FAQ: Geiger, Ion, Proportional counters - restoration and restoring #2- CDV 700 series

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Richard Hull
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FAQ: Geiger, Ion, Proportional counters - restoration and restoring #2- CDV 700 series

Post by Richard Hull »

We all have or will come into contact with the ubiquitous, yellow, Civil Defense Gieger counter. These were all made in the late 50's and early 60's. All were solid state and were produced by as many as six different manufacturers with over 10 totally different circuits, and designs. They used as few as 2 "D" cells to as many as 5 "D" cells.

This is a quick post on how to fix and test these items when one finds that new batteries installed in a recently purchased counter still leave it non-functional when checked against a known source.

The assumption is that you have a defective counter with known good batteries in the unit and that the contacts are clean and not corroded.

Does the GM tube look good based on what you can see through the grill in the probe?
If so, you should turn off the counter and remove the batteries. We need to verify that the GM detector tube is good. Check this by, first, unsoldering the center lead of the GM cable inside the GM counter from the circuit board. You can leave the shield wire attached.

With two clip leads connected to the freed central wire and the shield, you will need to hook up the tube to an external test circuit, ( as shown in the file given below), to make a crude counter. The high voltage from your supply should be brought up slowly from about 500 volts until clicks are heard in headphones attached to the counter. If you reach 1100 volts with no clicking, the tube is bad and the entire insturment is pretty worthless unless you can obtain an exact replacement somewhere for this old tube. (perhaps an old junked CDV counter.)

If the tube checks good, write down the voltage that it started to operate for later use. Resolder the central lead to the circuit board within the counter.
Now, you need to put the batteries back into the unit and turn it on. Using a 100 megohm input DVM, measure the voltage across the tube connection at the board. If it is 400 volts or less or zero, then your HV oscillator is not working. If it is over 400 volts, how close is it to the working voltage you measured in the test circuit? If far below, you still have oscillator issues.

The oscillator is controlled and driven by a single PNP germanium transistor. These are getting hard to find. A modern replacement may have too much gain and fail to function. I have found that only medium gain transistors will function in some cases. Thus, I collect older, early Germanium transistors so that I can repair these counters.

A good 2N404 substitute should get the oscillator working. To locate the oscillator transistor on the board refer to the schematic glued inside the base of the counter. (All CDV units had a small schematic glued inside.)

In almost all cases this will fix the counter! If the voltage comes back up at the tube, check the audio from the headphone jack. Place a radioactive sample at the opened counter tube and leave it lying close by. If clicks are heard, but you have no meter deflection, then the meter capacitor, the meter driver transistor or the meter, itself, may be bad. Usually it is the old PNP transistor that is the culprit.

NOTE: Work the range switch violently through all its ranges. This should clean the contacts after years of sitting idle. A spritz of tuner cleaner, "electro-wash" or electro Kleen will make sure it is clean as the switch is rocked through all its positions many times.

Check all circuit boards for cracks that can hide broken circuit traces. Check all solder joints around heavier components like the oscillator transformer, large capacitors, etc.

As a precautionary measure, change out all capacitors if they look even slightly discolored or leaky. Most CDV's only have one or two larger capacitors in them.

I have repaired about 30-40 of these counters. 90% were due to failed HV oscillators. Only 2 counters had bad GM tubes. All the rest were due to dirty contacts, dirty switches, bad solder joints, cracked or damaged boards and meter amp circuit issues.

The entire CDV series were minimalist systems and are nefariously cantankerous, being the epitome of government low bid parts and components. Their germanium transistors were early examples of this technology prior to higher gain, more stable, silicon based systems. Years of storage exacerbate all of this to make many CDV units appear intermittent at the most needful moments.

In spite of all of this, a working CDV series unit is not only a very serviceable GM counter, but a real collectable, a true piece Americana during a period of both fear and hope in "the nuclear age".

Richard Hull
Attachments
The general schematic of a CDV-700.  Note, different letter models had different circuits.  This is for the model 6B
The general schematic of a CDV-700. Note, different letter models had different circuits. This is for the model 6B
6B-1.jpg
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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Carl Willis
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Re: FAQ: Geiger, Ion, Proportional counters - restoration and restoring #2- CDV 700 series

Post by Carl Willis »

Hi Richard,

I'll add some links to CDV-700 forums that should be good sources of info on these units:

http://tech.groups.yahoo.com/group/Geig ... thusiasts/
http://tech.dir.groups.yahoo.com/group/CDV700CLUB/

The first of these is George Dowell's group. He is one of the most knowledgeable people about the CDV units and has offered refurbished, calibrated, and improved variants on the original to hobbyists for reasonable prices.

I have rarely repaired CDV-700s, but I have a couple on which I have replaced the probe cord with a detachable BNC-terminated coaxial cable. For field use, this pretty much must be done because the original gray cords tend to fatigue off.

In addition to the germanium transistors, another hard-to-find part is the 900V voltage regulator "corotron" tube. As an aside, some of these corotrons have radioactive material in them to help ionize the gas.

-Carl
Carl Willis
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TEL: +1-505-412-3277
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Richard Hull
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Re: FAQ: Geiger, Ion, Proportional counters - restoration and restoring #2- CDV 700 series

Post by Richard Hull »

Thanks for the input Carl. I have never seen a bad corotron in one of these units, but I have only worked on about 40. I have never seen a bad meter either. Many problems circle about the contact viability of often corroded battery holders and the rotary on-off/range switch. As noted earlier, the #1 purely electronic problem is the HV oscillator failing to come up.

I have modified about 5 of my own CDV units with audio amps and speakers (screw those lousy headphones). I have also added LED blinkers to some and BNC detachable cabling as you have also done.

I used to sell some refurbed CDV's with warranties and custom modified a number of them to the buyer's special needs. I gave it up when no one wanted to pay what I asked. I just couldn't make a decent profit at the work. Most folks are pretty low budget in this area and don't appreciate value added in my opinion.

Thanks for the additional refs and URLs they are great

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
keelan
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Re: FAQ: Geiger, Ion, Proportional counters - restoration and restoring #2- CDV 700 series

Post by keelan »

@richard hull
I got a Victoreen as my second Geiger counter ever and I have added a BNC connector, and an old Eberline HP-330 pancake probe for it. The whole thing works well but I have been wondering whether an old Eberline-520 with the internal tube, but I can’t find any reviews on them. Between the CDV-700 and the Eberline-520 (if you’ve tried them both) which has a longer service life and which is better. If you’re wondering why I’m even considering these older meters, it’s because a Ludlum is a little out of my price range right now, although I do have an old 177-58 benchtop meter Ludlum. What I mean is a Ludlum survey meter.
"What Mr. Einstein has said is not so stupid"-Wolfgang Pauli
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Richard Hull
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Re: FAQ: Geiger, Ion, Proportional counters - restoration and restoring #2- CDV 700 series

Post by Richard Hull »

The 520 must have the included external tube before buying one. The internal tube is a worthless tiny little gamma only tube on the X1000 range. With no external probe the 520 is worthless for anything except for post nuclear exchange work where you choose to die slowly seeking a much lower 30 rem/hour place to sleep the night over a 200 rem site.

If working with a functional external probe, the 520 is far better than the model 5. Assuming similar prices.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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