[CuriousMarc] is at it again, this time trying to undo the damage from a poorly designed power circuit, that fried the internals of his HP 9825 computer. (Video, embedded below.)
The power supply on this particular model has a failure mode where a dying transistor can lead to 13 V on the 5 V line. This causes all the havoc one would expect on the internals of a 1970s era portable computer. This particular computer is rather rare, so instead of calling it a lost cause, our protagonist decides to replace the faulty transistor, install a proper overvoltage protection circuit, and then start the tedious hunt for which chips actually let their magic smoke out.
It helps that [Marc] has a second, known-working machine, so he can isolate individual parts and identify problems. There were plenty of problems, and a handful of false positives, but after replacing four blown chips, the machine finally boots again. There are eight episodes of troubleshooting, and they’re full of information and troubleshooting tips for this era of hardware. Set a couple hours aside, and enjoy the trip down the retro repair rabithole.
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