Color Classic Sound issue

[GibLP]

Hi All,

Last year I converted a Color Classic into a Mystic (575 + VGA Mod) and there is this high pitched squeal coming from the machine. I originally thought it was the flyback transformer, but it's actually the sound circuit. When I unplug the speaker or plug headphones in the noise goes away. (Noise still present in the headphones though). The 575 board I got was supposedly recapped already.

Where do I begin troubleshooting the sound circuit, on the logic board or analog board? Aside from the high pitched squeal in the background the rest of the sounds are fine.

Thanks in advance!

 

[pizzigri]

The Color Classic has the sound power amp on the AB, and the circuit is the same as in the 575, except in the CC it only has a mono output (the 575 is stereo).
it is very simple, a couple resistors, two caps and a single TDA7052B ic.
however, If the noise also comes from the rear headphones jack on the LB, the problem is most definitely in the 575 board.

 

[GibLP]

The Color Classic has the sound power amp on the AB, and the circuit is the same as in the 575, except in the CC it only has a mono output (the 575 is stereo).
it is very simple, a couple resistors, two caps and a single TDA7052B ic.
however, If the noise also comes from the rear headphones jack on the LB, the problem is most definitely in the 575 board.

Thank you, that makes sense to exclude the amplifier section on the AB. What would be the most promising culprit on the LB: resistors, capacitors associate with the sound circuit or the sound chip itself?

Thanks again

 

[pizzigri]

The audio chip on the 575 LB is U1, DFAC-II. Here’s a snippet of the audio circuit on that LB.

 

[pizzigri]

The lines labeled HPOUTL and HPOUTR (14) are the lines that go to the AB for amplification.

 

[GibLP]

The audio chip on the 575 LB is U1, DFAC-II. Here’s a snippet of the audio circuit on that LB.

Thanks, this is very helpful. I kind of wish the issue was with the AB as that only has a handful of components... Where would you start troubleshooting on this board? Replace the main caps? I've heard of removing the DFAC, cleaning around and re-soldering.

Thanks again.