Hey guys,
Bought one of these little amplifiers to play around with about a year ago, while building and tweaking it I accidentally shorted some of the contacts on a metal plate and it bit the dust. I recently pulled it back out to repair it, I got it back up and running and have signal flowing through it again. Only problem is now i have a nasty 120hz hum coming from the output.

Below are the schematics and data sheet. I have tested past the Op-amp. It seems like everything is clean all the way up to the STA540. The inputs to the 540 show a clean wave but when I hook up to any of the outputs I see the noise. The noise is very dominant and is not altered by the volume pots in any way.

One thing im not sure is correct is I have a 7.5 volt reading on the signal out. Im thinking there is a short between output and vs. I've worked with the diagnostics pin and it seems to come back clean.

Any ideas on what to check or look out for?


540 Data Sheet
https://www.sparkfun.com/datasheets/Components/General/STA540.pdf
Schematic
https://www.sparkfun.com/datasheets/Kits/STA540 Audio Amp v11.pdf

 

If there is DC and a strong mains hum coming out the STA chip it has a shorted driver. You need a new STA chip.

Exactly what were the first symptoms, and exactly what did you do to fix it?

 

The first symptoms delt with the rest of the board. Couple blown out tracers and a bad LED.

On power up vcc 1&2 are shorted with all outputs along with signal and power ground.
That would prove the shorted driver theory?
Thanks for the response!

 

If a + or - power rail is shorted to the amp IC output, yes the amp chip is shorted. That is a common failure mode.

As a further test you can remove the chip, or disconnect the output pin, and test if it is still shorted (ie short is definitely inside the chip) or if the short has vanished (ie short was elsewhere on the PCB).

 

Yep. Removed from PCB and still shorted. Ordered another one just now. At least they are fairly cheap. Thanks for the help!

 

Remember to never short the speaker wires, or disconnect the speaker wires when powered.

That means your speaker wiring needs to be properly attached and secured, no loose connections etc.