Help troubleshooting a 386 motherboard \ VOGONS

Help troubleshooting a 386 motherboard

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First post, by tony359

Posted on 2021-03-16, 11:49

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Hi all

I have acquired this... D-YING (LOL!) motherboard as "faulty". When I received it it showed the usual battery leak corrosion. The BIOS VCC socket was so corroded that it had broken. I bent it a bit and the MB booted up fine. Cool!

So I washed the board as I usually do (It's a professional fluid designed for PCBs which works like a charm and removes corrosion and oxidation) and then replaced the bad sockets. As I usually do, I take all precautions such as ESD protection and rinse the board with distilled water when done. Then it dries for a day or two before I power it up again. I never had an issue to be honest.

Unfortunately, the MoBo does not boot up anymore now.

This is what I have done/noticed
- There is no BEEPS, so that tells me the CPU does not talk to the BIOS or it does not initialise somehow.
- There is no activity on Address/IO lines besides a short pulse when I power up (like a square wave?) but then nothing.
- I see the clock on the CPU but it's 50Mhz and the CPU is an AMD 386 DX 40Mhz?
- I removed the sockets and check every pin for continuity, following the traces and I did not find anything
- As far as I can see, everything is getting it's VCC
- I've checked the traces where the battery used to be and so fare they are ok
- I see a RESET pulse on the CPU happening, even though it seems to get high properly and then "slowly" going LOW.
- I've tried reading the BIOS chip with my programmer: it reads but I do not have a way to check if it's somehow corrupted?

I'll keep testing. But I was wondering if anybody on this forum has any input for me. It's a bit frustrating that the board was working and now it's not...

Thank you!

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Reply 1 of 17, by Nexxen

Posted on 2021-03-16, 11:58

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Nice board!!

Last edited by Nexxen on 2021-03-16, 13:14. Edited 1 time in total.

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Reply 2 of 17, by theoryboy

Posted on 2021-03-16, 12:03

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Not very familiar with your board, but your 50Mhz comment gave me an idea, what about the osc circuit there are no jumpers present and after a cursory look

Would this be a clue ?

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Reply 3 of 17, by tony359

Posted on 2021-03-16, 12:29

tony359 Offline

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Theoryboy

You are a star! I thought of finding the jumper setting for the board but I had forgotten. IT WORKS! Clearly the jumpers must have come off when I washed the board.

Thank you so much!! You - and this forum - are incredible!!

My Youtube channel: https://www.youtube.com/@tony359

Reply 4 of 17, by theoryboy

Posted on 2021-03-16, 12:41

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Huzzah! 😀

Reply 5 of 17, by tony359

Posted on 2021-03-16, 19:40

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Out of curiosity, with the jumpers set to 40Mhz I read 80Mhz at the CPU. And it works. Can someone explain me how that works please? Am I not supposed to see 40Mhz at the CPU?

My Youtube channel: https://www.youtube.com/@tony359

Reply 6 of 17, by tony359

Posted on 2021-03-21, 21:31

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Right, I've realised there is another (new?) fault on this board. The Keyboard doesn't work anymore, the BIOS says "Keyboard error". I cannot be sure it was working before to be honest.

I've poked the traces (the battery leaked so there is a bit of damage there) and I cannot see anything obvious. I have replaced a 10KOhm resistor which split in half while poking around but that did not resolve the issue.

I can see pulses on a pin of the keyboard connector and those pulses reach the Keyboard BIOS. The Keyboard BIOS does not seem very active to be honest. I don't seem to see much activity anywhere.

Any other great inputs on what to do next? Is there a datasheet for that chip - or does anybody have any advice on what to look for?

I did replace the Keybard BIOS' socket too but I also removed it to double check all the traces (when the board refused to boot) and everything seemed to be ok. Is the BIOS chip a standard chip I can dump with my programmer to compare with something to see if it's corrupted?

Thanks!
Tony

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Reply 7 of 17, by Deksor

Posted on 2021-03-21, 23:40

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Yes it's a standard type (27C512 probably). Actually if you want to share the ROM I'd be interested to add it to UH19 (see my signature) 😀

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Reply 8 of 17, by tony359

Posted on 2021-03-22, 11:47

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Thanks - I shall do.

My Youtube channel: https://www.youtube.com/@tony359

Reply 9 of 17, by tony359

Posted on 2021-03-22, 14:59

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I cannot read that chip using a 27C512 - I have tried a few different brands on the software. I get pin error. Could that be a different type of chip?

My Youtube channel: https://www.youtube.com/@tony359

Reply 10 of 17, by Deksor

Posted on 2021-03-22, 15:38

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Pin error or ID error ?

In last resort you can pull the chips' sticker to check what's inside.

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Reply 11 of 17, by tony359

Posted on 2021-03-22, 16:26

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Pin error.

Maybe I was not clear, apologies. I mean the Keyboard BIOS - the longer chip - which I guess it's where the keyboard is connected to? That does not have a sticker. It only says "Keyboard BIOS" on it.

My Youtube channel: https://www.youtube.com/@tony359

Reply 12 of 17, by weedeewee

Posted on 2021-03-22, 18:01

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that's a microcontroller... sortof . definitely not an eeprom chip.

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Reply 13 of 17, by tony359

Posted on 2021-03-23, 15:09

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Ah, ok, thanks. Is there any datasheet, pinout, document available that can help me troubleshooting it?

My Youtube channel: https://www.youtube.com/@tony359

Reply 14 of 17, by mR_Slug

Posted on 2021-03-23, 22:17

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Attached is the datasheet. It is a microcontroller programed as a keyboard bios. All keyboard BIOSs are compatible with the IBM AT. But most 386 onward keyboard bioses have some other functions programmed into them. So swapping the keyboard bios may not work. Some motherboards frustratingly wont boot without the keyboard bios.

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Reply 15 of 17, by tony359

Posted on 2021-03-24, 10:14

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thank you a lot. I will see if I have another one from another board to test but this is a good starting point!

My Youtube channel: https://www.youtube.com/@tony359

Reply 16 of 17, by tony359

Posted on 2021-03-24, 13:40

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So, I found another MB with the same chip - not socketed. So I started comparing and noticed that the faulty board had the Sync missing. I looked better and... well, I guess that if I forget to solder two pins on the socket, that may not work well with the system?? 😀

Thanks for your help everybody!

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Reply 17 of 17, by Nexxen

Posted on 2021-03-24, 15:34

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Solder on into your adventure and earn your tin badge!

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