Having tracked down a few problems in the IBM 1401, it's basically a matter of figuring out what the overall problem is, and then looking for a signal that indicates the problem. Then you work backwards through the circuitry, probing signals with an oscilloscope until you find something that might be the root of the problem. Then you can swap the potentially-bad card and hopefully it fixes the issue.

Of course, some times this is easy, and some times it is hard. Once you find the bad card, the restoration team likes to fix the bad card rather than just replace it. This can have its own challenges. I spent a long time trying to fix a card from the printer that ended up having a cracked trace that was intermittently bad. All the signals looked good, yet the card didn't work.

For a detailed look at a problem, I wrote a blog post about a core memory problem that turned out to be an inductor that failed open. After fixing that, the computer wouldn't power on. We replaced some weak transistors in the power supply, but that didn't help. Finally we found an undocumented fuse that had blown, probably from one of the boards we swapped. Replacing that got everything running again.

http://www.righto.com/2017/12/repairing-1960s-mainframe-fixi...

"restoration team" sounds like everything I like about old computers and general science fiction. Like a reference to Doctor Who or Philip K. Dick.

Anyway, thank you for those detailed information on the article.

Wow this link is fascinating! How fun that you got to be a part of that. I'm trying to imagine an IBM field tech back in the day being called to a customer site and having to go through similar with a customer who was renting one of these for exorbitant sums. It must have been stressful. I'm sure there was some sort of SLA.

I've talked to old-time IBM service engineers and IBM would do just about anything to keep a customer's computer running. An engineer could call his boss and say that a customer needed a very expensive replacement core memory, and IBM would send someone on the next flight to deliver it. If the customer was in the middle of nowhere, someone would drive out with a part and the engineer would drive to meet them halfway. There's a reason people say that nobody ever got fired for buying IBM.

IBM is the only company that will do a proper service contract with folks in rural areas. Heck, the IBM service tech arrived at our door with a new RAID card for our machine running IBM i before I knew there was a problem.