An annoying misadventure with generic OPTi 82C924 ISA sound card

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An annoying misadventure with generic OPTi 82C924 ISA sound card

Postby Eep386 » 2018-2-08 @ 03:24

I recently received a rather generic OPTi 82C924-based card from an eBay sale. After swapping the TL084 op-amp for a TL074 (yeah I know, it's practically the same thing, but the '074 is spec'ed for somewhat lower noise...) it worked and sounded pretty good (and the low-pass cut-off for the FM is set to 41 kHz, which is a bit lower than 49.7 KHz but not really a problem), but then I discovered that its onboard IDE channel was conflicting with my DTC EIDE Ultima adapter (to which my CD-ROM drive was connected), even when I disabled the IDE channel in the DOS driver. No matter how it was set, it would cause the CD-ROM drive to detect with a random mojibake string.

Unable to keep it from messing with the EIDE Ultima, I pulled the LS245s for the IDE channel, and cut the /IRQ_15 trace. That permanently disabled the sound card's IDE port and fixed the conflicts, no more of the CD-ROM drive coming up as gibberish under VIDE-CDD.SYS.

But this wasn't the end of my headaches. As I was playing with it, I suddenly heard it pop and crackle when it was just sitting idle. As time went on, it started making random sounds under Windows MIDI, and began messing up the music really badly on Titus the Fox: To Marrakech and Back.

Thinking it was the bus caps going marginal, I recapped the card. Not surprisingly that made no difference at all, and I wound up breaking a via as the caps I used for the output had leads that were just a little too big in diameter. Fortunately, I was able to patch the damage with a length of wire, but still...

After fussing for a while with it, I couldn't figure out what the issue was, so I pulled the parts that I felt were worth reusing (YMF262, YAC512, caps, etc.) and was about to send it to the recycler, when I realized something: Yamaha YMF262 chips have a little bit of DRAM inside them, so when that DRAM fails...!

So I put everything (minus the IDE port LS245s) back on it, but this time swapped the YMF262 FM chip with a known good one taken from a Diamond K2Y-PRO16 (OPTi 929-based) sound card, and...

No more pops, crackles or broken sounds. FM music is perfect, even after letting it run through several games. Sound Blaster Pro support is pretty good too (though I don't care much for how OPTi cards sometimes play DAC samples at a lower pitch than a Creative card).

So now I have a rather nice OPTi 82C924 card that plays well with my system. Compared to the OPTi 82C929 card I was using, its drivers play nicer with Windows 3.1 (it no longer hangs on shutdown) so it proves to be more useful to me in the end. Later, I'll try to get me a replacement YMF262-M chip for the K2Y-PRO16 I scavenged from. (One of those badly compromised OEM CT2800 ViBRA cards that come without the TEA2025 would make an optimal donor, so long as I find one broken for cheap...)
My synthesizer hierarchy: Novachord > DX-7 > YM2151 > YM3812 > CT1747/YMF262/YMF289 > CQM > PC Speaker > AudioPCI "FM" (retch)
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Re: An annoying misadventure with generic OPTi 82C924 ISA sound card

Postby CkRtech » 2018-2-08 @ 08:24

Oh man. What a headache! Aaaand I laughed out loud at the choke hold you put on D06/ IRQ 15.

Thanks for sharing the story, and I hope you next adventure is a little easier...

... so how is your CQM <-> YMF switch proceeding, btw? :wink:
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Re: An annoying misadventure with generic OPTi 82C924 ISA sound card

Postby Jo22 » 2018-2-08 @ 10:05

Oh my. That must have been frustating for sure!

I'm glad you finally found the issue and were able to fix it, though. :)

Also thanks a lot for the details. I wasn't aware the YM262 could suffer from such a damage.
Now that we know about the culprit, there's hope for other faulty cards.
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Re: An annoying misadventure with generic OPTi 82C924 ISA sound card

Postby Eep386 » 2018-2-08 @ 21:43

CkRtech wrote:... so how is your CQM <-> YMF switch proceeding, btw? :wink:


Thanks! As for the CQM<>YMF switch, it's on hold for now, until I find me a CT3600. Since that's the most common AWE card out there from the looks of things, methinks it'd be worth my while to try to OPL3-ize it so I can post instructions on how to do so.

Conceptually the CQM/OPL3 switch would be simple: all I'd have to do, is cut the /CS line feeding both the CT1978 and YMF289/YMF262 pads, wire a SPDT switch between the (-) side of /CS and the (+) /CS inputs of both chips. We still probably won't be able to switch FM chips mid-tone, as the internal register 'state' will not be shared between the chips, but we should be theoretically be able to switch them while nothing's playing.

One pitfall I have to consider though, is the effect of wiring the outputs of both the CT1978's TDA1387 and the YMF289's YAC516 together. I'm wondering if I shouldn't decouple the outputs of both chips using 0.1uF ceramic SMD capacitors first. I _could_ add a DPDT switch to select which chip outputs, but then I'd just be introducing antennas with all the wires I'd be adding.
My synthesizer hierarchy: Novachord > DX-7 > YM2151 > YM3812 > CT1747/YMF262/YMF289 > CQM > PC Speaker > AudioPCI "FM" (retch)
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