Easy and cheap CDV-700 tube adapter

I own a CDV-700 geiger counter of which the tube stopped working. It turns out that replacement tubes are both hard to find and expensive. The simplest option is to buy the LND 720 which basically is a drop-in replacement that should cost about $100.

It’s also possible to use cheaper, easier to find tubes with the CDV-700.  I found the following post from radio amateur K0FF on the “Geiger Counter Enthousiasts”  group on Yahoo groups. It describes a simple circuit to adapt tubes of varying voltage to the CDV-700. I built one, and it works!

I created a schematic to help people build this very handy circuit:

Parts:

R1 = 3.3 megaohms resistor 0.25 watts
R2= 1.8 megaohms resistor 0.25 watts
C1 = 10 nanofarad capacitor, rated 2 kilovolts
D1 – D3 = any number of zener diodes, the values added together  should be the desired voltage to regulate to

You can build this circuit in any way you want, keep in mind that there is high voltage involved, make sure that everything is properly insulated and that bare conductors are not closer to eachother than 2mm to prevent any arcing.

This circuit is basically a voltage regulator such as a 7805 built with discrete components and for higher voltages. To regulate to 500V for example, I used one 200V and two 150 zener diodes. 500V is a common voltage used by GM tubes.

With this circuit you are no longer limited to the more rare 900V tubes and you can connect all kinds of tubes to the CDV-700.  Keep in mind, however, that the CDV-700 may no longer display correct measurements in milliRöntgens per hour since the scale is calibrated for use with the 6993 tube.

 


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