Hello everyone!

I'm having an annoying issue with this old IBM computer I got recently.

It's an IBM PC 730-100 (Machine Type: 6875 with ISA/PCI).
Motherboard has an onboard audio card (windows sound system compatible, working fine) and an onboard 1MB video card (S3 Vision868) with slots for one more meg. It also came with another S3 video card (a Virge GX with 4megs) installed on the riser card (which can take 3 different ISA/PCI cards) and Windows 95 already installed. The original options were IBM DOS 6.3 and Windows 3.11 or OS/2 Warp 3.

There's no switch on the motherboard to enable/disable the onboard video card, as stated here:

https://www.ardent-tool.com/sandy55/PC750/PC750.html#Video

I upgraded the processor to 133mhz (maximum for this socket 5 motherboard) and installed the latest BIOS. Computer is working fine and Windows 95 detects the two different video cards, stating that the Vision868 one is disabled on hardware...

Sisoft Sandra and NSSI also detect both cards.

Then I decided to take out the pci video card and use the onboard Vision868 card instead, so i could use the extra PCI slot for an USB adapter card, maybe... I took out all cards from the riser card and restarted the computer. The IBM logo shows up, giving me the option to press F1 and enter the BIOS. If i do, or if i wait for it to start Windows, monitor screen goes black and there's a loud continuous beep... then nothing. Before restarting, i entered the BIOS and made sure all options were set to Default, especially the ones about IRQs, etc. Nothing happens, whatsoever! I had to reinsert the new PCI video card just to install DOS 6.22 but afterwards i get the same loud beep. There's also no error code showing...

Maybe the motherboard is damaged? Would be kinda weird if everything else was working except the video card? How can i test this? Any ideas?

Thank you so much for any help on this!