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486 motherboard doesn't boot

vbug

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Jan 2, 2021
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Hi everyone, new round :) I have that old 486 motherboard from the PC of my childhood, but it doesn't seems to work anymore. It was working, and one day for no particular reasons it just doesn't boot again. It was stored for years on a closet, and now I'm trying to restart it. I removed the leaking battery, it could have damaged keyboard circuits (the keyboard is powered as the led blinks once when I turn the computer on, but no clue on signal circuits). First time I started it since years one capacitor closed to the power supply connector burned. I replaced the damaged capacitor (and a second one I was suspecting to be faulty too). Traces of burns on the power connector are only due to my soldering iron (and my tremors !) when replacing capacitors. But the motherboard still doesn't boot. I have an ISA analyzer pc card. According to leds, the power supply seems right. When I turn the pc on, the card gives error codes : First, 0D-0C for maybe one second, then 06-05 and it stays blocked on that code. According to the analyzer card documentation, 0C code could be related to keyboard initialization. Then I'm wondering if the keyboard controller chip is working or not, because when the capacitor burned I have seen a spark between capacitor and keyboard controller chip (LT38C41 chip closed to the power connector). But maybe I'm wrong and the issue is somewhere else. Another information : I get thoses error code the first time I start the computer for a (relative) long time, after that first start if I start the computer again the card doesn't show any code (and doesn't boot neither), I have to press the reset button to get thoses error code again. Any tips or suggestion is welcome :) Thanks.
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modem7

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I have an ISA analyzer pc card
Which most people here will refer to using the generic name of 'POST card'.

Your particular POST card (shown at [here]) is of the type that shows two codes: the left display shows the last code received, and the right display shows the previous code received. It is the left code that it is the important one.

Be aware that the codes shown in the document supplied with your POST card, are not comprehensive. For example, BIOS authors changed the codes periodically. AMI is an example - see page 34 of the document at [here] where you can see that the meaning of code 03 differs between AMI's Hi-Flex and WinBIOS. If you have the manual supplied with the motherboard, the POST code list in that can be reasonably relied on.

When I turn the pc on, the card gives error codes ...
It is best not to think of the codes that you are seeing as 'error' codes. They are best thought of as 'progress' codes; codes output at various stages of the POST. If I was to use a POST card on my many fully functional motherboards, I would see many codes flashing past on the POST card as the POST progressed.

... First, 0D-0C for maybe one second, then 06-05 and it stays blocked on that code.
So the POST in your motherboard is progressing and then stops shortly after it has output code 06.

Any beeps from the speaker? If 9 beeps, then you can be confident that the 06 shown on page 34 of the document at [here] is what you need to be looking at.

But I see no RAM in the SIMM sockets. I think that you should, before doing anything else, populate at least bank 0 of those sockets. (Bank: Four 8-bit SIMM's providing 32 bits for the 486.)
 

T-R-A

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But I see no RAM in the SIMM sockets. I think that you should, before doing anything else, populate at least bank 0 of those sockets. (Bank: Four 8-bit SIMM's providing 32 bits for the 486.)

Noticed that myself when looking at the photo. 3 things a motherboard has to have to boot: RAM, output (be it video, teletype or whatever), and power.
 

Chuck(G)

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I'm having trouble believing that the OP would try to boot the system with no memory or peripherals. I'm assuming that the photos posted are by way of illustration of the layout of the motherboard.
 

the3dfxdude

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I'd buzz out those keyboard, keyboard ic and power connector traces near the battery.
 

nevadaOZman

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Unfortunately many vintage boards REQUIRED the battery to be "good" in order to boot properly. Since it is missing, mmm, could be an issue.
 

vbug

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Hi, and thanks for your answers and links to documents. As an information, I mount computers since I'm 8 (I'm 33 today) then of course I know a pc need RAM to works :mrgreen: I removed everything on it when I remove it from its case, but when I test it I add RAM and video card. I know that RAM and video card are correctly working, same for the CPU. When I turn on, there is no POST, screen doesn't turns on, no BIP from the speaker, nothing. I'll try to solder a new battery/external battery, just in case, but afaik a motherboard will always POST without a battery, but maybe print an error message on screen about that. If no change, I'll try to remove the keyboard controller and see if there is something different on codes given by the POST card. And I'll try to dig about the "error" codes signification. I'll keep you informed, thanks :)
 

vbug

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Hi, then I installed a new battery, and tried to boot again, with or without the keyboard controller chip, no change. But after some time (around 1min), the 0D-0C code on POST card turns to 00-0D. I'm a bit lost about the signification of those codes, and there is nothing about it on the motherboard user manual (really basic manual). At least I identified and repaired data circuits of the keyboard connector. I will try to change the keyboard controller chip as I have a doubt on it, we'll see, after that I'll probably be out of idea !
 

creepingnet

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I would'nt bother with the keyboard controller chip. Typically how those POST cards work is the last executed code is the one on the left (in your case 00) and the one on the right is the one that it halted on (0D) - which, according to the picture of the manual you posted, means it's either not finding the CPU, CPU is not responding, CMOS 14h failed to find the video adapter, or no video card is installed when you started it up. Either way, it sounds like things just took a step forward here - a tiny one - but a step nonetheless.
 

vbug

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I would'nt bother with the keyboard controller chip. Typically how those POST cards work is the last executed code is the one on the left (in your case 00) and the one on the right is the one that it halted on (0D) - which, according to the picture of the manual you posted, means it's either not finding the CPU, CPU is not responding, CMOS 14h failed to find the video adapter, or no video card is installed when you started it up. Either way, it sounds like things just took a step forward here - a tiny one - but a step nonetheless.

Hi, I'm still not sure about the real signification of those codes because of different signification it could have depending of the bios version/age. I checked everything on another working machine, everything is working properly, CPU, memory, video card. I tried with different ones of each, no change. I'm sure about the jumper configuration for the CPU too. I'll try to check again the CMOS on another motherboard but last time I tried it was working.
 

vbug

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CMOS checked on another motherboard, it works perfectly. I have no more clue and no idea on what I can do next :unsure:, any suggestion welcome :)
 

Agent Orange

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CMOS checked on another motherboard, it works perfectly. I have no more clue and no idea on what I can do next :unsure:, any suggestion welcome :)
Time to settle back and check some some caps. Tedious work but most probable cause. Anyway that you check or swap out the CPU and RAM?
 

maxtherabbit

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if any of those tantalums were bad it's highly likely they would either just explode or shut down the PSU, I seriously doubt caps are relevant here
 

Agent Orange

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From what I understand previously, it doesn't power up. If so, maybe he needs to check board voltages first.
 
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