BNC Mod to CDV-700 Probe Cable

Many common external geiger counter probes have a BNC female post connector. shows Eberline HP 260 pancake probes The Office of Civil Defense CDV-700 was not designed for probe swapping, but there are 900 volt probes like the HP-260 easily obtainable on eBay.

By putting a male BNC connector on a CDV-700, the beta/gamma pickle probe can be upgraded to an alpha/beta/gamma frisker.

Step 1: Parts You Will Need

The aluminum tube is made from an electrical crimp connector (21mm OD x 16mm ID x 91MM L) available on eBay for a couple of dollars.

The threads on the CDV-700 pickle probe are 13/16" x 20 threads per inch. This is "extra fine."

The die needed is listed above.

The connector is BNC to F (regular coax).

I happened to have a 3-pin socket extender made for the CDV pickle probe, but a wire ending in a molex pin would work just as well.

Step 2: Assembly

The tube is chamfered a little and threads die cut on one end. You only need about 5 threads. After threading, cut the tube in half with a common plumbing wheel cutter. The BNC connector has a couple of metal washers added to make it roughly the same inside diameter as the tube and a nut added to the end.

Using a rotary grinder, the threaded end of the tube is enlarged inside so that the tube will screw down over the socket base inside the CDV you are mounting it on. The rest of the inside of the aluminum tube must just allow the socket and BNC adapter to slide through. That way when the tube is screwed down the inside assembly will not twist up the connecting wires.

A 4700 ohm resistor is used to connect the center pin of the F end to the #1 pin. Pins #2 and 3 are bridged and grounded behind the nut on the BNC. In case you aren't sure, the #2 and 3 pins are the ones closest together.

The 4700 ohm resistor is used to protect the CDV-700 power supply in case of a short. A better place to put the resistor is just coming off the HV supply on the CDV circuit board, but that may not be possible.

I drilled and tapped 3 holes for set screws to lock the inside assembly once the tube has been threaded into the CDV-700 probe base. If you intended this adapter to be left mounted on a particular CDV, then a few drops of high expansion Gorilla glue in the set screw holes before putting them in will make the assembly permanent.

Step 3: Preventing False Counts

The whole assembly needs to be insulated. The pickle probe base on the CDV-700 was not grounded originally. #2 and 3 pins go down the cable braid, but the socket potting material and wiring under socket base leave the whole metal pickle probe isolated from the CDV electronics.

Using a metal tube and threaded BNC to F connector grounds the pickle probe base. The CDV-700 shell is positive gounded to one side of the batteries inside and to the cable braid. Positive grounding was common in the day these were made.

Simply touching the probe base can result in false counts depending on your body's capacitance. I used a short piece of heat shrink tubing to insulated the adapter and make the assembly look more professional.



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