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Help Booting up a 486 motherboard + Identifying motherboard

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First post, by DeggMan

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I purchased a 486 from someone awhile back, and tried to boot it up; nothing happened. I've had some free time lately, so I tore down the machine to the motherboard and want to boot it up outside of the case as simply as possible just to rule out what's happening. Having never built a 486 I'm confused by where case wires go, and what is needed to do a minimal hardware boot. I tried my hardest to identify the motherboard following some methods here (to find a manual) and this was the closest match I could find: https://th99.bl4ckb0x.de/m/U-Z/35297.htm. I should mention there was a power button on the case but it appears that there's actually no wires coming from it on the inside (you'll be able to see this in the attached photos). Thanks for all the help, all of this is alien to me! Was far too young to have ever had or touched a machine this old. Though I want to preserve it.

Here is an imgur gallery with all useful images of my hardware: https://imgur.com/a/UUAxIlV

Should mention I have all necessary cards for displaying video, they just aren't photographed.

Reply 2 of 10, by Ydee

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Hi, You need connect the switch ON/OFF - 4 wires from PSU (black, white, brown and blue with Fastons). The colors of the wires on each contact are described on the label on the PSU, usually is black and white wire hot and neutral from the AC line in jack on the back of the power supply. Brown and blue should be hot and neutral on the AC out receptacle on the back of the power supply, but check it on the label.
Connect the board power supply so that the black wires of the two connectors are together in the middle. If the power supply is correct, when the switch is pressed, the board should be powered up and start starting. Try boot in minimal configuration, i.e. board+CPU+RAM+VGA and see, what happens.

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Reply 5 of 10, by megatron-uk

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If you have never used an AT power supply before be very careful - there are two things to be wary of:

1. Mains level voltage (be that 100/110v or 220/230v wherever you are based) flows through that front panel switch - be damn sure you don't touch it or play around with it while the PSU is connected to the outlet. It's not like ATX where you have a low voltage 'power on' signal.

2. The two connections to the motherboard - P8 and P9 can be plugged in the opposite way around and it WILL destroy your motherboard. The mnemonic to remember is "Black to black is good. Red to red you're dead".

ATX solved these problems by using keyed connectors that could only be inserted one way and by using the low voltage power on signal to the front panel, rathe than mains voltage.

My collection database and technical wiki:
https://www.target-earth.net

Reply 6 of 10, by DeggMan

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I've attached an image of how I have the wires hooked up to the power button. Let me know if those wires appear to be on the right way. Also, a better image of the power supply diagram.

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Reply 7 of 10, by Ydee

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Oh, the wiring scheme on the label is for power wires only. You have a more accurate schematic on the attached photo.
It's plugged in well - black is always under voltage if the power supply is plugged into a socket. By turning on the switch, the voltage passes through to the brown wire. Black and brown are working phases, so they have to be underneath each other. If the computer switches on at the OFF switch, flip the black and white down and the brown with the blue up.

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Reply 8 of 10, by DeggMan

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Good news it boots. Here's this bios string I believe: 40-0400-006257-00101111-080893-U4800VLX-P. I'll have to get an adapter or appropriate keyboard before I can carry on further. It was American Megatrends bios of course.

Reply 9 of 10, by Horun

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DeggMan wrote on 2021-06-06, 13:02:

Good news it boots. Here's this bios string I believe: 40-0400-006257-00101111-080893-U4800VLX-P. I'll have to get an adapter or appropriate keyboard before I can carry on further. It was American Megatrends bios of course.

Great ! The AMI bios string section 00101111-080893-U4800VLX matches this: http://www.win3x.org/uh19/motherboard/show/2602 as posted above by evasive

Hate posting a reply and have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor. 🤣 Second computer a 286 12Mhz with real IDE drive ! After that came 386, 486, Pentium, P.Pro and everything after....

Reply 10 of 10, by evasive

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Congrats on getting it to boot.

As for identification, OEM version, I don't have information on manufacturer ID 006257 right now. Will see if I can find more info.

In the mean time, if you can manage to make a backup of the bios (you can use this tool):
http://cd.textfiles.com/microhaus/mhblackbox3 … MORY/GETROM.ZIP

And post that somewhere that would be very nice

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