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CDV 700-6A DOA (Repair)

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Dragon64

Mon Apr 18 2011, 10:22PM

Registered Member #1438 Joined: Sat Apr 12 2008, 12:57AM
Location: Canada
Posts: 218

I've had a CDV-700-6A I bought a while ago but never got a chance to play around with it. As the seller sold it as is which was a clear indication that it was DOA but purchased it anyway in hopes to repair it in the future.

After reattaching the fallen off BNC connectors for the tube then re-powering it up, I could hear a light humming sound then checked the voltage of the high voltage terminal to the GM Tube which peaked around 800v. I tried testing the Geiger Counter with a smoke detector Americium source with the probe shield open. Unfortunately, there was absolutely no response to the meter at all.

I've tried to re-solder all of solders which had not wicked up the solder but still nothing.

I'm crossing my fingers that the GM Tube itself is not broken but I'm not sure how exactly I could test it. The model for the tube is a Victoreen 6993 and the bottom silicon is in bad condition.

If anyone has any idea's on testing the tube or to fix the CDV itself, please feel free to comment.

I've attached some pictures to show the conditions of the Geiger Counter.

Proud Mary

Mon Apr 18 2011, 11:02PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Hello Mr Dragon,

I can't think of any good reason why a Victoreen 6993 GM tube would respond in any major way to a tiny americium source. It can't detect alphas at all, and GM tubes are such inefficient gamma detectors that the feeble gamma emission of americium would mostly get lost in the background.

The 6993 has a nominal working voltage of 900V, but it's quite possible that your meter itself may be pulling it down to the 800V you have detected. Battery equipment is often designed to run at the lowest possible power, so your meter may be drawing much more than the few tens of microamps the high voltage supply was designed for.

It is a simple matter to test the GM tube independently. Connect a 10M resistor in series with the anode, and a 220K resistor in series with the cathode.
Connect a supply of 900 or a 1000V DC across this series string. You can then pick off the signal from the cathode by a unity gain buffer, or audio amp input, or what have you.

The pulses will be a volt or two, and not difficult to detect at all. So you should hear the occasional random click from background radiation, and you will know the tube is working.

Stella

James

Mon Apr 18 2011, 11:06PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

Americium is an alpha emitter if I remember correctly. I'm not familiar with that particular Geiger tube, but many of them are not particularly sensitive to alpha radiation.

If it's working, you ought to get a click now and then from background gamma radiation, especially if you take it outside. You can get gamma test sources too, maybe get in touch with a science teacher at a local highschool or university, they might be able to check it out for you.

As far as testing the unit itself, you should see several hundred volts across the connector, and if you briefly short it you should get a click.

Dragon64

Tue Apr 19 2011, 02:10AM

Registered Member #1438 Joined: Sat Apr 12 2008, 12:57AM
Location: Canada
Posts: 218

Proud Mary wrote ...

Hello Mr Dragon,

I can't think of any good reason why a Victoreen 6993 GM tube would respond in any major way to a tiny americium source. It can't detect alphas at all, and GM tubes are such inefficient gamma detectors that the feeble gamma emission of americium would mostly get lost in the background.

James wrote ...

If it's working, you ought to get a click now and then from background gamma radiation, especially if you take it outside. You can get gamma test sources too, maybe get in touch with a science teacher at a local highschool or university, they might be able to check it out for you.

I should have added this before, the CDV does not even pick up any background radiation. Turning the Geiger Counter on and leaving it for several minutes does not pick up anything and the meter needle stays absolutely stationary. I've just realized a test source on the side which still detected nothing. The same goes for some sample of potassium nitrate.

Proud Mary wrote ...

It is a simple matter to test the GM tube independently. Connect a 10M resistor in series with the anode, and a 220K resistor in series with the cathode.
Connect a supply of 900 or a 1000V DC across this series string. You can then pick off the signal from the cathode by a unity gain buffer, or audio amp input, or what have you.

Does the power supply have to be precisely 900 to 1000V DC?

Also I'm having a hard time locating the anode and the cathode of the GM Tube. Right now looking at it, there's three prongs which protrude out of the silicon and all of them seemingly look exactly the same distances apart.

Thanks

Proud Mary

Tue Apr 19 2011, 05:33AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

hydraliskdragon wrote ...

I should have added this before, the CDV does not even pick up any background radiation. Turning the Geiger Counter on and leaving it for several minutes does not pick up anything and the meter needle stays absolutely stationary. I've just realized a test source on the side which still detected nothing. The same goes for some sample of potassium nitrate.

It will not detect background radiation. It was designed to detect the high levels to be expected in nuclear war - tens of thousands of times higher than the peacetime background level.

hydraliskdragon wrote ...

Does the power supply have to be precisely 900 to 1000V DC?

Anywhere between about 875V and 1025V should do. Too low and the tube won't avalanche, too high and it will go into continuous conduction and be quickly damaged.

hydraliskdragon wrote ...

Also I'm having a hard time locating the anode and the cathode of the GM Tube. Right now looking at it, there's three prongs which protrude out of the silicon and all of them seemingly look exactly the same distances apart.

The pin/s connected to the outer metal skin is the cathode.

James

Tue Apr 19 2011, 04:56PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

If you can find someone with access to an x ray machine and put the tube under that it ought to go nuts when the beam is activated. You could also pick up one of the Russian geiger tubes that are all over ebay and try that with it. Even with the Fukushima hysteria those were made in such vast numbers that prices aren't horrible.

Conundrum

Tue Apr 19 2011, 07:15PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Or the good old "2 stage pump + sellotape on motor" technique, to get X-rays up to something like 60keV.

-A

plazmatron

Wed Apr 20 2011, 01:38AM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

It should pick up background, normally around 1 or 2 cps for the CDV-700, and the source on the side is pretty active. Solder in a small bare neon in series with the GM tube, and see if it flashes at all. If not, it is probably a duff GM tube.

Les

Dragon64

Wed Apr 20 2011, 02:41AM

Registered Member #1438 Joined: Sat Apr 12 2008, 12:57AM
Location: Canada
Posts: 218

Whoops, sorry I'm a little late to respond. I was under hypnosis by the release of Portal 2

Anyways, thanks everyone for the input.

After trying out what Proud Mary mentioned, there wasn't even a trace amount of pulses from the GM Tube.
I'm guessing that the GM Tube has suffered a crack and the age and condition of the CDV further confirms it, rendering it useless.

Instead of getting a new tube for the CDV, I might just plan on getting a gamma-scout digital Geiger counter.
Been having too much problems looking for ores using my cheap DRSB-88 Geiger Counter.

Adam Munich

Wed Apr 20 2011, 03:02AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Get a mica windowed tube and make a circuit to drive it. Personally I like the 5979 tube, it works rather well for detecting gamma and x-rays due to its rather thick metal wall. It's also halogen quenched so it'll never die on you.

A CCFL inverter works just fine to power them, and you can use an arduino+lcd to make a nice digital readout.

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