I found a 486 board circa 1991 on ebay that was having keyboard issues so I grabbed it not knowing much more than that, When I received it, it seemed to be in really good condition otherwise. It’s a Biostar MB-1420/25/33PB/PV
![](https://blog.hegars.com/wp-content/uploads/2020/12/image.png)
This is a no frills 486 board, no cache no nothing., it has a 66MHz clock tho so its got that going for it.
![](https://blog.hegars.com/wp-content/uploads/2020/12/image-4.png)
![](https://blog.hegars.com/wp-content/uploads/2020/12/image-14-1024x583.png)
Inbuilt CPU
![](https://blog.hegars.com/wp-content/uploads/2020/12/image-13.png)
https://www.cpu-world.com/CPUs/80486/Intel-KU80486SX-25.html
The Issue
As expected it was the good old leaky NiCAD trick which ate out the two keyboard CLK and DATA traces, (also had a good crack at the “Power Good” signal but it seems to be fine, for now)
![](https://blog.hegars.com/wp-content/uploads/2020/12/image-5-1024x460.png)
The Repair
Trace back
Hiding under the keyboard controller is the 7407
![](https://blog.hegars.com/wp-content/uploads/2020/12/image-9.png)
![](https://blog.hegars.com/wp-content/uploads/2020/12/image-8.png)
![](https://blog.hegars.com/wp-content/uploads/2020/12/image-6-1024x443.png)
![](https://blog.hegars.com/wp-content/uploads/2020/12/image-7-1024x608.png)
BIOS
![](https://blog.hegars.com/wp-content/uploads/2020/12/image-1.png)
BOIS Dump
![](https://blog.hegars.com/wp-content/uploads/2020/12/image-2.png)
First Boot
Port 0x80
This boards chipset doesn’t have port 0x80 support so the standard diag cards didn’t help here
Booting
was still getting 8 beeps, turns out the ram was imbalanced,
![](https://blog.hegars.com/wp-content/uploads/2020/12/image-10.png)
![](https://blog.hegars.com/wp-content/uploads/2020/12/image-11-705x1024.png)
Configuration
![](https://pcrebuilding.altervista.org/images/resized_my_pcrebuilding/th99_20/32447-1.png)
Going Forward
Overdrive Time!
The one I have is a DX2OPDR66, the 168pin “ODPR” version but this board has a empty 169th pin for the “ODP” version of the upgrade… shrug,,,
![](https://blog.hegars.com/wp-content/uploads/2020/12/image-15-1024x351.png)
https://www.cpu-world.com/CPUs/80486u/Intel-DX2ODPR66.html
![](https://blog.hegars.com/wp-content/uploads/2020/12/image-12.png)
![](https://blog.hegars.com/wp-content/uploads/2020/12/image-3.png)