First post, by vstrakh
I have a Bioteq MB-1333 motherboard.
It's am386dx-40 based ISA mobo, built on a Bioteq-rebranded UMC 82c491 chipset.
It's working ok and all, up until I try some games with pcm samples being played. It looks like audio samples gets garbled when the screen is intensively updated.
There is definitely some dependency between the amount of pixels being updated and the chances to get garbage in audio.
The most prominent example would be the Pinball Fantasies, it shows all intro screens (quite static) ok while playing intro music, but when entering the menu (lots of new images drawn with animation) not only I get audio damaged, I see garbage written on the screen, mostly in the areas that were actively updated at this time. The impression I get is that somehow dma transfers and writes into video memory are clashing. I've tried different combinations of audio and video cards, in all cases observing this issue. So this has to be the motherboard fault.
Disabling the PCM audio allows the game to run flawlessly. The video writes alone without background DMA transfers shows no issues.
Decreasing the ISA clock frequency in bios does not fix the problem, so it's not about cards not being able to react to signals in time.
Have you guys seen anything similar to this?
Since it's definitely related to the amount of data being pushed on the bus, I do hope it's just some bad bypass caps and/or worn out latch/buffer ICs, and not the chipset.
But I'd listen for similar symptoms in your troubleshooting experience.