CDV-700 Maintenance

by DavidN71

12K1636

Intro: CDV-700 Maintenance

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.

STEP 1: Part 2

If a Vic or Lionel probe voltage is too high, the corotron has failed. The corotron is the glass tube pictured. Any corotron with the Rad trefoil has failed because of age. Those used a radium pit with 22 year half-life. Less than 1/8 of the original radium is left by reason of age no matter how much or little use the unit saw. The solid state replacement shown next to the glass corotron is simply a string of 300v zener diodes turned backwards with one more diode added to make the string come close to 910 volts. The tolerance on most 300v zeners is 5%, so three are strung and the string measured, then one lesser value zener is added to make the proper drop.

The last models of CDV-700 were numbers 6, 6a and 6b. Almost all the units available surplus are Victoreen 6a and 6b, and the Lionel 6b. The ENI model should be avoided because the circuit was marginal even when new. ENI's use 4 D-cells, Vic's use 4, Lionels use 2. Anton Labs became Lionel, so Antons are at least one model older and use 5 D-cells.

Technology has changed so much in 50 years that you have to make certain substitutions when doing maintenance. Alkaline cells didn't exist when the CDV-700 was made, batteries were carbon-zinc chemistry. Carbon-zinc was lighter, had far less capacity and had a paper shell. Modern alkaline and "heavy-duty" manganese chemistry cells are cased in a steel can that is also the anode. The metal battery clips in the CDV should be wrapped in friction or electrical tape so the clip doesn't short out one battery. Because of the way both Vic and Lionel battery holders are made and wired, one battery will short out if the clip is tight enough to touch both batteries at the same time. It's also a good practice to get some AA to D cell plastic case converters and simply use AA alkaline batteries instead of D cells. AA alkaline cells weigh and have the same capacity as the original carbon zinc cells.

STEP 2: Part 3

The same tube (OCD-D-103) commonly know as the 6993 was used on all CDV-700 models. Number 1 pin was always hot, and either #2 or #3 was the anode, depending on who made the tube.On most models of CDV the #2 and #3 pins were tied together under the socket with a wire, but this is not always the case, so it it is possible to have a mismatch between tube and socket resulting in an open ground. Remove the batteries and tube and check between socket base #2 and #3 with an ohm meter. If it is not zero ohms, then check each pin to the CDV upper housing to see which is ground. A thin bare wire strand wrapped between #2 and #3 on the tube is the preferred fix. No soldering, or probe rewiring is needed.

Do not buy an OCD-D-101 tube for your CDV. Those tubes were made to increase the range of the CDV-700 so it overlapped the bottom range of the CDV-715/7/20 ion chamber series. It is a conversion and ruins the use of the CDV-700 for contamination checking because it dulls the response of the unit by a factor of 10 on all scales.

If you must disassemble the probe, remove the batteries, cut the cable flush with the probe bottom, remove the tube, screw the probe halves back together, and use a 4mm pin punch to drive the black probe socket out. This will destroy the socket because they were epoxyed in place. An Allied-Cooper Interconnect 78-s3s is an exact replacement for the black female socket insert.

The probe cable has no exact modern replacement, however any high quality RG58 a/u or RG174 can be used bearing in mind that almost 1kv will be present on the center conductor. Steel stranded cable is preferred but hard to find except as surplus. Belden 83264 RG179 will also work. "A/U" cable has a stranded center conductor. Do not use a cable with a solid center wire.

STEP 3: Part 4

The original high voltage rectifier diode was selenium based. Modern fast switching diodes are silicon and work just fine. The minimum value should be at least 2kv. The R6000 (6kv, 0.2amps) diode is the best I know of and fairly cheap.

The transistors used in CDV-700 were mostly germanium based. Only the 2N404 germanium remains commonly available MCM Electronics sells them cheap. They are usually used to make special effects boxes for guitars and amps. Russian military surplus MP16b PNP germanium transistors will replace and outperform a 2N404. The base of an MP16b is bonded to the metal shell. With the leads face up and the base to the left, the collector lead will be at the top. All three transistors in a Lionel can be replaced with an MP16b. Do not use any modern silicon transistor in any CDV unless the original was silicon. Silicon transistors require a different bias to oscillate.

STEP 4: Part 5

The meters were 50 milliamp full scale. The meters on the CDV ion chamber series have a differently labelled face but are also 50 milliamp full scale and can be used as a desperation spare if you ignore the range labels.

28 Comments

JasonB337 1 year ago

Well....I bet I've smoked that unnamed unobtainable component. I was using a regular old DMM to try and set that voltage one day and it suddenly quit making the high pitch whine. So it's probably toast. Anyone got some winding resistance readings for that unnamed irreplaceable component?

Erin Tyres 2 years ago

Thank you for your advice. You are completely right when you write that the high voltage can only be measured using a very high resistance. My Fluke meter only reads 355 volts on the Geiger counter probe, because its 10 Meg ohm input resistance loads down the circuit. When measured correctly, the Geiger counter voltage is more than 900 volts.

Since I do not have any meter that could read a fraction of a microamp, I measured the high voltage using a voltage divider. I connected the Geiger counter high voltage, a 1 gigaohm resistor, and an oscilloscope with a 1X probe in series. The scope's input resistance is about a thousandth of the resistor, so it reads about a thousandth of the high voltage. This approach will not load down the Geiger counter circuit, so it should be possible to get a correct measurement.

My Fluke meter shows that the oscilloscope and its probe measure 999500 ohms. So the exact formula for my measurement was to take the scope reading and multiply it by 1000999500/999500 or 1000.5

To get a precise reading from the oscilloscope, make sure to set the scale to 50 millivolts per division or less, then turn the position knob so that the voltage to be measured is displayed. Almost all scopes measure with 8 bits of precision. So if the scope's screen is 8 divisions high, the total range of measurement at 50 millivolts per division is 0.05 x 8, or 0.4 volts, and each reading will have a resolution of 0.4/128, or 3.125 millivolts.

To check that the measurement method works, I used it to measure the AC line voltage. My Fluke multimeter measured 122.05 AC volts. The scope and resistor showed a sine wave with a peak at 0.171 volts. That's good! 0.171 peak voltage divided by the square root of 2 gives an RMS voltage of 0.1209. Multiply that by 1000.5, and you get 120.98, which is close to 122.05.

When I connected the Geiger counter to the resistor and the scope, it showed a DC voltage of about 0.9 volts. I found that adjusting the counter's high voltage potentiometer gave a maximum of .908 volts, which means my CDV-700 high voltage circuit is about 908.5 volts.

simplyscience 3 years ago

Hi, Hope you still respond to inquiries here as the last one looks like it was at least a year ago. I have several questions as I have the Lionel version:

1) Both of my metering transistors are burned out from a bad probe cord shorting. Is the 2N404 (which I have in my parts boxes) a direct pin for pin replacement for these transistors?

2) Speaking of shorting, is there any way to protect the transistors to prevent such a future occurrence? There is another instructable on here saying to insert a 4.7K resistor in series with the HV lead, but I've not been able to find that anywhere else.

3) Is there any problem using transistor sockets for easier changeout should this happen again?

Thank you in advance for your response.

DavidN71 3 years ago

I assume you mean the HV transistors fried from a shorted probe cable. In that case the 2N404 is an exact replacement if you can still get them. Otherwise the Ukraine/Russian MP16b works. I bought a couple of hundred of the Russian germanium transistors years ago and they have a metal shell so they are less likely to blow from overdriving. But the metal shell is base so you have to be careful about it touching another component on the board. The main thing is the transistors must be PNP and germanium (not silicon) or the bias will not be correct and they may not oscillate correctly to build up voltage. The 4.7k ohm resistor is used when doing a BNC mod, although you should never hot swap probes anyway. If you just want protection for the HV circuit without doing the BNC mod, then insert a 4.7K ohm 1/8 watt carbon resistor inline with the hot lead. Follow the center lead from the probe cable back to the board and put your resistor inline somewhere there.
I have never used transistor sockets. If you blow them that often, you should fix the first cause.

mtcooppdx 4 years ago

hi, I just found this site - landed here googling for help on CDV 700. I have replicated your setup to measure voltage at the probe connector. I have the same meter, and using the 1 gigohm resistor as you did, I am only getting 0.54 milliamps. That's 540 volts - really low. odd thing is the 6993 hotdog probe seems to work fine. It clicks at the check source, very close to the calibration level, and it reacts to other radiation sources. Any suggestions what to check for the low voltage?

mtcooppdx 3 years ago

hi, another question about CDV700 transistors - specifically used on Victoreen 6B. There are two of them, Q1 and Q2. Do you know are these germanium? and replaceable by 2N404A or the Russian substitute? There is another Victoreen 6b update instruction on the internet and that author used MPSA56 as a substitute for the Q1, in the pulse/metering circuit. Thanks

DavidN71 4 years ago

If the CDV works correctly then you have at least 880v even if it measures otherwise.
I would suspect you are loading the circuit trying to measure it. The resistor must be clean and not resting on any surface and the jumpers need to be on something clean dry and non-conductive. 1kv will bleed off easily and overload the weak HV supply on a CDV

mtcooppdx 4 years ago

thanks for the response. I'll check out my setup again and rerun based on your comment.

dehunter302 4 years ago

I have a lionel cdv-700 model 6b. Cr-2 and Cr-4 listed as t8-127, low voltage, from lel are defective. Anyone have a reliable part number ? Mine are too corrupted to read. Tks

DavidN71 4 years ago

Sorry for the delay in replying. CR means diode. CR-4 and CR-3 are headphone diodes. The headhones are mono clickers, not audio headphones. Pulses on that circuit make them click one per radiation event. The filtering capacitor on that circuit is 50v, so any common 50v (or higher) diode should do. The pulse is supposed to be about 12v, which will easily pass forward on even a cheap diode, the 50v rating is enough to block the backside of the pulse from causing a click.
https://www.jonshobbies.com/civil-defense-lionel-c... has a shematic of the CDV-700 Lionel 6B
CR-2 is coupled to a 50v capacitor on the low voltage side of the circuit that oscillates to provide a/c to the step up coil that drives the geiger tube. Since cr-2 is coupled to a 50v capacitor, a 50v common diode like CR-4 should be used.

Eddie667 4 years ago

CR1 is a 1n34, modern replacement is 1n34a. I got mine off Amazon. In my experience, CR1 dies when the probe hot is sorted to ground. In the past I have used a 1n4001 and the counter still worked, but I dont know if there are any issues replacing that particular diode with a silicon one.

DavidN71 5 years ago

It's a 1N34 and it's obsolete. It's was a germanium diode. That matters. The forward voltage is lower on germanium than silicon. The 1N34a is the modern replacement or use a 1N270. Just make sure you are getting a germanium diode. eBay has both in germanium. They are commonly used in fuzz boxes for guitar effects.

ShaunS69 5 years ago

can any one tell me on cdv 700 6a what diode cr2 is. radio receptor pa305a victoreen 489-18. is or and interchangeable equivalent. I've searched every item and can't find a thing...

Eddie667 4 years ago

Looking at the board component side up, with the meter terminals facing away from you, CR2 is right under the transistor on the top right. Circled in photo, looks like a resistor.

jalmada 5 years ago

In reading David's article above he states "The original high voltage rectifier diode was selenium based. Modern fast switching diodes are silicon and work just fine. The minimum value should be at least 2kv. The R6000 (6kv, 0.2amps) diode is the best I know of and fairly cheap." - I am thinking that this is what you are wanting to know. David can best confirm this but I think it is indeed a R6000 diode that is the modern equivalent.

ShaunS69 5 years ago

Could you give some model numbers for the 300 v zeners and the lower zener you reccoment. Or specify some specs i can find 300v isnt easy to find. Like watt or any details. Im new.. if you couldn't tell lol

jalmada 5 years ago

Would you be able to provide a roughed out schematic of the solid state corotron you build using these 200v zeners (and/or 100v zeners)? Thank you.
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