We love our computers in hackday. We are always happy to see someone salvage a historical relic from the landfill. Sometimes, just replace the damaged power switch or leaking capacitor; At other times, you need to take out the oscilloscope and deeply study the internal circuit. But [Jerry Walker] has done a lot of work to get the HP 9830a back on its feet, which you rarely see.
If you’re not familiar with the HP 9830a, it’s a desktop computer from the early 1970s, built entirely from discrete logic gates. (Jerry) the machine on the table is completely dead. The fan doesn’t even turn. This is caused by an unreliable power switch, but replacing the switch is only the beginning: there are several bad parts in the power supply and a lot of wet dirt on the back of the motherboard. After the power supply and the four parts are completely replaced within the range of several specifications.
Next is the complex task of debugging the four PCBs that together make up the CPU. [Jerry] connects his logic analyzer to these circuit boards to visualize the internal state of the CPU as it passes through its microcode. Again, several TTL chips proved to be bad and needed to be replaced. When the CPU is working, it’s time to enter the main program ROM, which eventually consumes most of [Jerry’s] repair time.
The problem with a damaged ROM chip is not to find a substitute, but to get the correct copy of the content. Fortunately, there is a cycle accurate HP 9830a emulator that contains a copy of the entire firmware. Since several of the dozens of ROM chips are unreliable or dead, it is unrealistic to repair the original ROM board, so [Jerry] designed a complete replacement board. With a modern, reprogrammable EPROM chip and some glue logic, it is also a great help to the rest of the maintenance process because it allows him to run custom machine code on the computer.
By writing some test programs in EPROM, jerry can test all ram, ROM, registers and displays, and find that more damaged chips need to be replaced and wrong connections need to be repaired. Now that all major functions are working properly, the HP 9830a comes into play and responds to commands again. The only remaining problem is the tape drive, which was originally caused by a broken sensor. Once replaced, the maintenance process is finally completed.
[Jerry]’s continuous comments on its 26 episode video series give you a lot of insight into the complex computer architecture, so if you want to find something to watch, forget Netflix and replace it with [Jerry]’s video. They also show the importance of system troubleshooting: start with the simplest possible system, verify that it works exactly as expected, and then start adding other components.
If you can’t get enough old HP gear repairs, check the relevant HP 9825 repairs. The ongoing process of repairing Centurion minicomputers is also absolutely worth following.