IBM PC/XT (Model 5160): Part 4 (More RAM faults)
19More progress. The base 64KB (BANK 0) RAM issue was solved yesterday and the machine was happy booting into Cassette BASIC as there were no drives.
Before further testing, I dismantled the Model 5151 monitor to give the case a good scrub - has come up quite nicely.
Added the hard drive & floppy drive controllers today and started getting "PARITY CHECK 1" failures.
So back to the diagnostics ROMs - I used mostly the Supersoft one:
https://minuszerodegrees.net/supersoft_landmark/Supersoft%20Landmark%20ROM.htm
It expects there to be the full 640KB of RAM, however as I'm only using the 256KB of system board RAM I can ignore any RAM errors for addresses above 0x40000.
I'm only interested in:
BANK 0: 0x00000 to 0x0FFFF (already fixed)
BANK 1: 0x10000 to 0x1FFFF
BANK 2: 0x20000 to 0x2FFFF
BANK 3: 0x30000 to 0x3FFFF
The diagnostics were flagging "PARITY ERROR IN ADDRESS xxxxx" so I thought I'd just go and replace the remaining three parity bits in BANK 1/2/3.
In hindsight I think this was unnecessary - it looks like changing the BIT 0 chip in the relevant bank resolved that parity error. Changing BIT 0 in BANK 2 for parity error in address 0x2C000 and BIT 0 in BANK 3 for address 0x3C000 fixed the parity errors.
Overall, I just kept changing the RAM chips as indicated by the diagnostics - took a few attempts as there was no log of failed addresses - I had to try to quickly write down each address - so I just kept iteratively replacing RAM and re-diagnosing.
Once the RAM issues were cleared the machine booted off the hard drive successfully - MS-DOS 4.01. Disappointingly there were no 30-year-old documents, school assignments, love letters, text adventures or other artefacts ... just Lotus 1-2-3 (which I didn't have enough RAM to run anyway!).
The floppy drives work as well - I was able to boot off the official IBM Diagnostics disk (vers. 2.05).
The case is fully dismantled now, ready for refurbishing (powder coating?) - just the top and bottom case. Giving the front fascia a good scrub was enough for that part.
Next up will be attempting the repair the damage on the Persyst "Time Spectrum 384" multifunction card caused by the leaking Varta battery.
More videos on troubleshooting IBM PCs & compatibles:
https://www.youtube.com/playlist?list=PL5pwokf56wMLjhEgssoRyZJO1VWeQ09ij