Missing resistor on Oak Tech 16-bit ISA VGA card, what should it be?

[GearTechWolf]

Chipset: OTI067, FCC ID: IDW87082025, board-number: Rev B 1087082025, S/N: 21M0502279, the missing resistor is R26, in-line between the oscillator pin on the ISA bus and U5/OTI068. (the first '8' in both the FCC ID and the board-number looks like a '6', but the FCC ID is invalid if it's input as a '6')
Oddly, it still works, but is a bit unstable. Thoughts on what to sub in?
(I'll add pictures of the card later, to help with identification of the specific card)

 

[mogwaay]

Is it like this card on VGA Museum:
https://www.vgamuseum.info/images/palcal/oak/273_1067082021_rev.c_top_hq.jpg (taken from https://www.vgamuseum.info/index.php/cpu/item/325-oak-oti-067)?

I'm wondering if it's just a 0 Ohm-ish, jumper-style resister if the Crystal is missing to provide a 14.318 MHz clock to the OTI-068 Clock generator. It could also be a small value, like 22 Ohm, to reduce reflections and improve signal integrity. I couldn't find a
I couldn't find a OTI-068 or OTI-068 datasheet, but I did find a OTI-087 'databook' which has the pinout of the OTI-068: http://www.bitsavers.org/components/oakTechnology/OTI-087_Feb1993.pdf (PDF page 67)

The reference schamtic doesn't even mention the OSC pin from the ISA slot and is just using the 14.318 MHz crystal input to the OTI-068 chip.

Looking at some of the OTI-068 cards on VGA Museum, many don't connect the OSC line, the ones that don't have 14.318MHz OSC on the card (they all see to have it, so they are also probably just including the OSC line as an option for the PCB?).

Does your card look like the resister was missing from the factory or do you think it had been removed?

 

[modem7]

Oddly, it still works, but is a bit unstable. Thoughts on what to sub in?

Does your card look like the resister was missing from the factory or do you think it had been removed?

Exactly. Maybe the card is not meant to have a resistor fitted (like the missing R5 on the XT-CF card at [here]).

 

[GearTechWolf]

Is it like this card on VGA Museum:
https://www.vgamuseum.info/images/palcal/oak/273_1067082021_rev.c_top_hq.jpg (taken from https://www.vgamuseum.info/index.php/cpu/item/325-oak-oti-067)?
Does your card look like the resister was missing from the factory or do you think it had been removed?

Yup, that's the card! Don't know if it was missing from the factory or not, as my younger self installed single-pin sockets in that location, but probably was!
Yeah, I've got a couple other similar, but not the same, Oak cards and neither of them have that pin connected, or even included, on the card-edge.
Guess when I get set up to test cards I'll find out why I marked it 'funky/works' in the past.
Might try disconnecting the crystal and installing a low-rating resistor to the ISA Oscillator pin just for giggles.
Thanks for the help, folks! Great find on that PDF, mogwaay!

 

[Eudimorphodon]

Guess when I get set up to test cards I'll find out why I marked it 'funky/works' in the past.

I bought a batch of five “infuriating“ Oak VGA cards from an eBay auction a few years ago, sold as such because the vendor got a huge pile of them surplus but they would never work consistently. And yeah, three out of five didn’t work when I got them and one of the others seemed flaky…

Cutting to the chase, the cards had socketed ROM and DAC chips. Reseating them firmly fixed every card. Maybe you’ve got similar issues.

 

[GearTechWolf]

A good possibility! I didn't know of such fixes way back when I last worked on/tested this card.
Got another that's missing the OTI-069 DIP-20 chip, gonna put some machine-pin headers in place of the missing socket as there's some trace damage from when younger me removed the old socket for unknown reasons. Then I'll just have to gently remove and socket said chip on another card so I can test the damaged one. No idea what became of the missing chip!

 

[mogwaay]

Yup, that's the card! Don't know if it was missing from the factory or not, as my younger self installed single-pin sockets in that location, but probably was!
Yeah, I've got a couple other similar, but not the same, Oak cards and neither of them have that pin connected, or even included, on the card-edge.
Guess when I get set up to test cards I'll find out why I marked it 'funky/works' in the past.
Might try disconnecting the crystal and installing a low-rating resistor to the ISA Oscillator pin just for giggles.
Thanks for the help, folks! Great find on that PDF, mogwaay!

No worries, I've fixed up a couple of old VGA/EGA cards and quite enjoy looking through the nice pics on VGA Museum to match up boards and try reverse engineer bit on them

I bought a batch of five “infuriating“ Oak VGA cards from an eBay auction a few years ago, sold as such because the vendor got a huge pile of them surplus but they would never work consistently. And yeah, three out of five didn’t work when I got them and one of the others seemed flaky…

Cutting to the chase, the cards had socketed ROM and DAC chips. Reseating them firmly fixed every card. Maybe you’ve got similar issues.

I've heard of other people having 'unstable' cards and the DAC was the issue - my money would be on that. From previously looking at other boards on VGA Museum there are often serval compatible DAC chips, so you might find another compatible one if you can't source the OAK one - see what other DAC chips you can see on VGA Museum - the Schamatic I found might have the pinout for it too which you can then match up with other candidate DAC chips.

Exactly. Maybe the card is not meant to have a resistor fitted (like the missing R5 on the XT-CF card at [here]).

Unrelated but 'big thanks' to modem7 as I had incorperated this very board - Serge's XT-CF-Lite V4 - into my integrated 8088 PC homebrew and I hadn't noticed that R5 was un-populated! It's been working fine anyway with it in, but I might try and remove it as I have no pull-ups on the data lines and it might take a little load off the bus drivers... so cheers!