One of my little chip amps has failed!

G'day all, this evening one of my little DIY chip amps based on an LM1875T stereo amplifier chip suddenly failed without warning whilst listening to a radio program.

The .5 amp mains fuse blew and a new one resulted in 'sizzles' from the power supply area. I have a spare DIY amplifier of the same type that I'd previously built which I've put into service.

I've had a quick look at the faulty unit and all wiring looks normal and intact but one thing I found which is odd is that one of the solder connections on the legs of one of the power supply filter capacitors seems to have lost all solder! I'm sure that it had been properly soldered. Did it vaporise or melt? Regards, Felix.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
I doubt it. A (good) soldered joint would take all the current a chip amp PSU could throw at it.

Probably the chip has failed either as a result of some problem or on its own. As long as the bridge is OK and the chip is good there's little else left really.

Use a bulb tester (and no speakers) for powering up after repair.
 

Mihkus

Member
2012-07-22 8:01 pm
Yep bridge rectifier failure is common.

Once i said to a guy who had blown amp and strong mains noise in the output
"Your bridge is shorted"
And noone belived me, after few days he came back after very long testing and measuring "You were right"

But only one side was blown half bridge per rail...
 
G'day all, just an update. It's fixed and working well again. The fault turned out to be one faulty LM1875T and I replaced both for the sake of uniformity, one bridge rectifier (one diode was shorted) and one leaky electrolytic filter capacitor.

The originals were working every day since about 2005 so that's a pretty good lifetime. I don't know what failed first but here's hoping for another good few years of normal operation. Thanks to all here for their help with fault finding on this. Regards, Felix.