Last Friday I picked up this beast (and almost did my back in - it's HEAVY):

It's an IBM 3270AT. Basically, an early IBM PC/AT with some special hardware to talk to System/370 mainframes and a decidedly non-standard video solution (cards+monitor) driving it. The monitor is quite special, supporting 8 colours at 720x350 resolution over some kind of TTL connection (pinout clearly not CGA or EGA-compatible)

Here's some info I've been able to trawl up so far:
https://www.seasip.info/VintagePC/5271.html
https://en.wikipedia.org/wiki/IBM_3270_PC
There's a bit more on the 3270PC (an XT-based predecessor), but info on the 3270AT seems very thin on the ground.

Now, the machine is in optically excellent condition and was supposed to be working, but when the seller tried to show me this, it wouldn't boot (PSU shorted out) and on closer inspection a tantalum cap in the Host Connect Card was smoking. There's more than that amiss though, as even with the offending card removed it no longer shorted out but still showed no signs of life. That helped my negotiations on price considerably 😉 I wanted it regardless for the Model F 122-key Battleship, but seeing as I have the whole shebang anyway I might as well see what it can do and how much I can fix. I'm pretty well-versed in general troubleshooting, but lacking even basic documentation on this old beast means I'm pretty stuck.

Here's the guts:

From left to right:
- big old 190W Gresham PSU
- dual 5.25" FDD (one definitely DS/HD)
- full-height 5.25" MFM HDD (not identified yet)
- MFM/FDD controller card
- Host Connect card
- 3270AT Keyboard controller
- Serial+parallel card
- 128kB RAM expansion card (512kB on motherboard)
- 3-card combo Display Adapter

What I've determined so far:
- PSU is working fine and can boot a known good system.
- The 3-card video solution will give me output when transplanted together with the keyboard controller to another system.
- With the keyboard controller in the other system I can't get any response from the keyboard. This may be due to it outputting XT (not AT) to the DIN port. If I stick in a regular AT keyboard the other system does respond to it even with the keyboard controller card present.
- The Host Connect Card is indeed toast (fortunately I have no overriding need to connect to a System/370 mainframe at present 😉 )
- Supposedly the system can work as a normal AT without all the oddball cards, at least the 3270PC is documented to be able to work as an XT, so it is likely this one can too.
- The AT system board also looks dead. If I remove all cards and just stick in an ISA VGA adapter and a POST diagnostic card, it gets to 0A and hangs, In fact it hangs on 0A regardless of what is in there.
- No obviously dead caps on the system board.

Now, my first preference would be to get this old tank running again. Failing that, plan B would be to use the IBM 5272 monitor for something sensible, at least if I could find a way to feed it with something smaller than the three-card monstrosity it comes with, ideally sending it EGA if possible.

Questions for the main goal:
- Does anybody have technical documentation on the 3270AT, i.e. the 5273 system unit and the 5272 monitor?
- Can anyone confirm it should indeed work as a normal AT with the 3270 keyboard controller (and its custom ROM) removed?
- What does "0A" POST code mean on an original PC/AT? This suggests it should be "Soft Reset Tested", but that doesn't sound like something to hang on.

Questions for the secondary goal:
- What is the pinout of the DE9 connector from the 3270PC/AT Display Adapter to the 5272 monitor?
- Any experiences with/resources on using it with any other card than that?