Quick IBM IO Tester Restoration (Patreon)

You probably remember seeing us unbox this big gray box recently. It is a donation from viewer Brad Proffit (thanks Brad!). 

He did not know what it was, but Carl quickly identified it as an IBM IO Tester. It was a piece of debugging equipment from the 1960's, for the IBM 360 peripherals. You could hook it up to a supported printer, card reader, or disk controller, and it would give the customer engineers a blinkenlight panel on which to debug them. A primitive logic analyzer really.

Ken quickly went on his usual reverse engineering path and twitted about it: https://twitter.com/kenshirriff/status/1447666855447957508 

It's simple, yet more complicated than you'd expect, with a big voltage power supply and many SLT modules, IBM's famous metal squares. They are hybrid on ceramic circuits, made with individual transistor and diode chips, and predate integrated circuits. The tester uses the exact same building philosophy as the IBM 360.

Nice SLT modules and IBM multilayer PCBs. 

Ken was able to power it up on its second attempt using external power supplies.

He managed to demonstrate bit sensing on a few lights.

Wow, blinkenlights! This is the light test, and quite a few are not working. Almost none of the switches did work.

I repaired the special IBM lights, which curiously were 3V and only 15mA, not that bad compared to a LED really. I was able to find replacement internal bulbs but they were not made to be internally replaceable and it was a royal pain to change them.

Switches are high quality, C-H branded, and you can take them completely apart. I had to machine a few new parts to repair some that were completely broken, but most needed contact cleaning only.

Al Kossow of CHM and bitsavers.org fame chipped in, and gave us several of the front panel inserts that we were missing.

I reprinted the faded bottom insert, and voila, looks and works like new:

Now what to do with it? We could attach it to my IBM 2501 - we actually tried, and indeed we found the matching service plug for it at the back. But the card reader is far from working yet, still in the garage.

In the meantime, I am thinking of making it display a clock when not in active use, and Ken is working on a secret, more elaborate program (maybe it will mine a bitcoin?).  What do you think we should do with it?

Marc