the board. We are looking for one probably associated with common/ground, specifically at a narrow point burning open like a fuse. Can be hard to spot.

[rf7]

I checked all of the traces on the PCB and I could not find any bad areas. I also beeped out the ground traces all over the board and everything checked out OK.

The amp does a low pitched thump at power on and at power off (which I think is normal), but after the low thump at power off, it does a loud higher pitched snat, which is what I'm chasing.

[Jazz P Bass]

I would replace the + & - 40 volt power caps.

[rf7]

I would replace the + & - 40 volt power caps.

OK, I thought about that. I'll try it. Thanks!!

[km6xz]

The problem with turn-off thump and decaying oscillation is usually caused by the rails of a push-pull amplifier fading at different rates. One side might be pulling more current or the filter cap on the faster fading side is lower then normal capacitance. Check it by monitoring both rails with a dual trace scope and see how they decay when the power is turned off. That would give clues as to where to look.

[John G]

Hi,
If my memory serves me right I do remember having to slug the +16volt rail with an additional 2,200uF cap to make the low volts rails decay at the same rate.
Additionally I remember having to put one (maybe two) resistor/cap snubbers across the mains switch.
Was a while ago now but maybe worth a try.
Cheers,
John G

[rf7]

The problem with turn-off thump and decaying oscillation is usually caused by the rails of a push-pull amplifier fading at different rates. One side might be pulling more current or the filter cap on the faster fading side is lower then normal capacitance. Check it by monitoring both rails with a dual trace scope and see how they decay when the power is turned off. That would give clues as to where to look.

The negative rails are falling much slower than the positive rails. I replaced the main Filter caps, they were 20 years old, but it did not change the popping.

Hi,
If my memory serves me right I do remember having to slug the +16volt rail with an additional 2,200uF cap to make the low volts rails decay at the same rate.
Additionally I remember having to put one (maybe two) resistor/cap snubbers across the mains switch.
Was a while ago now but maybe worth a try.
Cheers,
John G

I put a 2200uf cap across the +16V rail and it dropped the 2nd loud snat down by quite a bit. I was trying a snubber cap across the power switch, but that only helped the turn on pop, not the turn off pop.

[km6xz]

If the charge is equal held in the filters is equal than the rate of discharge difference is based on current being unbalanced. Find out why one rail is pulling a lot more current.

[rf7]

What is the best way to go about doing this? Basically, the 16V rails are powering opamps and a few foot pedal switching transistors. Just pull opamps one at a time?

[rf7]

I just put AC snubber cap and resistors across the power switch and that fixed it. Thanks John G!