Repairing an HP 5370A Time Interval Counter

Summary

Introduction I bought an HP 5370A time interval counter at the Silicon Valley Electronics Flea Market for a cheap $40. The 5370A is a pretty popular device among time nuts: it has a precision of 20ps for single-shot time interval measurements, amazing for a device that was released in 1978, and even compared to contemporary time interval counters it’s still a decent performance. The 74LS chips in mine have a 1981 time code which makes the unit a whopping 44 years old. But after I plugged it in and pressed the power button, smoke and a horrible smell came out after a few minutes. I had just purchased myself hours of entertainment! Inside the HP 5370A It’s trivial to open the 5370A: remove the 4 feet in the back by removing the Philips screws inside them. remove a screw to release the top or bottom cover (Click to enlarge) Once inside, you can see an extremely modular build: the center consists of a motherboard with 10 plug-in PCBs, 4 on the left for an embedded computer that’s based on an MC6800 CPU, 6 on the right for the time acquisition. The top has plug-in PCBs as well, with the power supply on the left and reference clock circuitry on the right. My unit uses the well known HP 10811-60111 high-stability OCXO as 10MHz clock reference. The bottom doesn’t have plug-in PCBs. It has PCBs for trigger logic and the front panel. This kind of modular build probably adds significant cost, but it’s a dream for servicing and tracking down faults. To make things even easier, the vertical PCBs have a plastic ring or levers to pull them out of their slot! There are also plenty of generously sized test pins and some status LEDs. High Stability Reference Clock with an HP 10811-60111 OCXO Since the unit has the high stability option, I have now yet another piece of test equipment with an HP 10811-60111. OCXOs are supposed to be powered on at all time: environmental changes tend to stress them out and result in a deviation of their clock speed, which is why there’s a “24 hour warm-...