I'm trying to repair a small Behringer CE500A active speaker. Upon switch on, the speaker made a loud humming sound, followed by a pop and smoke.

Attached are photos showing the damage. It seems one of the 47µF capacitors exploded and the audio Amp. IC also has a short?

What would have caused this?

 

The wrong supply voltage?
The wrong supply voltage type (AC/DC)?
Incorrect supply polarity?
Supply voltage connected to the wrong place?
It is old or has been stored unused for a long time - capacitor died of old age?

 

The wrong supply voltage?
The wrong supply voltage type (AC/DC)?
Incorrect supply polarity?
Supply voltage connected to the wrong place?
It is old or has been stored unused for a long time - capacitor died of old age?

…Counterfeit capacitor? In any case, it's a good idea to replace all electrolytics.

https://en.m.wikipedia.org/wiki/Capacitor_plague

 

The wrong supply voltage?
The wrong supply voltage type (AC/DC)?
Incorrect supply polarity?
Supply voltage connected to the wrong place?
It is old or has been stored unused for a long time - capacitor died of old age?

It was left for some time before being powered up. It has the original toroidal transformer connected via a polarised board connector.

…Counterfeit capacitor? In any case, it's a good idea to replace all electrolytics.

https://en.m.wikipedia.org/wiki/Capacitor_plague

I think the speakers were manufactured in 2010 so after the capacitor plague. They aren't bulging but I will remove them and test the capacitance and ESR.

What would cause the burn marks on the Amp. IC pins?

 

It was left for some time before being powered up. It has the original toroidal transformer connected via a polarised board connector.

Shorted rectifier diode?

 

The catastrophic failure of the capacitor could have been the initial point of failure or a symptom of a power supply failure that delivered excess voltage or unrectified AC to it.

The integrated amplifier IC failure was probably secondary to whatever primary failure occurred and could even have been caused by a bit of metal from the capacitor being flung into it's pins.

It would be a good idea to test any diodes, bridge rectifiers or separate power supplies before repowering the unit.