Since it was not preventing the computer from posting or giving any error codes, I believe the card and its BIOS are fully working... Could it possibly be the DAC chip?

Possibly. The RAMDAC is socketed, so if you have a card with another socketed RAMDAC it's trivial to do a swap.

DEAT wrote on 2025-03-05, 06:37:

Possibly. The RAMDAC is socketed, so if you have a card with another socketed RAMDAC it's trivial to do a swap.

The other is a VERY different DAC chip... and I don't wanna risk it. So I ordered a replacement DAC for few bucks on AliExpress. It should be here in few days and I'll report back...

Crap! Swapping for a new DAC didn't work...

By further testing the card, I noticed something: each time the resolution changes (ex: from DOS to Win 3.1 and the other way around) there seem to be a video signal of some sort since the monitor's power light stops blinking.

Do you have a digital multimeter that supports frequency mode? (or an oscilloscope?)

If so, what you can do is measure the frequency of data coming out from the Hsync, Vsync, Display Enable and Pixel Data pins on the Feature Connector. These pins go directly to the RAMDAC as well, so it could help estimate whether data is coming out with right frequencies from these pins, or if already the card is outputting incorrect video (or no video at all)

You can find the pinout of the Feature Connector at https://oummg.com/manual/#appendix_b_vesa_sta … rough_connector

When you boot the system to DOS (the 80x25 text prompt), the frequency rates that are present on the different lines should be:
- HSYNC: ~31.5 kHz
- VSYNC: ~70 Hz
- DISPEN: ~29.0 kHz
- PIXCLK: ~28.5 MHz
- pixel data lines DATA0 - DATA2: at least some hundred kHz (the DOS boot text mode will have some content with text color 7=gray). If the DOS boot is working, try pressing enter a couple of dozen times to fill the DOS boot prompt with the "C:\>" text on every scanline. That should increase the data rate here.
- pixel data lines DATA3 - DATA7: these will be constant zero in the DOS boot prompt, so no worth examining.

Make sure you have -12V present on the ISA connector. Without it the DAC will only output a black screen.

clb wrote on 2025-03-16, 09:12:

Do you have a digital multimeter that supports frequency mode? (or an oscilloscope?) […]
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Do you have a digital multimeter that supports frequency mode? (or an oscilloscope?)

If so, what you can do is measure the frequency of data coming out from the Hsync, Vsync, Display Enable and Pixel Data pins on the Feature Connector. These pins go directly to the RAMDAC as well, so it could help estimate whether data is coming out with right frequencies from these pins, or if already the card is outputting incorrect video (or no video at all)

You can find the pinout of the Feature Connector at https://oummg.com/manual/#appendix_b_vesa_sta … rough_connector

When you boot the system to DOS (the 80x25 text prompt), the frequency rates that are present on the different lines should be:
- HSYNC: ~31.5 kHz
- VSYNC: ~70 Hz
- DISPEN: ~29.0 kHz
- PIXCLK: ~28.5 MHz
- pixel data lines DATA0 - DATA2: at least some hundred kHz (the DOS boot text mode will have some content with text color 7=gray). If the DOS boot is working, try pressing enter a couple of dozen times to fill the DOS boot prompt with the "C:\>" text on every scanline. That should increase the data rate here.
- pixel data lines DATA3 - DATA7: these will be constant zero in the DOS boot prompt, so no worth examining.

I don't have an oscilloscope... But I about time I get one!

Thanks for the tips! I'll report back once I get my readings...

maxtherabbit wrote on 2025-03-16, 13:12:

Make sure you have -12V present on the ISA connector. Without it the DAC will only output a black screen.

I highly doubt the problem come from power input since my other TVGA 8900CL works on that same slot... Unless some designs of the same card are fundamentally different.

I got my hands on an oscilloscope (Fnirsi DSO 152)... I might need some time to fully understand how to get my readings right, but I trust I'll get there...

However, I noticed that my card has pins numbered this way:
...,5,4,3,2,1
...,17,16,15,14

Instead of the newer standard:
1,3,5,7,9,...
2,4,6,8,10,...

So it's not really clear to me if I should follow the pin NUMBER or the pin POSITION to probe for frequency?