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.

 

[GibLP]

Hi All,

Bumping this back up again. Well I've had some more time to debug my Color Classic Mystic (575 + VGA Mod).

1. Audio works perfectly through the speakers and headphone jack, but there is a high pitched background hum/noise present through speakers.

2. The background noise/hum goes away if I put foam on the underside of the 575 board where there are clips so it prevents touching the case shell metal shielding. BUT - this makes the audio cut in volume and sound staticy when playing sounds (both in the speaker and headphone jack).

This excludes the Analog Board circuit as it's present in both speakers and headphones. This makes me think it's a grounding/shielding issue, rather than the audio circuit on the LB. Anyone have any ideas, this is very frustrating!

Thanks in advance!

 

[nathall]

There are two ground planes (at least) on the CC. One is for the sound, the other is for “everything else.” I’m surprised you get any sound at all with the ground clips not touching the shielding, but then, you also have a 575 logic board not a CC or CCII logicboard. My CCII had intermittent sound that seemed completely random until I realized I’d accidentally dislodged the clip responsible for grounding the audio circuit when I re-installed the logic board last time and it was making intermittent contact with the shield.

My CCII is stock so I don’t have first-hand experience with all the mods you have, but it sounds (no pun intended) like there is something sharing the audio ground plane that shouldn’t and introducing noise.

 

[GibLP]

There are two ground planes (at least) on the CC. One is for the sound, the other is for “everything else.” I’m surprised you get any sound at all with the ground clips not touching the shielding, but then, you also have a 575 logic board not a CC or CCII logicboard. My CCII had intermittent sound that seemed completely random until I realized I’d accidentally dislodged the clip responsible for grounding the audio circuit when I re-installed the logic board last time and it was making intermittent contact with the shield.

My CCII is stock so I don’t have first-hand experience with all the mods you have, but it sounds (no pun intended) like there is something sharing the audio ground plane that shouldn’t and introducing noise.

Thanks @nathall for the reply - Apologies for the late response, I missed the notification in my email. Will poke around a bit more. Thanks

 

[djc6]

Your post reminded me of this video - I forgot the details but it was some weird ground issue I think:

 

[nathall]

Yeah, that video basically goes into a bit more detail of the same thing I was describing in my post above.