My former AP Chemistry teacher recently asked me to look at these two CDV-700 Model 68 Geiger counters he would use to talk about radioactivity in class. Back in the day, the American Nuclear Society's local branches in different states would distribute this particular model around to various schools.
The goal of these programs would be to provide students with a physical understanding of radioactivity. If I recall correctly, I remember seeing these exact devices in class when I was a student in high school a few years ago.
Now I am a student of nuclear science and engineering. So I guess the ANS project worked.
The two CDV-700 devices I was given to check over were not working properly. The first thing I did was open them up. Opening the CDV-700 is easy because the device was designed to be easy for hobbyist repairs, the top (including the meter and GM tube holder) comes off to expose the battery holders, circuit board, and level selector switch.
The manufacturer left a circuit diagram in the metal case. The circuit diagram includes troubleshooting points (locations in the circuit where certain DC voltages are expected). I found that the HV calibration was a little low. I was not sure if this was intentional or happened when somebody dropped the device. I reflowed the leads to the flyback transformer that creates a 900V standing rail for the GM tube.
When ionizing radiation enters the GM tube, it momentarily excites the gas in the enclosed tube. This causes a short circuit and the flyback transformer deposits some electrical energy into a circuit that both creates a pulse for a speaker as well as charges a needle in the device's meter. The amount of charge deposited to the needle (over a selected resistor for scale) is related to some amount of effective dose (in mR/hour).