CDV-700 Maintenance

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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.

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    16 Discussions

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    ShaunS69

    9 months 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...

    2 replies
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    jalmadaShaunS69

    Reply 5 months 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.

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    ShaunS69

    Question 9 months ago on Step 1

    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

    6 answers
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    jalmadaDavidN71

    Reply 5 months 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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    DavidN71jalmada

    Reply 5 months ago

    You don't make a corotron replacement. You measure the unregulated voltage and then add zener backwards and check again. If the CDV works and the voltage is below 930v then you are probably safe. I have one ENi that works fine at 880v. Using zeners backwards to drop voltage is an art. They were not rated to do that, but will if the current is kept low enough. CDV's can't push much current so that works well.

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    ShaunS69DavidN71

    Reply 9 months ago

    thanks for your help much appreciate it

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    William CJShaunS69

    Reply 6 months ago

    1N4993 is not in stock that I can find but you might also try Z300A currently available on EBay: 162082810127

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    William CJWilliam CJ

    Reply 6 months ago

    Also try P6KE300A they are readily available. The last two I used these one worked well.

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    jalmada

    5 months ago

    I have the Anton CDV-700 and I'm curious about the capacitor C8 - The service guide parts list calls out for a .0025f +100% - 20% 1.4 KV capacitor. The one on my Anton looks like a ceramic disc capacitor but looks more like a mylar film cap to me. I've looked around parts houses and can't find a match - Any advice? Do I just leave the existing one in and hope for the best or is there some supplier out there with an equivalent?

    1 reply
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    DavidN71jalmada

    Reply 5 months ago

    I avoid working on Anton's unless they are broke. They are much more complicated than ENi's and Lionel's. When replacing a capacitor that is known bad the voltage matters, as does the capacitance and whether the cap is polarized. Like you said, I don't know of a mylar cap with that rating. Maybe someone tried a fix and it didn't work. https://www.ebay.com/sch/i.html?_nkw=high%20voltage%20ceramic%20caps eBay is a pretty good place to start. The blue caps are not mylar, they are ceramic. Buy at least 2kv, 3kv is better. The current rating system is the actual voltage the cap will fail, the old system when things were made in the US was more lenient.

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    DavidN71ShaunS69

    Reply 9 months 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.

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    TiernanW

    1 year ago

    Which manufacturers used the #2 vs the #3 pins as the anode? Is there any way to find out without testing first?

    1 reply
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    DavidN71TiernanW

    Reply 1 year ago

    You don't need to. Just take a thin strand of bright copper wire and wrap it between pins 2 and 3 if you are not sure. You do not need to solder it, just wrap it and put the tube in the socket as you would normally.