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Captain America PCB: No Audio?

djsheep

djsheep

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Hoping someone can point me in the right direction. Bought a minty looking Captain America PCB with no audio. It looks like the pots have been changed, but am unsure that's the issue as the seller I bought it from seemed to know his electronics.

If I touch the caps in the sound section, I do get the usual interference. JP1 is set to MONO... haven't tried STEREO from the external header though. Have seen some other people online with the same issue, so assuming it's a somewhat common problem on this board. Hoping someone can chime in?

cap2.JPG


cap1.JPG
 
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djsheep said:
If I touch the caps in the sound section, I do get the usual interference.
Click to expand...
that implies that the audio amp is working but something upstream feeding the amp is not.

Considering this board has both OKI and Yamaha sound CPUs my guess is that that fault is somewhere in the pre-amp mixing . Then again I've seen some PCB setups where a dead sound CPU prevents the other sound CPUs from running properly and killing sound completely.

I don't know enough about this board's specific audio setup to say.

if I had to guess it's one of the OKI or one of the Yamaha chips here that has failed.
 
Thanks for chiming in. I’ve seen multiple boards with no audio. I might try replacing those components in the sound area and see how I go…
 
if you're going to shotgun it I'd start with the YM3012 and the surrounding logic ICs as that's what's performing the digital to analog conversion before heading to the amp.

you could try a logic probe and/or piggybacking to see if that lends any clues before putting an iron to it.
 
I’m still very much learning how to repair. I have a logic probe but haven’t really learned how to use it. I will do some reading and see how it goes. Thanks again.
 
Typically the recommended tool for diagnosing audio problems is an Audio Probe.

AFAIK no one sells these, people just build their own. It's essentially just a small speaker with an audio amp and the input tied to the probe so you can "listen" to different pins in the circuit.
 
djsheep said:
It looks like the pots have been changed,
Click to expand...
I was going to say what a stupid idea they had at Data East installing pots requiring a screwdriver to adjust.

Anyway, I would first check sound CPU is running ('45' custom chip?), dump sound ROM and probe sound RAM.
Then audio probe the sound circuitry to hear where the sound disapears (if ever generated).
 
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