Hewlett-Packard 9825 Repair Part 14—Installing a Crowbar Circuit

After spending months repairing the damage done to a Hewlett-Packard 9825 laboratory computer when a single transistor failed in its power supply, slamming 13 volts onto the 5 volt supply rail for the logic circuitry, destroying a total of 29 chips throughout the machine.

With repairs complete and full functionality restored, it’s time to say “never again” and install protection into the machine to prevent a recurrence. A crowbar circuit monitors the output voltage(s) from a power supply and, if an overvoltage is detected, instantaneously shorts out the supply rails, protecting downstream components and blowing the power supply fuse to shut things down and indicate there’s a problem. As it happens, Hewlett-Packard later appreciated the risk and added a crowbar circuit to later models of this computer, so it was sufficient to reverse engineer it and make copies to install in this older machine and others in the collection.