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MB-4DUVC & PCI48AG-0 486 motherboards not posting

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First post, by bassybeats

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Hey Team, first post here.

I have two boards of which I am trying to get one of them going. I have tested all the parts separately so I know its just the boards ( tested PSU, ram, cpu, vga card, cache, bios on one of them in working unit )

I have two of the MB-4DUVC boards ( the UMC chipset one ) and have ruled out the bios chip, CPU, Cache, ram, PSU and VGA card in the working unit. My ISA post code tester showed up and it is saying that the reset circuit is stuck high. Where do I start looking? I have attached a video of the working unit going through the post codes.

The other one is a PCI48AG-0 board. Again I have ruled out the CPU, Cache, ram, PSU and VGA card in a working unit. I cannot for the life of me find a BIOS file for this board ( i have attached what I dumped off the chip to see if someone can tell if it is corrupted). The post code board shows no activity but the CPU does get warm. Is this likely a BIOS issue?

Any direction would be helpful. The first time playing with 486's since I was a kid and I have forgotten most of it all. But I'm pretty damn sure the CPU's are configured correctly.

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Reply 1 of 5, by Deksor

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I've had a motherboard with the reset signal stuck high. The issue was the 14mhz crystal missing. Maybe it's been removed on your board or it's bad ?

Also would you like to help us improve our database by providing a photo of your motherboard ? http://www.win3x.org/uh19/motherboard/result/?name=4DUVC

http://www.win3x.org/uh19/motherboard/show/4957

When I go back from work I'll see if I can check your bios

Trying to identify old hardware ? Visit The retro web - Project's thread The Retro Web project - a stason.org/TH99 alternative

Reply 2 of 5, by waterbeesje

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MB-4duvc is the perfect example of Jumper Paradise. Miss one jumper and all may fail... Nott sure what the board does internally, but it just won't post on some settings. I had this once on my board when testing stuff and it wouldn't post any more. I ripped off all jumpers for speed and type setting and started from scratch. Th99 is your friend 😁
Also try to reseat stuff again and again, it might help.
Now it's running a Cyrix 5x86-100 very happy 😀

I see you are using an overdrive. Do you have a CPU without any tricks on the bus speed? I see them SX on the background. Or maybe a DX or DX2?

Stuck at 10MHz...

Reply 3 of 5, by bassybeats

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@deksor, I will get a good photo of the boards for you. I measured the crystal and it is putting out the correct frequency ( measured at the crystal ). Is there somewhere else I should be measuring it?

@waterbeesje, I have two of these boards and configured them both exactly the same. One worked and one didn't. The overdrive is in there atm to see if it worked with a different chip instead. I was using an AMD DX2-66 at first.

Reply 4 of 5, by Deksor

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I have checked the BIOS from the TMC. I couldn't get it to POST in my emulator, but many 486 BIOS don't POST in it ... However I could open it in modbin (a DOS tool to explore old Award BIOSes), so I think it's fine.

I added it to the matching page in UH19

For your reset signal issue I can't really help you much more ... Maybe the signal isn't going where it should, maybe something else is broken ... But there's definitely something causing this.

Trying to identify old hardware ? Visit The retro web - Project's thread The Retro Web project - a stason.org/TH99 alternative

Reply 5 of 5, by waterbeesje

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A bit of a long shot: These old boards do have a lot of thru hole solder joints. Maybe there are some joints bent and shooting something?
Even longer shot: might there be a trace or joint damaged that is supposed to pull down a reset signal?

Stuck at 10MHz...

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