Monitor 4, "Radiation Alert"
S. E. International Inc, Summertown TN

These little GM counters have a typical failu the 500VDC voltage falls low, as the zener regulation diodes become damaged.

In the power supply circuit on the PCB, three zeners in series are used as feedback reference: one of 1N5272 at 110V, and two of 1N5281 at 200V (to create a 510V zener.)

I found that all three were damaged and leaky, resembling resistors of value between 1M and 4M. They still checked out as diodes. But they were conducting at much lower voltages than their rating. They caused the -HVDC value to be around -260V, so the geiger tube wasn't counting.

On the PCB, the first diode is connected between a 470K resistor and a 4.7M resistor which leads to the GM tube. All three diodes are in series.

I didn't have any 1N5281 on hand, so I tried a 1N5279 (180V, not 200,) and the unit woke up and worked fine. It might not give calibrated meter readings though, since the energy of each pulse might be down to ((510-40)/(510))^2, or 85% (since the HV supply is down by about 8%, if low by 20+20=40V)

Note that there's a trimmer pot on the PCB. Possibly for meter calibrate? I didn't change its setting, measured at 362 ohms.