Introduction: Alternate Probes for a CDV-700 Geiger Counter
The US government issue CDV-700 geiger counter came with a hard-wired pickle probe that had a 900v beta/gamma halogen quenched thin-wall metal tube. There were a limited number of 900v tubes issued by FEMA that had 1/10 the sensitivity of the originals to increase the range of the CDV. The more sensitive tube was the OCD-D-103 [aka "6993"] and the less sensitive the OCD-D-101. There was also a very limited number of direct connect alpha wands issued named CDV-700RP. The CDV-700RP can still be bought special order from SE International.
CDV-700's do not have a power supply capable of directly driving most scintillation probes. The power supply was made weak to extend battery life, as the CDV-700 was made for nuclear war/end of civilization survival.
It is possible to swap the original 6993 tube for any common geiger tube as long as the driving voltage of the substitute is no higher than 900v. 1 watt or 5 watt axial Zener diodes driven backwards will supply more than enough current for most common geiger tubes.
NEVER hot swap a probe! CDV's have weak transistors powering the high voltage and audio circuits and it is very easy to short them out. Turn the unit off and pull the batteries before doing a probe swap.
The array of probes in the box on the right is 12 Soviet SBM-20's with the 500v of zeners inside to allow for direct 900v operation. Even a Lionel 6b can supply enough voltage to drive the cluster.
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Step 1: Zener Diode and BNC Adapters
Most commercially built probes come with a BNC connector. Ludlum and Eberline had their own BNC type fittings, but adapters exist to mate those proprietary adapters to a standard BNC.
The top of the pickle probe unscrews. The CDV-700 geiger tube seats in a 3-pin tube socket. It simply pushes in or pulls out. NEVER twist the tube or squeeze it while inserting or removing. Sometimes the gasket under the tube gets gooey with age and the tube has to be rocked gently while pulling.
1N5378BG [100v] and 1N5368BG [50v] zener diodes are cheap and easily available from eBay or Octopart. Those values and some 10M ohm 1/8 watt resistors from Radio Shack are needed to make the adjustments to voltage and anode resistance to make alternate probes work.
The small black adapters in the center picture were made from a BNC panel connector and an Allied-Cooper Interconnect 78-s3s three-pin socket. Pin 1 is hot, 2 and 3 are both ground. Because CDV-700 probe voltage varies from 900 to 950v, two adapters are made.
One adapter has no diode, and one has a 50v zener diode is installed inside inline with pin 1 and with the diode arrow facing the CDV-700. That allows a quick 50v drop if the host CDV is 950v instead of the assumed standard 900v. To check the voltage on a CDV-700, turn it off, pull the tube and ground pin 1 to the shell using a 1 gigohm resistor. Measure the current flowing with a meter that can read to 0.01 micro amps. 0.95 uamps would be 950 volts.
Step 2: Cables and Adapter Construction
The cable in the left photo has five 100v zeners inside in series turned to face the CDV. That make a fixed voltage drop cable for probes that can't be modded internally with enough zeners to make them work on 900v.
As an example, a Soviet SBM-20 tube works well on 400v. Either I need five 100v zeners in the cable or I need them inside the probe. The Soviet SI3-BG and a Phillips 18504 alpha tube also work on about 400v so the same cable would be used.
The 10M ohm resistors can also be added to a probe or cable as needed. CDV-700's have a 3.3 Mohm resistor inside, but the Phillips tube needs about 10M to be stable. The two small probes at the top right were made from gutted LED flashlights bought at Home Depot. The Phillips 18504 inside has a 10M ohm resistor added inline with the anode [ground].
The black box in the middle with a toggle switch is wired to pass through or drop either 300 or 500 volts by flipping the switch. That adapter takes the place of two cables.
Note that all my probes and cables have permanent labels on them. The label has the name/make of probe, it's proper voltage, and zener drops inside and if it needs extra anode resistance. I have a Soviet neutron probe that is twitchy unless it has at least 30M ohms of anode resistance, so I made a special BNC cable with 30M ohm inline for it.
Step 3: In Action
The left photo shows a Vic 6a with 2 adapters powering an LND 7807. The smaller adapter has no diode and the switching adapter is set to drop 450v. The writing on the middle of the PVC says "unshielded section." The probe inside has a metal shield to block betas. The photo on the right is the same Vic 6a with stock 6993 tube. Both are shown reacting to a piece of orange Fiestaware with natural Uranium glaze. The yellow can unscrewed in the right photo is a passive piezo speaker made for CDV-700's that I bought off eBay.
No tools are required to make the swaps.
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