Fender Rumble 40 Power PCB capacitors blown

[Switchy]

So 2 capacitors are blown in my power PCB, I don't see anything else fried.
The power light was cycling on and off, and then they popped.

If I replace them, should that be the whole fix required?

EDIT: Does the power led cycling on and off signal anything specific?

 

[Jon Snell]

Not sure how long that piece of string is.
Maybe is about as close as I can guess without knowing what is in that SMPSU in the way of filtering.

 

[ZenGuitarist]

There must be a reason why the capacitors blew in the first place. The Rumble 40 v3 is a fairly new model so the age of it shouldn't be the problem.

If it was me I would (a) check to see if it's still under warranty and if so, have Fender fix it, or (b) determine what caused the capacitors to blow before replacing them.

 

[corliss1]

Fender doesn't fix those

If it 's in the warranty period they hand you a new amp and you're all set.

 

[Paul G.]

Fender doesn't fix those

If it 's in the warranty period they hand you a new amp and you're all set.

That tells you something, don't it?

 

[Switchy]

There must be a reason why the capacitors blew in the first place. The Rumble 40 v3 is a fairly new model so the age of it shouldn't be the problem.

If it was me I would (a) check to see if it's still under warranty and if so, have Fender fix it, or (b) determine what caused the capacitors to blow before replacing them.

I bought it 2nd hand, so I don't have the receipt.

 

[corliss1]

I'd replace the caps and see what happens. You need special tools to work on SMPS systems, and if that is the issue anyway you would just replace that whole power supply unit, and by that I mean just buy a new amp since it'll be cheaper.

Put in the caps and see what happens would be my plan.

 

[Switchy]

Not sure how long that piece of string is.
Maybe is about as close as I can guess without knowing what is in that SMPSU in the way of filtering.

There's a string on the handle, it's tied in knots, so it would be hard to get a measurement

But if you are interested in the PSU, here is the schematic:

 

[Lowerleftcoast]

I would think somehow the C400, C401 caps received too much power. D321 could be suspect along with a dropping resistor (maybe R335). (right upper corner of the SMPS PCB). The caps in that area may be suspect as well.

 

[Switchy]

I would think somehow the C400, C401 caps received too much power. D321 could be suspect along with a dropping resistor (maybe R335). (right upper corner of the SMPS PCB). The caps in that area may be suspect as well.

The Rumble 40 doesn't contain those components per the parts legend.

 

[Lowerleftcoast]

Looking again. Maybe D320.

 

[Switchy]

Looking again. Maybe D320.

I did a visual on the PSU board, and everything looks perfect. No heat marks on the resistors, and all the caps look perfect.

 

[Dismalhead]

I've got the same amp. Damn. I hope it doesn't happen to mine.

 

[Lowerleftcoast]

I did a visual on the PSU board, and everything looks perfect. No heat marks on the resistors, and all the caps look perfect.

The diodes clamp the voltage. Looks like D320 is supposed to clamp at 30V. If the voltage can get above the 35V capability of the caps they will blow.

Check your voltages. If the voltage is too high not only will caps fail. The transistors will fail as well.

 

[Switchy]

The diodes clamp the voltage. Looks like D320 is supposed to clamp at 30V. If the voltage can get above the 35V capability of the caps they will blow.

Check your voltages. If the voltage is too high not only will caps fail. The transistors will fail as well.

Understood

 

[Jon Snell]

According to the piece of string theory, if your decoupling (VSS/VDD) capacitors have failed, I would check/replace the two large electrolytics decoupling the VSS and VDD (C318/9) within the power supply as that could cause the smaller values in the power amplifier to overheat and fail as described.
They all should be 105℃ or 125℃ working for reliability.

 

[Dacious]

I would still approach Fender - even if out of warranty they may replace it.

 

[Switchy]

According to the piece of string theory, if your decoupling (VSS/VDD) capacitors have failed, I would check/replace the two large electrolytics decoupling the VSS and VDD (C318/9) within the power supply as that could cause the smaller values in the power amplifier to overheat and fail as described.
They all should be 105℃ or 125℃ working for reliability.

Yeah, I wish I had an ESR meter, I'm gonna take a close look, but mostly I'm going to chance it and switch out the 2 popped capacitors.

 

[Switchy]

According to the piece of string theory, if your decoupling (VSS/VDD) capacitors have failed, I would check/replace the two large electrolytics decoupling the VSS and VDD (C318/9) within the power supply as that could cause the smaller values in the power amplifier to overheat and fail as described.
They all should be 105℃ or 125℃ working for reliability.

Considering just ordering replacement filter caps. Do you know if faulty filter capacitors would cause the PSU to cycle on and off?

 

[Jon Snell]

If the decoupling capacitors had gone high impedance then the opto-isolater that is used to regulate the output will see a high peak (no smoothing) and shut off then back on again with the drive when the voltage drops.
I think that has all the hallmarks of your issue and cure.