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Last week, the vintage IBM 1401 computer at the Computer History Museum started behaving strangely: it wouldn't halt. More specifically, if you had two HALT instructions in a row, it would halt for the first, but when you continued, it crashed mysteriously. Here's how we fixed it.... 1/N

The IBM 1401 computer lets you run code a single cycle at a time, so you can see what happens at each step. (The arrow on the control panel.) But when I tried to single-step through the bad HALT instruction, the computer immediately died in a distant memory location. Now we had two problems. 2/N

I hooked up an oscilloscope and discovered that the computer wasn't running a single instruction cycle. Instead, it ran a few cycles (the pulses below), stopped briefly, ran a few more, irregularly stopping and starting, "building up steam" until it ran thousands of cycles. But why this pattern?

It turned out that the computer's START button had a strand of wire that shorted one of its contacts. As a result, it would keep re-starting as long as you held the button, blowing through the HALT or single-step until it crashed. The contacts bounced a bit, causing the random gaps we saw.

With the switch fixed, everything worked. At least until next time... I worked on this with @curiousmarc.bsky.social, @tubetime.bsky.social, @ymatyt.bsky.social, and others. I'll leave you with this documentation of the relevant circuitry with inscrutable IBM symbols. end/N

Nice work, that must have been a satisfying fix!

I was so happy I got to see that machine and everything else at the museum back in April. I was in awe at everything I saw and keep thinking back on it now and then. Ya'll do amazing work. :)