A500 sound issues, op-amp problem?

[unbrain]

Having a problem with my A500 (rev 6a) where sound is extremely quiet and badly distorted. Some poking around with a meter in the audio circuit shows that the vcc+ pin of the TL084CN is receiving +12 volts. The vcc- pin, however, is not receiving +0.84 volts.

Does anyone one know what voltage should be showing at the vcc- pin?

 

[demolition]

Vcc- should be close to -12V as you can see from this schematic:

Missing -12V would give the issues with the audio that you describe. My guess is that -12V is missing from your PSU, although it could still be a fault with either U14 or R302.

 

[unbrain]

Vcc- should be close to -12V as you can see from this schematic:
View attachment 127620
Missing -12V would give the issues with the audio that you describe. My guess is that -12V is missing from your PSU, although it could still be a fault with either U14 or R302.

thanks very much for your rapid response
ive checked the -12v line from the PSU and it is definitely giving -12v, so I assume the problem must be elsewhere stopping it getting the correct voltage.
very useful information you’ve provided. Now I actually know where to start looking

 

[demolition]

Here's the full schematic in case you need more:
http://www.amigawiki.de/dnl/schematics/A500_R6.pdf

 

[unbrain]

Awesome! Thanks a ton

 

[unbrain]

Here's the full schematic in case you need more:
http://www.amigawiki.de/dnl/schematics/A500_R6.pdf

Further checks show that although the PicoPSU is showing -12v with no load just checking the outputs, under load it’s definitely not supplying -12v. Guess I’m sending that back to the supplier!

- - - Updated - - -

That’s that problem solved. Now I’m only getting two out of the four channels of audio. The volume and distortion issue fixed tho.