BioStar MB-1420/25/33PB/PV Repair

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

This is a no frills 486 board, no cache no nothing., it has a 66MHz clock tho so its got that going for it.

Inbuilt CPU

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)

The Repair

Trace back

Hiding under the keyboard controller is the 7407

using 30AWG hooked one back to the board trace then the other sat in the PTH
the DIN connector just sat over the top of the wires without issue



Texas Instrument TMS27C512

First Boot

Port 0x80

This boards chipset doesn’t have port 0x80 support so the standard diag cards didn’t help here


was still getting 8 beeps, turns out the ram was imbalanced,


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,,,