HP 33120A Repair: Shutting Down the Eye of Sauron

When a friend of [Tom Verbeure] came into possession of two HP 33120A 15 MHz function/arbitrary waveform generators, he could not resist giving them a try. Although not exactly high-end units, the HP 33120A makes for a pretty nice unit for a home lab. During the first test run, however, [Tom] discovered that one of the units had a dead output, which made it rather useless. Undeterred, [Tom] set to work diagnosing it, helped by the repair manual and full schematics.

While the cause was quickly tracked down to the general area around an exploded MLCC, fixing the fried Zener diode that may have initiated the short on the -15V rail revealed an unpleasant surprise. To [Tom]’s horror, he saw a portal to Hell itself open when part of the PCB caught on fire due to an internal short. After making sure to capture a video of this event, he then proceeded to use a thermal camera to track down the hot spot and uses a drill to remove the short.

While one can argue with the use of a drill to remove shorts on inner layers of a PCB, ultimately the fix was effective. A look on the schematic and comparison with the functioning 33120A unit later, all it took was two bodge wires to restore functionality. After this event, [Tom]’s friend gave him the repaired unit as thanks, and definitely not because [Tom] had begun to refer to it as ‘his precious’.