Just a guess...
From Diamond Speedstar VGA ISA video card, quick test here, I see jp7 has a jumper.
If that jumper enabled or disabled the bios, that could prohibit the card from being detected by your computers bios and thus result in the one long, three short, gfx ram problem beep code.

edit: after looking at the th99 info, it might just be the dipswitches on the back of the card, setting for 8/16bit bios.

Push down on the three large socketed chips to make sure they're firmly in there, a problem I've had before

JP7 is the "alternate timing jumper"

AFAIK, it MUST have a jumper either in position 1-2 or 2-3 - on your board it's missing completely

I set a jumper to 1-2 and the system stops beeping at power on but there is no video or BIOS post beep. Setting the jumper to 2-3 causes the same one-long three-short error beep.

I reseated both the VGA BIOS chips and every other chip on the board for good measure and the result is the same: no beeps but no video or post.

I also tried setting the switch block as indicated in the manual to see if it would boot with alternate settings.

The socketed quad pack chip gets a little warm. Is that normal?

There is also a crystal that is weirdly soldered. It's not flush with the board and has a small bit of foam underneath that appears to be from the factory.

Also notice the chips hiding underneath the very large socketed chips. Very clever!

Simple things first: try cleaning the contacts on the bus connector.

Be on the lookout for lifted pins on the ET4000 chip. I've had to reflow the pins on various ISA VGA cards not long ago.
Also, are any of the little rectangular TTL chips getting abnormally hot (some such as the 74F245 and 74F244 chips can get a bit warm, but I'm talking like bare-chip-486DX33 like hot)? Or, conversely, are any TTLs (that aren't CMOS) not warming up at all despite running for a while?

The only chip that gets warm but not alarmingly so is the socketed quad pack. Everything else is room temperature or otherwise unremarkable.

Kahenraz wrote on 2021-08-02, 23:26:

There is also a crystal that is weirdly soldered. It's not flush with the board and has a small bit of foam underneath that appears to be from the factory.

That's pretty common, they put that separator to protect the component from heat during the wave soldering process. Crystals are very heat sensitive, it's not a problem when soldering it by hand but when you get a wave of hot solder on the back of the PCB for a few seconds everything gets really hot.

Regarding your problem, I agree with the previous post. Clean the contacts thoroughly and check all the legs on the main chip one by one in case one of them is loose or not properly soldered to the pad.

I examined this card closely today under a microscope and found that all of the glass capacitors had cracked! I've never seen anything like this before but apparently it is a thing. There is even a report about it that was written by NASA.

All of these capacitors are rated for 50V though. That all is very high. I wonder what could have caused them to crack.

Those look like plain old ceramic capacitors (mlcc) in a glas housing. wth?

mechanical stress seems doubtful.
thermal stress there shouldn't have been, though maybe left out in a cold garage and brought in to a hot house?
ultrasonic induced stress ?

that's just weird, but could definitly be a cause for non-functioning/malfunctioning, since most of them seems to be for decoupling .
I guess they ran out of exploding tantalums for this board 😉

This is silly. The glass is partially filled with an epoxy resin. They only appear cracked as an optical illusion where the ceramic capacitor is not fully encapsulated.

Mystery solved! Sort of. The problem ended up being the motherboard.

I swapped the problematic board that had a VIA VT82C692 and replaced it with an Intel 443EX. The card works perfectly now.

Both boards are from the same era and I used the same components with each motherboard, including the power supply. Maybe the VIA doesn't like pairing with an ISA video card. This is the only one I have so I can't confirm.

Another surprise was how slow... and how fast this card can be at the same time. Doom is extremely laggy but Quake is fast and smooth (35fps) at 320x200. I would think that Quake would be even slower than Doom but this was not the case.