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Old 10-12-2020, 01:35 AM   #1
Paar
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Join Date: Jun 2020
City & State: Opava
My Country: Czech Republic
I'm a: Knowledge Seeker
Posts: 14
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!
Attached Images
File Type: jpg 20201011_204728.jpg (1.49 MB, 40 views)
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