My rebuilt IBM 5170 PC/AT

Last Updated on January 12, 2022 by Dave Farquhar

I have what appears to be an IBM 5170 in my collection. I’ve owned it since the mid 1990s. There’s not much original about it. Part of that is due to the ravages of time. But it’s mine. And since I did some fairly major repairs to it myself, I’m pretty attached to my Frankenstein PC/AT.

It’s a Frankenstein because it has parts from at least six different computers in it.

Cast off in a computer store

Frankenstein IBM 5170
This is my Frankenstein IBM 5170, soon after I got it working again. The case and both floppy drives are IBM, but it’s not exactly true blue inside.

I bought the case at Laclede Computer Trading Company, a long-running used computer store in St. Louis. I bought it sometime in 1995 or early 1996, and I remember I paid $10 for it. IBM AT cases were cheap and plentiful then, as the machines had mostly outlived their usefulness by that point. Sometimes they were bare cases, and sometimes they still had boards in them. I don’t think mine had a board or a power supply. I did find a power supply that fit, but it wasn’t an IBM. It was a DTK model, rated at 250 watts, which was higher than normal for the time. IBM used a 200 watt model. I don’t recall what I paid for the power supply but I’m pretty sure it was a separate sale because both the case and power supply still have their inventory stickers on them. The power supply also has a sticker on it from Houlihan Office Systems, which was a local IT company in the 80s and 90s.

I built a lot of Pentium-class systems into IBM 5170 AT cases in the mid 90s. The cases were good quality and plentiful, but they were cheap. They were large, and people expected to have more drive bays in a case that size. For budget builds, they made sense. You could get a case for $40, but it would look cheap. An IBM case for half that price didn’t look cheap.

Second life as a Pentium

I put a 75 MHz Pentium board in mine, along with a 3.5-inch floppy and an IDE CD-ROM drive, and eventually I swapped that board and CPU out for an Abit IT5H to give myself the option to use something faster than 133 MHz. I ran OS/2 and Windows 95 on it. At one point I had it triple-booting OS/2 4.0, Windows 95, and Linux. It looked like an outmoded IBM 5170 PC/AT at first glance, but with the upgrades it was a capable computer for its time.

I got several years out of the system with those two boards in it. Even when it was outmoded as a primary system, I still used it for testing. I never had anything faster than a 200 MHz Pentium in it, though. I had a Socket 370 board in an AT form factor that I desperately wanted to put in that case, but the case had a beam under its hard drive bay that interfered with the DIMM modules in that Socket 370 board. So I never had the 500 MHz IBM PC/AT sleeper I wanted. I thought about trying to modify the case with a Dremel to increase the clearance, but I never got around to that. Instead, the case ended up in my boneyard. Today I’m glad for that.

The boneyard

I accumulated a lot of cast-off parts over the years. Some of it was from buying used systems cheaply and salvaging the usable parts. Some of it was my own stuff that was no longer useful. And some of it was cast-offs from other people. I raided the boneyard for projects fairly often, but some stuff never made it back out. At some point I’m sure I had at least one 5170 motherboard, if not two or three.

I thinned out my boneyard after I got married. There were quite a few things I didn’t expect I’d ever use, and those got tossed or recycled. I got rid of a lot of cheap minitower cases, and I expect those IBM 5170 boards went around the same time too. But I did keep that case. It ended up with a Pentium board and a 1.2 GB Seagate hard drive in it. Somewhere along the way I ended up with some genuine IBM 5.25-inch floppy drives too, so it ended up with the proper YE Data-manufactured 1.2 MB drive in it. I also put a black 360K drive from an IBM 5155 Portable PC in it. Eventually I found a proper beige YE Data 360K floppy to complete it. It wasn’t cheap, but also wasn’t crazy expensive.

I had a 12 MHz AT clone motherboard that escaped my purge. At various times I had the idea to put that board in the AT case. It wouldn’t be exactly correct, but it would be a reasonable match. But the board had battery damage so I kept putting it off.

Why 286 systems had 360K 5.25″ floppy drives

It wasn’t long before people realized that swapping disks between 360K and 1.2 MB 5.25″ floppy drives could be problematic. The heads on 1.2 MB drives are narrower, so if you write to the same disk with both types of drive, the 1.2 MB drive doesn’t erase the whole track. The leftover data confuses 360K drives.

The workaround was to install a 360K floppy in AT systems as drive B: to facilitate swapping data with older PCs. They weren’t standard issue in every IBM 5170, but they were common, for a time.

Over time, this became less of a problem as people discarded their XT-class systems. Meanwhile. 3.5″ disk caught on, so eventually, AT owners swapped their 360K 5.25″ floppies for a 3.5″ drive. Not all ATs supported a 1.44 MB floppy but all of them supported 720K. Even a 720K 3.5″ drive was more useful by the early 1990s.

Having a 360K drive could be useful, so that’s what I put in the second bay.

Rebuilding my IBM 5170

Finally, in 2019, I got the board out, along with the case. I cleaned the battery damage, repaired the traces, rigged up a battery, and gave the board a final cleaning. I figured I didn’t have much to lose. Best case, I got the board working. Worst case, I’d have a broken board and would have to spend $40 to get something comparable to replace it. I got the board working. I added an ISA IDE controller, a 128 MB compact flash card, a VGA card from 1990, and an NE2000 network card. My DOS 5.0 boot disks wouldn’t work so I ended up putting DOS 6.22 on it. I also found an AST multifunction card, with 640KB RAM expansion and serial and parallel ports in my boneyard, so I added that. The board had 1 MB of RAM on it, so the AST board brought me up to 640K of conventional memory and 1 MB of XMS memory.

The motherboard

The motherboard is a bit of a point of pride. When I was in my 20s, I had more than one 40-something try to call rank on me by asking me if I could solder. It’s not something I had to do often, but I could do it, and did. There’s no reason a Gen Xer or Millennial or younger can’t do things like solder or follow a schematic. Or for that matter, follow traces and fix things when a schematic isn’t available. Soldering is a learned skill, not a generational thing.

Now that I’m in my 40s, someone younger than me could do a better job because they have a steadier hand and better eyesight. And I’m fine with that. But I’m also glad I haven’t lost the ability. Someone else could have made that board prettier, but this one works, and it’s mine.

Of course, the board gave me some resistance along the way. The board wouldn’t POST the first time I tried it, and a POST card indicated it was hanging up after the CPU test. But the next day when I tried it again, it completed POST and gave me a CMOS error. That’s much easier to fix–just run the BIOS setup.

Eventually I tracked down a proper IBM motherboard. It was broken and I fixed it. Hilariously, I bought it from my boss’ dad without either of us having any idea who the other person was. My boss asked me one day out of the blue if I bought an old IBM motherboard off Ebay. I said yes, then asked how he knew. He said because his dad sold one to a guy named Dave F. in St. Louis and he thought there couldn’t be very many people other than me who match that description.


Why did I go with only 128 MB? Because this motherboard doesn’t have a user definable hard drive type, the largest drive it can recognize is type 9, which works out to about 117 megabytes. In the 1980s, that was a huge drive. The drive is silent, of course, and very fast. It transfers over 800 KB per second and has seek times well under 1 ms. The card is certainly capable of more, but the ISA bus limits it.

The CPU and video are about 8-9 times as fast as the IBM PC/XT, and so is the compact flash card. Hard drives of the time were only about twice as fast as the IBM PC/XT’s drive.

Since I don’t have a lot of software that malfunctions on my 486, I don’t see the 117 MB limit as a huge problem.

Adding sound

I wanted this system to play games that ran too fast on my 486. Most of the stuff in the system is period-correct, but I didn’t have a period-correct sound card, and at this point anything older than a Sound Blaster 16 is expensive. As in, $100 or more. I found a way to hack a Sound Blaster 16 to work in a 286 and added that. Now I could play Railroad Tycoon and have Adlib sound instead of the PC speaker. It doesn’t necessarily sound exactly like an Adlib from the late 1980s, but it’s close. I ended up with a 1996 sound card in what’s supposed to be an approximation of what would have been a nice PC in 1989. That’s not too far off. If I ever find a period-correct sound card at a good price, I can replace it. If I don’t, I’m happy enough with what I got.

A good enough IBM 5170?

If I find a box of parts I forgot about and it happens to have some genuine IBM boards in it, of course I’ll work those in. The 12 MHz board is nice, as it benchmarks as 50% faster than an 8 MHz AT. If I ever have the need, it has jumpers to slow it down to 8 MHz or even 6 MHz to match the speeds of a genuine IBM 5170. But if I find a 5170 motherboard I didn’t know I had, of course I’ll put that in there. I can put the clone board in any other AT case.

I have no way of knowing how many PC/ATs ended up with third-party motherboards in them, but I imagine it was a fair few. It was no secret that clone motherboards fit the IBM AT case. Putting a 12 MHz 286 in one wouldn’t make a lot of sense unless it was a repair. Most people would opt to swap in a 386 or 486 board, I’m sure. But what I have looks like a PC that would have been nice to have in the late 80s or early 90s. As I transferred files to it from my 486 to get it working, it felt like working on PCs in the 90s felt. It’s not exactly an IBM PC/AT, but it feels pretty close.

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