Calling all PC-III family and 286 motherboard repair gurus 🙂
I have spent the past 6 or so months repairing a badly battery damaged PC40-III and have finally got it mostly working and usable, bar one strange fault I just can't work out with the 2nd bank of RAM which makes the system a lot less useful.
After replacing dead drams in both banks all RAM is now detected in bios. Where it goes pear shaped, if the bios is set to 640k, the system POSTs fine but will not boot, either from the hard drive or a floppy, loads for about 1 second (a few seconds with floppy) then just stops dead, but still responds to ctrl-alt-del and ctrl-alt-esc so isn't completely locked up. Now the weird part, if you set the bios to 512k so it's only using bank0, it boots just fine, is totally stable, and ram tests in Landmark and CheckIt will test the full 640k+384k address range with no issues over multiple passes on full/long test modes. (For those not familiar the system uses 2 banks of 512kb, addressed as 640k base 384k extended). The Extended ram disable logic is functioning, the extra 384k disappears and appears correctly based on the jumper position, and most of the rest of the ram logic is shared between both banks, so I am absolutely puzzled where to even look next. The 74F series chips are not so easy to find in DIP format so I don't really want to be pulling chips out and replacing willy nilly without knowing what the issue actually is.
So I guess I'd be appreciative of any advice from others who have similar board level repair knowledge where to maybe looks next. What other ram tests I may have forgotten that might help. What would pass all ram tests yet be so critical the system won't boot with it enabled.
Any assistance greatly appreciated, the extra 128kb makes a big difference to what can be run haha.