Simplex Clock Correction Issues

I’m feeling a bit nostalgic this morning. Readers of a certain age will remember the classic Simplex clocks from school days. The clocks throughout the building had a special feature – a receiver and small control system that would permit the clocks to be synchronized or changed throughout the building, using a master control device. Useful to keep all the clocks at the same time, easily adjust for seasonal daylight savings time changes, and to reset the clocks in the event of power loss.

There’s a good overview of these systems here:

The Synchronous Wire system is the most popular system in the United States. Clocks are run using a power circuit that acts as its time base. The clocks receive an hourly correction which synchronizes both the minute and the second hand. Every 12 hours the clock receives a daily correction to keep the clock perfectly synchronized.

When I first started working in the medical imaging field (circa 1989), we’d run into issues with these clocks, a lot. Hospitals and health-care facilities were big users of these (there’s a clock in every patient room, hallway, and procedure room), and one particular piece of equipment (a Phillips “Classic” generator) was particularly susceptible. The generator used a motor-driven, linear variable autotransformer (think Variac or Powerstat) to adjust for line voltage changes – and the signal injected onto the mains by the clock controller (typically around 3500 Hz) would mess with the voltage regulation circuit, and the motor would “hunt” for the duration of the pulse (usually 5 or 10 seconds), The generator would be disabled or locked out while the motor was moving, and this would drive the docs and techs crazy (since it happened hourly).

Simplex Clocks

A waveform sample from a PowerLines trip report circa 2007, using Rx Monitoring Services power analyzer to capture the clock correction pulse.


I’ve also come across a few old power quality threads discussing these clock pulses causing standby / hybrid UPS systems (notoriously sensitive to anything that might indicate the start of an actual outage) to switch to inverter improperly.

Back in the day, the old BMI-4800 power analyzer had a “high frequency noise” detector which looked at broad spectrum harmonics or noise, and output a distinctive “picket fence” 24 hour log when these clocks were present (I still miss this diagnostic / reporting feature on modern power analyzers). I’m sure I have an example of this graph kicking around somewhere but can’t put my paws on it at the moment. I suspect any graphs I recall pre-date digital images (when I would create reports with blank boxes, and manually paste in photo-copied disturance graphs) so I’m not finding anything in trip reports or old PowerPoint presentations)

Resolving these issues? Sometimes we’d consult with the facility engineer – oftentimes these were turned up to “10” (maximum) and we could get the amplitude turned down to the point where it worked but did not cause problems. Sometimes we’d get the clocks reprogrammed to only correct 2x a day (noon and midnight) when it would be unlikely to affect the equipment. Some resourceful field techs developed a filter circuit to protect the regulation circuity; although that was sometimes not permitted (FDA requirements for x-ray equipment forbids modification or retro-fitting).

I don’t hear too much about these lately. Clocks are now often digital, controlled wirelessly or via ethernet. Switched-mode power converters have replaced old analog systems.