> bridging +5V and -12V
I don't think so. These capacitors are GND relative. If you find both -12V and +5V shorted to GND then probably two capacitors are shorted.
Did you unsolder the capacitor to test? If not, it can still be anything else on the card that is shorting as most components are connected to 5V and GND in parallel.
You can test the card with such capacitor removed. The value of this particular one is 10µF/16V.

TL431 from another card will be fine. You can also often find them in switching power supplies near the optocoupler. On the VGA it is used as current source for the video DAC.

h-a-l-9000 wrote:

> bridging +5V and -12V I don't think so. These capacitors are GND relative. If you find both -12V and +5V shorted to GND then p […]
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> bridging +5V and -12V
I don't think so. These capacitors are GND relative. If you find both -12V and +5V shorted to GND then probably two capacitors are shorted.
Did you unsolder the capacitor to test? If not, it can still be anything else on the card that is shorting as most components are connected to 5V and GND in parallel.
You can test the card with such capacitor removed. The value of this particular one is 10µF/16V.

TL431 from another card will be fine. You can also often find them in switching power supplies near the optocoupler. On the VGA it is used as current source for the video DAC.

I have not desolderd the capacitor yet. Can I replace it with an electrolytic capacitor with the same values and if so does it need to be low ESR?

You are correct -12 is only shorted to ground and not to +5v, I just rechecked. From B07 to B01 (or B10 also GND and where the cap goes) my multimeter shows 0.5 ohm and measured over this resistor (still on the card) it also shows 0.5 ohm while B01 to B03 shows no connection. I will desolder it now to make sure it's this one.

I must have confused measuring resistance between B07 (-12V) and B01 (GND) vs B03 (+5V). I only measured on the fingers quickly to see if there indeed were any shorts at all then I moved on to checking the caps so with luck it's only the cap and no other shorts.

Success, no more short between -12V and GND! 😀

And the cap is still shorted when removed from the board.

This was a bit hard to do with one hand for the camera!

This was extremly hard to do with one hand for the camera, some tape to hold the cap in place was needed! The multimater read 0.5 to 0.6 ohm when i held it securly with both hands but 0.6 - 0.7 ohm with one unsteady hand.

Electrolytic is no problem (aside from aesthetics).

Secret hint: GND is on the slot cover - easier to stick the prober there.

I though I had a broken OTI037C with both a non shorted 10µF/16V tantalum capacitor and the correct TL431C shunt regulator. As a bonus the middle lead on the TL431C looked long enough to reach solder pad "3" on the 8bit OCTEK UM587F. I have tested this card in 3 systems with 3 different screens with the same result, the screens shows that they get a signal but the screens remains black with the systems booting as normal.

This is the "broken" OTI037C card, the sought after components are in the upper right corner.

I have two other very similar OTI037C cards and they both work fine with the LCD screens I have tested but I thought I should test this card with a CRT just to make sure as all 3 systems I tested the card in uses LCDs (including the XT clone). I put the card in the XT clone and conneted an old VGA screen I dug out from the mess and sure enough the stupid card works fine, just not with LCDs.

Why did you have to work?

I guess I will have to order a new TL431C shunt regulator when I order capacitors from Mouser or ELFA. I dont want to desolder parts from this card now that I know it works as it looks almost new but this means it will take a while before I can fix the 8bit OCTEK UM587F. I did find a PSU in my scrap pile with bloated caps and a TL431 but it's soldered very snug against the board and the middle lead will not be long enough to reach around to solder pad "3".

I will continue to seach my scrap piles too see if I find any other donor hardware.

h-a-l-9000 wrote:

Electrolytic is no problem (aside from aesthetics).

Secret hint: GND is on the slot cover - easier to stick the prober there.

Good! I have plenty of small electrolytic caps if I should find a suitable TL431 but not a 10µF/16V tantalum capacitor.

Thanks for the help! 😀

It almost sounds like the card only worked in the XT machine because the ISA bus would be running at 4.77Mhz, and not something faster like 6Mhz or 8Mhz (or more...). This card quite simply could just be not working due to the faster bus speeds on your other motherboard...

orinoko wrote:

It almost sounds like the card only worked in the XT machine because the ISA bus would be running at 4.77Mhz, and not something faster like 6Mhz or 8Mhz (or more...). This card quite simply could just be not working due to the faster bus speeds on your other motherboard...

The XT clone ran at 10 MHz and this system was one of the three systems I have already tested this OTI037C card in, a 386 25 MHz and a Slot-1 system beeing the two others. This is one of the cards I got in the scrap lot I posted pictures of in the "Bought these (retro) hardware today" thread a week ago.

You are correct that it wasnt a LCD vs CRT screen thing though as I just retested the card with the LCD I have been using with the XT clone and the combination now works. Things I tried before to see if I could get the card going was to reseat the removable chips and cleaning the fingers but it diddnt seem to help. It seems the card just wanted some attention but scaring it with using it as spare parts was the correct procedure. This is not the first time things start working when I test them the "very last time" before salvaging useful components, it's usually 486 motherboards though.

Haha yeah this old stuff is always a little bit tempremental.

For example, my IBM PS/2 had a faulty floppy drive until I drove it from Adelaide to Melbourne (around 700Km) and it now just magically works...

Gotta love this stuff. It all has such a quirky character to it.

orinoko wrote:

Haha yeah this old stuff is always a little bit tempremental.

For example, my IBM PS/2 had a faulty floppy drive until I drove it from Adelaide to Melbourne (around 700Km) and it now just magically works...

Gotta love this stuff. It all has such a quirky character to it.

Well it's hard to complain when stuff magicly fixes it self. 😀

Yeah! Would like to know why it happened though so it doesn't happen again haha

Hi Skyscraper,

I have exactly the same vide card. Do you have the settings for the modes ?

Skyscraper wrote on 2016-04-27, 17:42:

Shorted tantalum capacitor..JPG
Now broken transistor.JPG