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Weird KI2 conversion board audio issue

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Ioncannon

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Bought a conversion board (KI with KI2 adapter and drive) a while back that had "no sound" in the description. I assumed it was probably some pin that was lifted or some-such but loading it up on my Astro City I noticed weird crackling when things happened on screen. Then I turned up the volume both in the test mode and my Astro City and could faintly hear the audio was indeed working. It was just 1) Really quiet, 2) Crackling/Hissing/Popping a lot (though that may have been due to setting the volume to max).

Any idea where I should start looking first? Sounds like something with the amplifier (does the KI board even have one?) or sound circuitry?
 
ekorz said:
Worth checking the sound roms since they’re known to go bad on these. But if you’re getting some audio then yes I’d probe the amp and op-amp. Do you have an audio probe?

https://github.com/mamedev/mame/blo...343180113ef3ca/src/mame/drivers/kinst.cpp#L33

TL084 op amp https://protosupplies.com/wp-content/uploads/2019/07/TL084-Block-Diagram.jpg

TDA7240 amp https://circuitspedia.com/wp-content/uploads/2017/12/20-W-Audio-amplifier.jpg
Click to expand...
Thanks for the chip info. Hoping it's not the caps around the amp since most if not all are SMD. Gonna build myself a probe but I *think* I can use the LINE_OUT solder pad to test pre-amp audio. At least the same pad works on MK3 boards.

Got a replacement on the way just in case. I did see this Astro City was reading -4v on the rail, and hoped that was the issue. Recapped the board, replace the regulator at IC2, and got it back to -5v but still happens... so sounds like it is the KI2 circuit.
 
ekorz said:
Worth checking the sound roms since they’re known to go bad on these. But if you’re getting some audio then yes I’d probe the amp and op-amp. Do you have an audio probe?

https://github.com/mamedev/mame/blo...343180113ef3ca/src/mame/drivers/kinst.cpp#L33

TL084 op amp https://protosupplies.com/wp-content/uploads/2019/07/TL084-Block-Diagram.jpg

TDA7240 amp https://circuitspedia.com/wp-content/uploads/2017/12/20-W-Audio-amplifier.jpg
Click to expand...

Welp, replaced both amps and still no dice. Interestingly the weird buzzing feedback sound "resets" when I ground one of the caps surrounding the TL084. My ESR reader reads them as open circuit too but not sure if that is just the case with SMD caps.
 
C25 and C32. While sound is unamplified, the buzzing slowly comes in, getting loud and louder until it becomes a squeal/buzz. If I ground that cap it disappears and then starts ramping up again.

The squealing changes in pitch as the HDD loads.
 
CC8FE5AF-40ED-4995-80C9-A38DA436A897.jpeg

Those two caps are grounded where I highlighted in red. are those pads not grounded on your board? Maybe reflow that solder?
 
Ah it would be C25 only then. Hmm will double check if it still has a connection to ground.
 
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