Kustom Sienna 30 output IC issues

[Mickey]

Hi Guys,

Have a Kustom Sienna 30 on the bench which has a faulty output IC (TDA7296), replaced this and as I wound up the variac one of the HT filter capacitors exploded in a cloud of smoke and horrible stink.
Now I have replaced numerous faulty electros over the years but usually they are of the higher voltage variety, typically with a working voltage of 300V DC or higher, but the ones in this custom are rated at only 35V DC.
My question is are these type of filter capacitors prone to internal shorts which I guess caused this electro to explode or is there something I'm missing. I've checked the diodes and they measure OK but I will replace them as a precautionary procedure.
Your input will be appreciated.
Mike from Australia

[The Dude]

There can be other reasons caps explode, but first on the list is over-voltage. You might want to monitor the voltage across the cap as you bring up the variac next time and make sure the caps rated voltage is not exceeded.

[Enzo]

Over voltage or reverse voltage.

[Mickey]

Over voltage or reverse voltage.

Thanks guys. The voltage from the centre tapped transformer is per specs, the faulty caps were original now replaced with higher voltage types of the same capacitance, the diodes measured OK but I wonder if they were letting AC pass when connected in series with those caps and under load? Maybe another "Chinese Junk" issue. Have you guys experienced these type of cap blow outs with low voltage caps?
Cheers and thanks again for your input. Mike from Australia

[Enzo]

I have occasionally had a cap can shoot past my ear as it erupted. In fact that has always been low voltage caps. High voltage caps for me have usually failed open. But ANY cap can explode. Don't waste mental energy trying to find a pattern.

When I say reverse voltage, I am more afraid a cap has been soldered in backwards or a diode same, than I am some part developed a weird arcane failure mode. As the wise man once said, when you see hoof prints in the dirt, think horses, not zebras.

[Mickey]

Thanks again guys! After fitting the caps and new diodes and checking the power transformer for leakage I slowly powered up the Kustom, what a relief everything appeared to be OK. The voltages were as on the circuit diagram and after adding an earth strap to all the pots the amp is working as designed.
Now to my next repair, a Laney LA65 which also has a blown output IC, hope the power supply filter caps don't explode !!!!

[J M Fahey]

Cool, but please confirm exploding cap was actually upside down/reverse.

It is not so useful to know that it now works but what was the actual problem.

Otherwise we learn nothing from this.

[Mickey]

Hi, no cap was original, never touched or replaced and as I noted earlier I believe it was just "Chinese Junk"! The diodes were all good and as the caps are at the beginning (before regulation etc) there was/is nothing that could affect their performance. Mike