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Old 10-12-2020, 01:35 AM   #1
Paar
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Default Old 286 motherboard cries for help

Hi! I have problem s with a 286 motherboard I have acquired some time ago. The battery needed removal and corrosion from it's gunk cleaned, and after applying some patch wires all seemed to work. I could boot from floppy and HDD and was able to play some Prince of Persia.

Then I tried to connect an external battery (3V coin cell from my 486 rig). Things looked promising as the CMOS settings were being saved so I turned off the computer and let sit for maybe 20 minutes.

The problems started when I turned on the computer again. I kept seeing graphical corrution even in POST (missing letters or letter replaced by different symbols) and even though the computer seemed to work fine otherwise, it was very unstable and produced garbled graphics even in games like Commander Keen 1. After a while it stopped working alltogether, instead of POST I got only some pink areas where a POST text should be, without the PC doing anything.

So I've checked the motherboard and noticed that one patch wire was soldered wrongly. I have corrected it but that unfortunately haven't fixed my issues. I've been probably being sending 3V to a wrong part of circuit and fried something. There are two transistors sitting near each other (2N3906 and 2N3904) and after some checks by multimeter I have discovered that I have connected the battery to 10kOhms resistor which had it's other side connected to the collector lead of 3906. Now it's correctly connected to connector lead of 3904 transistor through a filtering tantalum cap.

Interesting thing is that if I wait awhile, let's say 2 minutes, the PC starts normally but gets unstable again after a minute or so. Is there a part which could get damaged by 3V battery? I'm new to this so I don't know where to check.

I am attaching a photo of the board with the affected area. Red circles show the bad connection and green the fixed connection. Thank you in advance for any tips!
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File Type: jpg 20201011_204728.jpg (1.49 MB, 40 views)
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Old 10-13-2020, 03:34 AM   #2
Paar
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Default Re: Old 286 motherboard cries for help

I have some news about this. To put it simple, the motherboard works fine again, at least is seems to. Just decided to give it another go the next day and it booted fine without any glitches. Played some games on it, including Wolfenstein 3D, Prince of Persia and Commander Keen series. Let Commander Keen 4 loop for about an hour. Worked great.

I started to wonder what the problem was and I have this theory: the board has two CMOS setups - ordinary CMOS setup for date, time, floppy and HDD configs, and XCMOS setup for memory timings, bus clocks etc. When I put the external battery in there I played with both setups and I vaguely remember setting some BIOS and video memory shadowing and possibly memory interleaving. I'm not 100% sure but it is possible that I saved those settings and that was the cause of my issues. Some incompatibility.

That theory could be right but that would mean the XCMOS settings take far longer to erase themselves without a battery. It seems unlikely to me but it's possible the settings are stored elsewhere, maybe on the PAL chip that is on the board (the chip should contain some memory according the datasheets). Or it may be a part of a different chip of the chipset. I don't know. Just hope it will work fine from now on .
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Old 10-13-2020, 08:27 AM   #3
pecrie
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Default Re: Old 286 motherboard cries for help

From what I see , the most left resistor of 10K , seems to have a bad solder joint.
It would be good to check the board for bad solder joints ...
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Old 10-13-2020, 10:50 AM   #4
R_J
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Default Re: Old 286 motherboard cries for help

I see the corrosion on the board and it has likely damaged the feedthroughs connecting the different layers, check that they are ok and not open
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Old 10-13-2020, 09:17 PM   #5
RJARRRPCGP
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Default Re: Old 286 motherboard cries for help

I suspect a bad solder joint, like a used Radeon 9800 Pro I had in 2007.
(That card seemed to be fine at first, but then turned around, and when just in a utility program, the screen was corrupted! Then on reboot, kept getting a corrupted BIOS screen, random coloured letters and I thought I saw blinking letters as well.
A dummy at the time, I was hoping an alcohol bath would fix it. I noticed that moving it around in the AGP slot, would result in a change of what's on the screen.)
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Old 10-14-2020, 05:17 AM   #6
Paar
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Default Re: Old 286 motherboard cries for help

I cannot solder it properly from the top side because of the corrosion. But it's soldered nicely from the bottom and has good continuity according to my multimeter. These leaking batteries are no joke and I am always very disheartened when I see such a corroded motherboard. Affects all board up to 486. Luckily, some newer 486 boards started using coin batteries.

I'll probably try to reflow all joints just to be sure and figure out how to strenghten the connections on the upper side. It's probably fine now but you can never be too sure. I'll put the board through more testing with external battery and see if works fine or not. Thanks for your advice!
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