Have you tried taking out the VRAM expansion chips? I have a few ViRGE cards that didn't work correctly until i removed them.

When the card is installed slightly twist the card, maybe cause of the problem is a broken trace or a solder joint.

I would also suggest checking traces between pin 1 on the VGA connector and pin 168 on the ViRGE/DX chip with a multimeter, including ferrite beads (marked as FBx on the PCB). In your case, check if FB1 and the resistor placed at Q1 are 0 Ohm and R33 (the load resistor) is measuring as 75 Ohm - failure of any of those might be the cause of missing red color. Be sure to also check the resistance between VGA pin 1 and ground - should be 75 Ohm as well 😀

Thank you all for your hints. Since I don't have my test bed assembled at the moment I went ahead with the suggested measurements and voilà, we have a likely culprit. Traces separately were OK for R, G, B; resistors were OK according to their nominal values, ferrite beads were OK for G, B, but NOK for red channel (infinite resistance)! Also resistance test against ground failed for red, OK for green and blue with around 75 Ohm. Looks like this ferrite bead for red went bad somehow. Is it advisable to short it out as a fix, or should I find some SMD replacement?

elcrys wrote on 2023-09-08, 18:30:

Looks like this ferrite bead for red went bad somehow. Is it advisable to short it out as a fix, or should I find some SMD replacement?

yeah, you can short ferrite beads out; in fact, this (along with RGB capacitors to ground removed) usually fixes image blurring at higher resolutions on cards with bad RGB filters, but for a testing, it's best to only touch the ferrite beads. Just make sure that all three R/G/B lines have them shorted to have the consistent image across all three channels 😉

I found a replacement ferrite bead from other faulty card and after a bit of soldering the graphics card is fixed. It turned out the bead was cracked in half (probably due to mechanical stress during VGA cable connecting). Red is back and I wasn't able to detect any other problems with the picture, it's nice and sharp. Again, thank you for your help.