CDV-700 Maintenance

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Introduction: 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.

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    24 Discussions

    0
    mtcooppdx
    mtcooppdx

    3 months 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?

    0
    mtcooppdx
    mtcooppdx

    Reply 2 months 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

    0
    DavidN71
    DavidN71

    Reply 3 months 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

    0
    mtcooppdx
    mtcooppdx

    Reply 3 months ago

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

    0
    dehunter302
    dehunter302

    6 months 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

    0
    DavidN71
    DavidN71

    Reply 5 months 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.

    0
    ShaunS69
    ShaunS69

    1 year ago

    cr1 sorry

    0
    Eddie667
    Eddie667

    Reply 1 year 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.

    1
    DavidN71
    DavidN71

    Reply 1 year 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.

    1
    ShaunS69
    ShaunS69

    1 year 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...

    0
    Eddie667
    Eddie667

    Reply 1 year 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.

    20190713_183927.jpg
    0
    jalmada
    jalmada

    Reply 1 year 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.

    0
    ShaunS69
    ShaunS69

    Question 1 year 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

    0
    jalmada
    jalmada

    Reply 1 year 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.

    0
    DavidN71
    DavidN71

    Reply 1 year 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.

    0
    ShaunS69
    ShaunS69

    Reply 1 year ago

    thanks for your help much appreciate it

    0
    William CJ
    William CJ

    Reply 1 year ago

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

    0
    William CJ
    William CJ

    Reply 1 year ago

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