IBM 5150 Repair
Parity RAM Buffers Be Dip
So one of the people who ran the accomodation offices at uni had an old IBM 5150 sittin in her gararge. She did not want ot see it get dumped & wanted it to go to a good home. She already knew me as she use to work in my secondary school back when I was studying there & at some point moved to work at my uni.
So one day she drove it in for me. The system was very complete & came with the colour monitor, computer, keyboard & expansion unit. Sadly the software was long gone & the cable between the computer & expansion bay was also missing, but it turned out to be a DB62 cable. I am yet to oder one even to this day.
When nothing was on the screen the only sign of life was the clock signal.
Some but not meany of the address lines were doing something.
What the frequency counter could see. Had to be a nixie one of course.
The computer side expansion board. These are quite hard to get as a pair.
So I started replacing the buffer chips on the motherboard.
I started with the 74LS244 chips then moved to the 74LS245 chips.
Finaly I soldered on new 74LS373 chips as they were the only "buffer" chips left sending the signal to other pards of the machine.
Once all the chips were replaced I was greeted not with a full boot but this message. This was major progress as I finaly confirmed a few systems were functional such as the grapics card, monitor & keyboard.
Also found a fault with the electrical system in the gararge.
The BASIC ROM failed so I nabed the ROMs from a spare but broken motherboard I ordered for this project. Then was the long case of tracking down the RAM errors. Oh there were meany RAM errors. I removed the RAM expansion card, blewup a row of RAM chips as the ones on the mobo were -5V, the others are not. I also replaced the tanterlem capasitors as they are time bombs at this point.
I ended up salvaging the ROMs & RAM from this board to fix the IBM 5150. As well as buying a ton of RAM which is not cheap these days.
Thsi board had had someone remove something with a blowtorch at some point. I also used it for the IBM 5160 restoration project.
The dip switches on the motherboard. You can see all the socketed chips I replaced to. Only a few of the chips I replaced were still functional. I also made sure that all the RAM in the machine was socketed so I could test it more. To the point I used my IBM 5150 to test the RAM from other computers.
It was on the anoying 1054 201. But when I changed anything the computer would point to the correct chip. In this case 10D0 201. I had the service manuals printed off so i could at least follow the bread crumbs.
This serial & RAM card is dead as it had a NiCad battery on it that did what NiCad batterys do best. Spue their corosive guts everywhere. Still not got this card working to this day. Can't find a user manual for it to.
Theis error 1054 201 was the bane of my exsistance untill I did some more digging in the manual & found it was an external RAM card configeration error. So I set the dip switcheed on the mobo to the amount of RAM installed & the dip switches on the RAM card to the amount installed & boom the machine booted into BASIC.
I did some floppy & FDD testing with an old 286 machine I have that is from 1991, was used in some industrial machine. I was able to confirm my service & restor of the FDD drives was a success & the OS disks I bought were stilll operational.
Once the computer booted into BASIC with no errors I was exstatic. I instantly ran upstairs to get my IBMDOS 3.3 disks. Much to my delight the machine booted into MSDOS & was working happily. I also tested both FDDs by reading from both drives. They worked proving the interface card for the FDDs was fully functional.
The IBM 5150 booting into IBM MSDOS 3.3 for the first time. This was very much a holy site & made me so happy. I also achived this after breaking up with my first partner so needed a boost like this.
This was the most major part of the project complete as all what was in the expansion bay was HDDs which one was confirmed to be dead & the other I have never quite got to work. Should try & fix the main interface card at some point.
Now the IBM 5150is being used to repair other machines such as the IBM 5160 from the musium.
Here it is trying to formate 80s HDDs, an effort on futility but still worth a try to see if you can get the old things working. They are pretty much garenteed to loose their data after 30 years. HDDs do not make a good long term storage solution.