VOGONS


First post, by AppleDash

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I was given a VLB 486 motherboard by a friend, and wasn't told much else other than it "acted weird". I did some testing, and no matter what I do, as long as there is a valid memory configuration, it counts 3MB of RAM. It had no cache so I added some, and now it counts between 2 and 3MB of RAM at random. I haven't tested beyond this. The RAM is known good and so is the cache. What can I do to try and troubleshoot this? My first thought would be potentially bad capacitors? I'd love to get it working!

Main retro system: Am486 DX4 100MHz | 128K cache | 16MB RAM | VLB Mach32 | Sound Blaster 16 | HardMPU w/ MT-32 or SC-55 | MS-DOS 6.22; no Windows

Reply 1 of 14, by Ampera

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Maybe some of the address lines from the board are broken and it can only address a certain amount? Busted caps I wouldn't think do this, but you can try replacing them if you want to. Does it do this with less than 2/3MB SIMMs?

Reply 2 of 14, by AppleDash

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I got some more information from the guy who gave it to me - could it be a bad BIOS? He said that when he first got it it gave him a bad BIOS beep code, and then started half-working as it is now. Is there somewhere I can obtain another BIOS to test?

Main retro system: Am486 DX4 100MHz | 128K cache | 16MB RAM | VLB Mach32 | Sound Blaster 16 | HardMPU w/ MT-32 or SC-55 | MS-DOS 6.22; no Windows

Reply 3 of 14, by jade_angel

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What board is it? It may be possible to chase down another BIOS, or someone may have one lying around.

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Reply 4 of 14, by feipoa

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Did you check for corrosion on the BIOS DIP socket and try different densities and configurations of RAM? Although, I have my money on obtaining a freshly flashed BIOS.

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Reply 5 of 14, by AppleDash

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feipoa wrote:

Did you check for corrosion on the BIOS DIP socket and try different densities and configurations of RAM? Although, I have my money on obtaining a freshly flashed BIOS.

Sorry for the late replies! I just looked and everything seems fine. I've tried different RAM configurations.
I have no idea as to the model of the board unfortunately, I found something kind of close on TH99 that had all the jumpers in the same places and named the same, but it was listed as an unknown model.

The BIOS chip "AMIBIOS American Megatrends 486DX ISA BIOS (c) 1993", and also says "AB6118613" on it. The chipset looks to be made by "PC Chips" or "IPC Chips" - the logo is kind of ambiguous.

Main retro system: Am486 DX4 100MHz | 128K cache | 16MB RAM | VLB Mach32 | Sound Blaster 16 | HardMPU w/ MT-32 or SC-55 | MS-DOS 6.22; no Windows

Reply 6 of 14, by AppleDash

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This is the closest board I can find: http://arvutimuuseum.ee/th99/m/U-Z/32303.htm

Main retro system: Am486 DX4 100MHz | 128K cache | 16MB RAM | VLB Mach32 | Sound Blaster 16 | HardMPU w/ MT-32 or SC-55 | MS-DOS 6.22; no Windows

Reply 7 of 14, by SRQ

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Of some note is that I had this board originally and, if I recall right, it failed to work if a TAG ram module or cache was installed.
I think, I'm actually unsure but that might help find a solution.

Reply 8 of 14, by AppleDash

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New information gained from tonight's tinkering session!

Sometimes, when I turn the system off, it refuses to come back on (no video) for ~5 minutes no matter what I do, then after letting it sit it magically starts working again.

Could this be bad caps?

Main retro system: Am486 DX4 100MHz | 128K cache | 16MB RAM | VLB Mach32 | Sound Blaster 16 | HardMPU w/ MT-32 or SC-55 | MS-DOS 6.22; no Windows

Reply 10 of 14, by h-a-l-9000

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486 VLB sounds like electrolytics don't play much of a role (the high-capacity electrolytic caps came later).

What you can do:
- re-seat any socketed chips
- clean all sockets, connectors, plug ISA cards several times
- have a thorough look at traces and chip pins for damage (resolder them if you have the equipment)
- in case the board is rather dirty, brush it with soap and water, after that dry in the oven around 80°C
- bend the board or ISA cards while switching on, check if that makes a difference, like freezes immediately when put under mechanical stress

1+1=10

Reply 11 of 14, by kenrouholo

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AppleDash wrote:

New information gained from tonight's tinkering session!

Sometimes, when I turn the system off, it refuses to come back on (no video) for ~5 minutes no matter what I do, then after letting it sit it magically starts working again.

Could this be bad caps?

That is a symptom of bad caps, yes, though that's not the only potential cause.

Yes, I always ramble this much.

Reply 12 of 14, by AppleDash

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Visually inspected the system, cleaned everything I could, nothing is damaged or dirty. Replaced every capacitor on the thing and that did not help (did not hurt either).

Main retro system: Am486 DX4 100MHz | 128K cache | 16MB RAM | VLB Mach32 | Sound Blaster 16 | HardMPU w/ MT-32 or SC-55 | MS-DOS 6.22; no Windows

Reply 13 of 14, by Deksor

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Maybe the bad caps aren't located on the board but rather inside the PSU ?

I remeber that my PCChips 486 had stability issues. Once I recaped the PSU, it worked flawlessly. Mu 8088 crashed after a few minutes of usage. Once the PSU was recaped, again, problems gone !

To find out which board you have, you should rather look to the POST string. The POST string is located at the bottom of the screen where the BIOS counts the ram.

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Reply 14 of 14, by AppleDash

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I've tried with 2 different known-good PSUs. There's no useful information on the POST screen I'm afraid 🙁

Main retro system: Am486 DX4 100MHz | 128K cache | 16MB RAM | VLB Mach32 | Sound Blaster 16 | HardMPU w/ MT-32 or SC-55 | MS-DOS 6.22; no Windows