I managed to get the Geiger counter kitset working.
I believe it was a problem with the high voltage leaking through some flux. Either way, a good cleanup around my solder joints seemed to fix the problem.
The Geiger Müller tube is a is a Russian model, marked with the code СБМ20 – There are lots of results on google for searches for “CBM20”, but very little of use regarding the actual specs of the tube.
I’ve been trying to measure the voltage across the Geiger Müller tube. It turns out that with no capacitor regulating the high voltage, the 10MΩ internal resistance of my multimeter and oscilloscope is enough to drop the voltage significantly. The normal way to get a reading is to add 90MΩ in series with the meter. This effectively creates a 10:1 voltage divider while increasing the total resistance of the measuring circuit to 100MΩ.
Unfortunately, even with 100MΩ across the high voltage, the voltage on the tube seems very low. The image below shows the min voltage to be around 50V and the max about 100V. All I can assume is that the lack of a capacitor is creating this issue.
At the end of the day, the counter works and seems to “click” about 15 times per minute, which is in line with expected background radiation rates.
I’ve been searching the internet and have found a regulated high voltage supply using an inverter from a cold cathode lamp. I’m in the process of sourcing the parts and then will see if I can get a more regulated power supply. I believe I need about 400 Volts DC, but until I can find more information on this tube, even that is unknown.