VOGONS


First post, by 8bitbubsy

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I have one of these cards, it has an S3 805 chip. I think it's a perfectly fine card for DOS gaming on a DX2-66. It has an external RAMDAC that is placed close to the VGA output, so there is very little output noise. However, the RGB voltage levels seem to be out of spec (too high), giving an overexposed image on my TFT monitor. Grayscales were severely affected by this, resulting in crippled details. This is at least the case for my unit with an AT&T RAMDAC.

After tinkering around for a long time, I found out that replacing R7 (150 ohms) with 223 ohms makes it look about perfect!
I have attached an image showing the location of the R7 SMD resistor. The strange value of 223 comes from some experimentation with bodging two SMD resistors together. I assume you can find this value as one single SMD. If not, then go for 230 ohms (don't go below 223, and not above 230).

With that problem fixed, comes the poor DOS graphics performance... On DOOM, I get only 22.7FPS, whereas on my CL-GD5428 I get around 28FPS. One day I randomly thought of flashing VGA BIOS ROMs from other S3 805 cards, and I ended up finding 5 different dumps and I tested them all. One ROM in particular gave quite good results - 30.3FPS in DOOM! That's an increase of ~7.5 frames per second. But there's a catch - with this ROM I can't get most of the video modes to sync with the monitor in Windows 3.1x when using S3 805 drivers (works fine with the default VGA driver). This doesn't matter to me, as I use this machine for DOS gaming only. Also, the settings you may have chosen with VL24CFG.EXE (from STB driver) are not read by the card on startup anymore, but the default settings are fine.
[30.06.2021 EDIT] I don't recommend using this ROM after all, as it causes flickering in some games like for example "Jones in the Fast Lane".

To use this ROM, you should have an STB Powergraph VL-24 with an AT&T RAMDAC and video memory rated at 60ns.
It has to be flashed to a 32kB *EPROM chip.

Image is courtesy of wiki.preterhuman.net (with the red circle edit done by me). This is not a picture of my card.

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Last edited by 8bitbubsy on 2021-06-30, 16:09. Edited 1 time in total.

486:
- Motherboard: "SIS 486G 3.3/5V Ver:E" w/ 256kB L2 cache
- CPU: Intel 486 DX2 66MHz (no WB cache)
- RAM: 32MB FPM 60ns
- HDD: 8GB microSD
- VGA: S3 805 1MB VLB
- Audio: SB16 CT4180
- ISA PS/2 mouse card

Reply 2 of 6, by 8bitbubsy

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maestro wrote on 2021-05-10, 20:16:

Sweet, I bought one of these cards recently and will definitely try this out. Thanks for finding this and sharing.

Remember to make a backup of your current VGA BIOS first. Also let me know if you have the same overexposed RGB signals as I did (probably only on TFTs). The way you can tell is to look at things that are supposed to have a shade of gray, f.ex. the grey buttons in Windows 3.1x.

486:
- Motherboard: "SIS 486G 3.3/5V Ver:E" w/ 256kB L2 cache
- CPU: Intel 486 DX2 66MHz (no WB cache)
- RAM: 32MB FPM 60ns
- HDD: 8GB microSD
- VGA: S3 805 1MB VLB
- Audio: SB16 CT4180
- ISA PS/2 mouse card

Reply 3 of 6, by weedeewee

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You could also use a w27c512, add a jumper to pin1 (A15) to select first 32k or second 32k and program the modded bios in one half and the original in the other.

Just an idea .

Reply 4 of 6, by 8bitbubsy

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That's a neat idea. Could in theory add a toggle-switch in the back, which could be toggled when the computer is off.
Not sure if I could be bothered though, I don't need >16 colors, >640x480 and hardware acceleration in Windows 3.1x, as I rarely boot that on the computer.

486:
- Motherboard: "SIS 486G 3.3/5V Ver:E" w/ 256kB L2 cache
- CPU: Intel 486 DX2 66MHz (no WB cache)
- RAM: 32MB FPM 60ns
- HDD: 8GB microSD
- VGA: S3 805 1MB VLB
- Audio: SB16 CT4180
- ISA PS/2 mouse card

Reply 5 of 6, by maestro

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8bitbubsy wrote on 2021-05-10, 20:18:

Remember to make a backup of your current VGA BIOS first. Also let me know if you have the same overexposed RGB signals as I did (probably only on TFTs). The way you can tell is to look at things that are supposed to have a shade of gray, f.ex. the grey buttons in Windows 3.1x.

Good tip on the backup. I'll report back on whether I see an overexposed RGB signal, I don't have any CRTs to compare with but plenty of TFTs on hand.

EDIT:

Well I've hit a snag, my 486 isn't powering up and I think it's the power supply.

When I switch the power on, the CPU fan spins about 1/20 of a rotation and I see the power LED on my CF2IDE adapter briefly flash on and off. When I switch the power off, the same things happens.

- This configuration was booting the last time I had tried it.
- I've removed all of the components except the CPU and I get the same result.
- I've tried reseating the socketed Dallas, which is dead and which I had removed/inspected between last boot and this boot.

No change in behaviour is observed during my troubleshooting and unfortunately I don't have another CPU or PSU to test against. I'm still hoping it's a simple fix.

Does this sound familiar to anyone?

Reply 6 of 6, by 8bitbubsy

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Sounds like a short to me. Try to measure the resistance between 12v and ground, and 5v and ground, on the PSU connectors (going to the mobo).

486:
- Motherboard: "SIS 486G 3.3/5V Ver:E" w/ 256kB L2 cache
- CPU: Intel 486 DX2 66MHz (no WB cache)
- RAM: 32MB FPM 60ns
- HDD: 8GB microSD
- VGA: S3 805 1MB VLB
- Audio: SB16 CT4180
- ISA PS/2 mouse card