Geiger counters made by the US government for civilian use in the event of a nuclear war with the USSR or China are now readily available for purchase as surplus on sites like eBay and sites that cater to "Preppers."
Because even the last models of these geiger counters are 50 years old, any unit bought as surplus will require some overhaul before it can be trusted to work. There are model specific rebuild kits available from eBay and other places. Since you are here on Instructables, I'm assuming you want to do as much work as you feel competent to do instead of buying a reworked model.
The probe voltage should be checked first. All models of CDV-700's use a nominal 900 dc volt geiger tube. The current supplied is very very low, but if you don't feel comfortable with that voltage, then you should not do maintenance on the unit. The same high voltage exists on the circuits inside as well
In the photo above the probe voltage is measured by inserting a 1 billion ohm (1 Gigohm) 1% resistor into the #1 probe socket and grounding that through a microamp meter to the shell. There is no other way to accurately measure the probe voltage without expensive lab equipment. The meter must be capable of measuring to 10 picoamps (0.01 microamps). The meter shown was bought at Home Depot for $40. Lionels 6b's have a weak high voltage circuit and less than 150 million ohms load will noticeably affect the voltage. When using a 1 Gohm resistor, 0.97 microamps, as shown, means you have 970 volts on the probe. Between 900 and 920 volts is ideal, but 970v on a Lionel is acceptable. A Vic should not be operated with the voltage above 920 because of the way it's audio circuit is made. Overvolting a Vic 6b will destroy a component that cannot be replaced.