VOGONS

S3 83C805 with bad RAM - chip to address mapping, datasheet, ?

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First post, by philpem

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Has anyone repaired an S3 805 VLB VGA card with suspected bad RAM? I'd love to compare notes if you have!

I've got one on the bench which gives a blank screen (except for attributes) or corrupted text in text mode, but kinda-works in Speedsys's graphics mode (which I think is 640x480 16-colour?)

My card has eight RAMs on the board, plus the two RAM expansion sockets filled. I pulled the two expansion RAMs and it was still screwed up, so my thinking is it's one of the base RAM chips.
On my card they're all 256K, 4-bit wide, at 60ns, Hyundai HY534256ALJ-60 is the part number.

I could shotgun all eight chips, but that seems wasteful, and they're not cheap. I can't find any rated at 60ns, most are 70, 80 or 100.
I've tried to cross-reference it and found the NEC uPD424256LA-60, Samsung KM44C256, Panasonic MN41C4256 -- but the only sources I found for 60ns wanted more for eight chips than it'd cost to grab a new card on ebay.

I've been looking for the datasheet/databook but it's sadly not on Bitsavers or the usual places.
I've found a copy at the Stanford University library, but it seems to be in storage and is tagged as "not for public access" (https://searchworks.stanford.edu/view/14138923).

Thanks - Phil

Reply 1 of 3, by stamasd

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If you have at least one good equivalent chip, you could try chasing the bad chip on the board by the piggyback method. It has been explained multiple times on this site so a search should bring that up.

here's an example: Re: ISA XMS/EMS Memory Extension / Expansion cards: Now Running without Driver / Documentation :-)

I/O, I/O,
It's off to disk I go,
With a bit and a byte
And a read and a write,
I/O, I/O

Reply 2 of 3, by philpem

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stamasd wrote on 2024-10-01, 19:52:

If you have at least one good equivalent chip, you could try chasing the bad chip on the board by the piggyback method. It has been explained multiple times on this site so a search should bring that up.

here's an example: Re: ISA XMS/EMS Memory Extension / Expansion cards: Now Running without Driver / Documentation :-)

Unfortunately the chips are J-lead surface mount, so they can't easily be piggybacked. The equivalent chips are extremely expensive.

I figured that if it's affecting text modes then it's probably the low RAM which has issues, but it could equally be all of the RAM. I'm working on getting a copy of the 83C805 datasheet and figuring out the RAM mapping to try and identify the bad chip.

Reply 3 of 3, by Tiido

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You can momentarily short the data lines of different chips with each other and find out which pixel columns are closest to where problems are seen in graphics modes, that should help to narrow down which chip is giving the problem. It isn't polite to the chip though...

T-04YBSC, a new YMF71x based sound card & Official VOGONS thread about it
Newly made 4MB 60ns 30pin SIMMs ~
mida sa loed ? nagunii aru ei saa 😜

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