With no 75ohm load and nothing on the monitor detect pins, it might have gone into monochrome synch on green mode. If there's nothing happening on green then yeah, got a weird fault.

BitWrangler wrote on 2024-03-18, 15:54:

With no 75ohm load and nothing on the monitor detect pins, it might have gone into monochrome synch on green mode. If there's nothing happening on green then yeah, got a weird fault.

I tested it with monitor connected and checked for sync signals with oscilloscope directly on GD5429 pins.

Just in case if it matters

• RESET goes off
• CLK looks ok (at least I clearly see 14.318 MHz sine)
• Some activity on video RAM address lines present continuously (I haven't checked all the lines yet)
• Some activity on VLB is present while POST checks memory and prints something to the screen
• 74F245's are pretty hot (45-50°C)
• GD5429 is slightly cooler (40-45°C)

I had a similare issue and I had a trace cut in a way I couldn't see it, only at an angle it showed.
VLB video card "out of range" error, won't display image // GD5428 (solved)

Does your screen issue some kind of error warning? Could you post a pic?
Also hi res pics of the card? front and back.

Nexxen wrote on 2024-03-18, 16:13:

I had a similare issue and I had a trace cut in a way I couldn't see it albeit at an angle.
Does your screen issue some kind of error warning? Could you post a pic?

No picture at all.
Monitor doesn't detect VGA signal as it have no HSYNC/VSYNC.
No error beeping at POST. PC boots as if VGA works ok but with nothing on the monitor obviously.

froller wrote on 2024-03-18, 16:17:

No picture at all. Monitor doesn't detect VGA signal as it have no HSYNC/VSYNC. No error beeping at POST. PC boots as if VGA wor […]
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Nexxen wrote on 2024-03-18, 16:13:

I had a similare issue and I had a trace cut in a way I couldn't see it albeit at an angle.
Does your screen issue some kind of error warning? Could you post a pic?

No picture at all.
Monitor doesn't detect VGA signal as it have no HSYNC/VSYNC.
No error beeping at POST. PC boots as if VGA works ok but with nothing on the monitor obviously.

Maybe one of the SN chips is dead? Can you trace the signal to see if H/Vsync go to one of those?

https://pdf1.alldatasheet.com/datasheet-pdf/v … /SN74F245N.html

Nexxen wrote on 2024-03-18, 16:21:

Maybe one of the SN chips is dead? Can you trace the signal to see if H/Vsync go to one of those?

H/V sync is analog signal and it goes to VGA connector almost directly (with some RC-chains)
SN74F245N's are the bus transceivers (tri-state bidirectional buffers) used to isolate chip from the bus to avoid bus conflicts. They certainly can affect the connectivity between VLBus and GD5429 in a highly disruptive manner. So they are suspects No.1

If one of SN74F245N is dead then BIOS can't initialize GD5429 and I wonder if it is required to show something at all i.e. black screen with correct sync signals.

In some modes GD5429 waits for high level on ESYNC pin to produce sync signals. And this pin looks floating.
It can be configured as input w/o pull-up or even not used at all. However it is routed to VESA Feature Connector along with EVIDEO.

make a boot floppy with VESA univbe.exe in autoexec with output redirected to >file.txt. this way you will at least see if the card is initialized completely and fully

rasz_pl wrote on 2024-03-18, 17:09:

make a boot floppy with VESA univbe.exe in autoexec with output redirected to >file.txt. this way you will at least see if the card is initialized completely and fully

this is a great idea

rasz_pl wrote on 2024-03-18, 17:09:

make a boot floppy with VESA univbe.exe in autoexec with output redirected to >file.txt. this way you will at least see if the card is initialized completely and fully

Assuming the output of univbe is redirectable, this a a good solution for the specific problem at hand. What I typically do to diagnose such kind of video card problems is connecting a null-modem cable to a serial port, and then using CTTY in autoexec.bat (or typing it blindly) to get the system use that serial port as "default I/O channel". Before running CTTY, the port settings should be initialized with MODE. If I remember correctly, the typical sequence is

1MODE COM1:,9600,n,8,1
2CTTY COM1

After that, you can interact with DOS programs that only access the screen via DOS (but not via the BIOS or direct screen memory access) from a terminal emulator program on another PC. For example, DOS's DEBUG works perfectly in this scenario to allow deeper inspection.

If you happen to have an MDA/Hercules (or clone) and a way to display the video output of that card, adding a secondary video card is another way to get access to a PC that boots but does not provide video output on the primary (S)VGA card.