The Epic Journey Of Repairing An HP 9830A Desktop Computer system From The 1970s

We adore our retrocomputers below at Hackaday, and we’re constantly delighted to see another person rescue an historic artefact from the landfill. In some cases, all it takes is replacing a broken ability switch or leaky capacitor other periods you have to have to deliver out the oscilloscope and dig further into internal circuitry. But the large amount of work [Jerry Walker] put into bringing an HP 9830A back again on its ft is a thing you really do not see pretty generally.

If you are not familiar with the HP 9830A, it is a desktop computer from the early 1970s, absolutely built from discrete logic gates. The device on [Jerry]’s desk turned out to be totally useless, with not even the supporter spinning up. This was prompted by a dodgy ability switch, but replacing that change was just the starting: there were being quite a few terrible elements inside of the electric power supply as properly as a large amount of money of moist filth on the back again of the motherboard. Following a complete cleaning and the substitution of many unsuccessful parts, all four energy rails were functioning inside spec all over again.

Next came the advanced process of debugging the 4 PCBs that jointly make up the CPU. [Jerry] hooked up his logic analyzer to these boards to visualize the CPU’s interior point out as it stepped through its microcode. Once more, several TTL chips turned out to be damaged and desired replacement. With the CPU operating, it was time to get to the primary software ROM, which finished up consuming much of [Jerry]’s mend time.

The problem with damaged ROM chips is not so much finding a alternative, but obtaining a suitable copy of their contents. Thankfully, a cycle-exact HP 9830a emulator exists which has copies of the overall firmware. With many of the dozens of ROM chips dodgy or dead, repairing the initial ROM board was not a real looking proposition, so [Jerry] built a full substitution board instead. With a fashionable, reprogrammable EPROM chip and some glue logic it was also a huge assistance in the relaxation of the mend procedure, as it permitted him to operate customized equipment code on the personal computer.

The alternative ROM board is made up of just a single EPROM and some decode logic.

With a several exam routines prepared into the EPROM [Jerry] was equipped to check all the RAM, ROM and registers as perfectly as the exhibit, and found but a lot more broken chips to be changed and faulty connections to be repaired. With all the most important functions now functioning, the HP 9830A came to lifetime and responded to commands yet again. The only remaining difficulty was in the tape travel, which turned out to be brought about by a damaged sensor. The moment that was replaced, the repair approach was finally complete.

[Jerry]’s operating commentary on his 26-section movie sequence provides tons of terrific insights into the intricacies of personal computer architecture, so if you are seeking for something to binge watch, forget about Netflix and place on [Jerry]’s films alternatively. They also display the significance of a systematic troubleshooting technique: start out with the most bare-bones system probable, verify that it will work accurately as supposed, and only then get started adding other elements.

If you just cannot get more than enough of vintage HP equipment repairs, verify out this mend of the associated HP 9825. The ongoing process of restoring a Centurion Minicomputer is also unquestionably truly worth following.

Many thanks for the suggestion, [mgsouth]!