AC problem with LM3876

[bacon665]

I simply didnt have time to rip the chassis apart and remove the cards so i decided to do some other expiraments.

i had some spare panasonic 150uf 63 volt caps I decided to place one as an output cap on the right channel (the one with the dc) to my surprise a volt meter shows it isnt blocking it. theres still a now average of -5.3Vdc. I did the same to the left channel in hopes the noise might be small dc pulses. if anything it was a wee bit louder with the output cap.

 

[Rainwulf]

Ok, unless the cap is passing dc thats nearly impossible, but it will depend on how the voltmeter responds to an ac with dc superimposed onto it.

Your meter is showing 5.3 vdc between the output and Earth.
Through a cap.

What the hell..

Ok, check the earth between the speaker output and the zero volt connections of your psu.

 

[AndrewT]

The DC blocking cap needs a ground reference resistor.
Use 100k meantime. If this is omitted the leakage through the cap and the high impedance of the meter will give a real DC offset voltage that you are measuring.
100k won't reduce the leakage offset to zero but it will be much closer.

 

[bacon665]

so this refrence resistor just needs to go between the binding terminals right?

 

[bacon665]

I attached a crudely drawn image.(forgive me drawn at school in about 2 minutes)

i didnt have 100k if i do i dont know where it is so i used a 200k refrence resistor
a 10 ohm wirewound resistor with a bypass cap was used as a dummy load.
i flipped the amp on and grabbed the negative probe for my meter looked up and within those 3 seconds there was a giant ball of smoke coming from the resistor.
meter says the resistor is still good and i dont see any external charring so im guessing its still good. any more suggestions?

 

[AndrewT]

if that 10r smoked it could only be AC voltage heating it.
I suspect your amp is oscillating.

That power power passing through to the amp tells me you have not used the mains bulb tester while still proving that the amp is working properly.

 

[bacon665]

mains bulb tester?

theres a neon lamp on the mains for supression... other then that i dont know what your speaking of.