The Epic Journey of Repairing an HP 9830A Desktop Computer From the 1970s
We love our retrocomputers right here at Hackaday, and we’re at all times delighted to see somebody rescue an historic artefact from the landfill. Typically, all it takes is changing a damaged energy swap or leaky capacitor; different instances it’s essential deliver out the oscilloscope and dig deeper into inside circuitry. However the big quantity of labor [Jerry Walker] put into bringing an HP 9830A back on its feet is one thing you don’t see fairly often.
In case you’re not aware of the HP 9830A, it’s a desktop laptop from the early Seventies, totally constructed from discrete logic gates. The machine on [Jerry]’s desk turned out to be fully useless, with not even the fan spinning up. This was brought on by a dodgy energy swap, however changing that swap was just the start: there have been a number of dangerous elements inside the facility provide in addition to an enormous quantity of moist filth on the again of the motherboard. After an intensive cleansing and the alternative of a number of failed elements, all 4 energy rails had been operating inside spec once more.
Subsequent got here the advanced process of debugging the 4 PCBs that collectively make up the CPU. [Jerry] connected his logic analyzer to those boards to visualise the CPU’s inside state because it stepped by its microcode. Once more, a number of TTL chips turned out to be damaged and wanted alternative. With the CPU working, it was time to get to the primary program ROM, which ended up consuming a lot of [Jerry]’s restore time.
The issue with damaged ROM chips is just not a lot discovering a alternative, however getting an accurate copy of their contents. Fortunately, a cycle-accurate HP 9830a emulator exists which comprises copies of all the firmware. With a number of of the handfuls of ROM chips dodgy or useless, repairing the unique ROM board was not a sensible proposition, so [Jerry] designed an entire alternative board as an alternative. With a contemporary, reprogrammable EPROM chip and a few glue logic it was additionally an enormous assist in the remainder of the restore course of, because it allowed him to run customized machine code on the pc.
With a couple of take a look at routines written into the EPROM [Jerry] was capable of take a look at all of the RAM, ROM and registers in addition to the show, and located but extra damaged chips to get replaced and defective connections to be repaired. With all the primary features now working, the HP 9830A got here to life and responded to instructions once more. The one remaining situation was within the tape drive, which turned out to be brought on by a damaged sensor. As soon as that was changed, the restore course of was lastly full.
[Jerry]’s operating commentary on his 26-part video sequence provides a number of nice insights into the intricacies of laptop structure, so if you happen to’re searching for one thing to binge watch, neglect Netflix and placed on [Jerry]’s movies as an alternative. In addition they present the significance of a scientific troubleshooting technique: begin with probably the most bare-bones system potential, confirm that it really works precisely as meant, and solely then begin including different elements.
In case you can’t get sufficient of classic HP gear repairs, try this repair of the related HP 9825. The continuing strategy of restoring a Centurion Minicomputer can also be undoubtedly value following.
Thanks for the tip, [mgsouth]!
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