FAQ: Geiger, Ion, Proportional counters - restoration and restoring #1

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Richard Hull
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FAQ: Geiger, Ion, Proportional counters - restoration and restoring #1

Post by Richard Hull »

This will be a series of posts with this name and will be used to assit others in identification, and restoration of older radiation instruments.

I will limit the discussion here to Geiger Counters, Ion chamber instruments, Proportional counters and electronic digital counters.

Such instruments are found on E-bay and at hamfests all the time. Many, if not most, are non-functional for any number of reasons. Some older instruments have parts or batteries that are a mystery. You may find your new acquisition is.......

1. Just plain dead - no power indication
2. No batteries or special batteries required and no where to buy them.
3. Comes on, but not indicating or performing as designed.
4. Incomplete, (parts missing, damaged, or non-functional)

In this series of FAQ's we can all contribute fixes, workarounds and give tips on these orphaned items and supply images for identification purposes or exchange schematics.

If you are serious about restoring a number of instruments, you will need at least four decent power supplies.

1. two isolated and independent metered power supplies that are adjustable between zero and 100 volts DC at about 25ma capacity each. These replace missing "B" batteries for testing puposes.
2. One metered supply that can put out 0-25 volts at up to 1 amp
3. One metered supply that is fully and smoothly adjustable from 0-2000 volts at a few milliamps. (used to test GM tubes that are suspect.)

You will need a know source of radiation to test instruments. For this you will need at least one very hot long lived source. A large or particularly hot piece of Uranium ore would be nice. Failing this an old ward bird insturment dial of WWII vintage would be nice.

Finally, you must have some electronics experience to really dig into the "tough ones".

I will start the ball rolling with a few insturments I have worked on recently with explainations on each.

First, the rare El-tronics RS-3. This was in very sad shape and totally non-functional when I acquired it. This is one of the first couters ever released by El-tronics. It is very collectable. Unfortunately, it was a heavy beast at 9 lbs when ready to go into the field.

The RS-3 was only produced for one year (1948-49) as it had a severe appetite for battery's. It took 3 giant 45 volt/22volt tapped carbon zinc batteries and two special, large 1 1/2 volt batteries of extreme capacity. None of these batteries are currently available as Union Carbide no long produces high voltage carbon zinc batteries over 30 volts.

In repairing this unit I hade to replace, 4 capacitors, and the GM tube. I had to repair the range switch cam assembly and remove, clean and replace the filthy meter. The batteries that I removed had not leaked much at all, but were a bit swollen. (dryed out completely)

Interestingly the batteries had been in this counter, unremoved, for at least the last 55 years!!!! I turned the large "A" batteires over and found the admonition...."Install before January 1955". This meant they were probably installed in 1953 or thereabout.

The red dust and fine sand in the unit told of its most likely doing yeoman duty for some prospector during the height of the U boom then left moldered away in some location since at least 1955 for sure.

It now works perfectly! One of the amazing things about this old El-tronics RS-3 unit is that it used eight vacuum tubes! Most later 50's tube units got down to two or three tubes at most. Also, this unit has a critical zero adjust as it has an extremely long time constant integrator. I found it could be adjusted in both HV for the GM tube and the zero setting to operate on very low battery juice. It still worked at 75 percent battery voltage! Few other such units would do this as they did not have the compensation circuitry as this unit does.

The next counter is of a Nuclear Chicago 1613A "Classmaster" These were in every High school and college around the nation in the 1950's and 60's. In this unit I installed one new vacuum tube, (it has 6 in it), replaced all 7 electrolytic and paper capacitors, installed a new power cord and removed and cleaned the meter movement.
It now works just like the day it graced that first classroom it was placed in. A common counter, but a jewel nonetheless. I now have 4 different classmaster versions or models in my collection.

Another counter made by Oak Ridge Atom Industries, "Instructor". This unit is a complete ratemeter, scaler, counter with timer. It has its original GM tube with stand for sample counting. Counters like this one and the Nuclear Chicago unit about littered the classrooms of America in the "Atomic Age". This worked when I got it, but as a precaution, I replaced all capacitors in it with modern units. A shorted capacitor could burnout or overload a precious and irreplaceable power transformer. Better safe than sorry.

Finally, I will discuss my thoughts on a Victoreen 471RF ion chamber survey meter. I bought this unit as an unknown for about $20.00. It was amazingly clean, almost pristine, with no batteries inside. A test using power supplies showed flawless operation.
The five (5) 30 volt batteries needed are still available! I bit the $65.00 battery bullet and put in the two regular "D" cells. I now have a great ion chamber that I use in the lab sniffing around the fusor for x-rays. This is a working instrument. Probably, this is a late 60's or early 70's device as the front end is an electrometer tube but the amp and meter circuitry is all transistorized. This unit still bears the Corps of Engineers "Detroit District" cal tag on it. Note: 2019...The 30 volt batteries are no longer available. RH

I will continue to post here until the thread gets long enough to justify a #2 posting.

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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Carl Willis
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Re: FAQ: Geiger, Ion, Proportional counters - restoration and restoring #1 ION CHAMBER AND ELECTROMETER CLEANUP

Post by Carl Willis »

Hi Richard,

This promises to be a useful thread.

I see you have a Victoreen 471 ion chamber, which reminded me of a technique I use to clean up electrometer-based circuits (ion chambers, "cutie pie" survey instruments, Keithley 610s, etc.) that have symptoms of leakage. Very common problem in second-hand instruments of this type. The leakage is often caused by dust, grime, solder flux, or moisture and amounts to nano- or picoamps. How do you know you have a leaky electrometer? The reading is jumpy or drifts erratically.

I keep methanol in a wash bottle filled maybe 1/3 full with baked-out zeolite pellets intended for a vacuum foreline trap, and give the circuits of the electrometer input stage a vigorous washing with dry methanol and a toothbrush, followed by drying with a hair dryer. This applies to the circuit boards themselves, the glass bodies of resistors and electrometer tubes, and switch contacts. Make sure the high-voltage bias batteries, if present, are disconnected first! Most of these instruments also have a little desiccant can included inside the instrument that will need to be baked out in a toaster oven on low heat for a few hours until the pink indicator turns blue. Most craziness in electrometers can be tamed by a good cleanup.

-Carl
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Carl Willis
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Re: FAQ: Geiger, Ion, Proportional counters - restoration and restoring #1 GET A PULSER!

Post by Carl Willis »

Another repair FAQ topic, extolling the virtues of having a good pulse generator for testing Geiger counters and NIM gear. Richard had a good writeup years ago here:

viewtopic.php?f=13&t=5085#p34893

You simply cannot troubleshoot NIM signal chains without a pulser. For some reason though, they are a little scarce in the usual surplus outlets and they're relatively expensive. The essentials of such a pulser are:

1. Puts out a long tail pulse with a very short (few ns) risetime. This is often achieved by closing an RC circuit with a mercury-wetted relay.
2. Variable output amplitude, set with a locking precision potentiometer.
3. Switchable pulse polarity.

Some features are important, but may not be present on every instrument:

4. Switchable attenuators to change the range of the output up to 1000:1 or so
5. Variable internally-generated pulse rate, or input for external control of the relay

The pulser can be used to do many things, but among the most important are

-Testing the appropriate function of signal chain components
-Checking the linearity of amplifiers, ADCs, SCAs;
-Quantifying dead-time effects in Geiger counters or NIM systems
-Setting the "pole-zero" on NIM preamplifier-amplifier combinations
-Calibrating ratemeters (for serious or legal calibrations you'd have to send the instrument to a certified lab though)
-Measuring the charge output or noise from detectors or preamplifiers

Without a pulser, you're flying blind when trying to troubleshoot pulse-based nuclear electronics.

-Carl
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Re: FAQ: Geiger, Ion, Proportional counters - restoration and restoring #1 GET A PULSER!

Post by Richard Hull »

Thanks Carl for those tips and info on the pulser. I have three NIM pulsers in my collection. All use the mercury wetted relay.

I also have a large external NIM pulser made by Tennelec. It is a very high precision device with .01% range resistors and switch selecable impedance matchers, etc. I found it in a giant cardboard box under a table at a ham fest with a note $1.00 for anyhting in this box. Since then I look very carefully in junk boxes at these events.

I have used absolute ethanol for cleaning and maintaining the large number of Kiethley and General Radio electrometers that I have on hand. Maintaining strict control over moisture and deposited grime in and around all of the electrometer bias resistors and the tubes (5886's) is a chore not to be overlooked. Many of the better ion chambers had a totally sealed and hermetically enclosed electrometer integral to the range switched bias resistors, thus obviating the cleaning. An old Tracer Labs "cutie pie"
that I have is built in this fashion.

Carl is correct. We tend to use these meters on their most sensitive ranges and this means the largest bias resistor is in play and leakage is always a big, big problem. If your great deal in an ion chamber is unable to zero on the lowest range or jittering or all over the place, clean as Carl suggested and dry afterwards with the hair dryer.

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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Carl Willis
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Re: FAQ: Geiger, Ion, Proportional counters - restoration and restoring #1 ELECTROSTATIC VOLTMETERS

Post by Carl Willis »

A useful piece of instrumentation for Geiger counter or ion chamber repair is an electrostatic voltmeter so voltages can be measured without loading the power supply. They work on the same principle as a gold-leaf electroscope, but have nice accurate scales. They are effective for both AC and DC voltages and are polarity-blind. Leakage resistance is enormous compared to the usual resistive-divider high-voltage probes or VTVMs. They are essential for properly setting the high voltage on GM counters, proportional detectors, and ion chambers that may have load resistances in the gigohm-teraohm range. They can also be used to check old 300V "B" batteries for viability.

I have three: 0-500V, 0-1000V, and 0-5kV.

EBay and hamfests are full of these and they're usually a good bargain.

-Carl
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Re: FAQ: Geiger, Ion, Proportional counters - restoration and restoring #1 ELECTROSTATIC VOLTMETERS

Post by Richard Hull »

I have found my three electrostatic meters at Hamfests. These are capacitively operated and the needle is swung using misaligned capacitor plates trying to align or be drawn into alignment by the applied voltage (sic charge). The scales, off course, are non-linear, but they are easy to read. The average meter very ugly and ungainly. They are both heavy and extremely bulky. My 0-30kv meter weights 40+ pounds. They use no batteries or power. They are passive instruments.

Interestingly, I opened one of my oldest 0-5kv electrostatic meters and found a cal tag inside the unit that read "met-lab" cal 3-44. For the history buffs, this was the metallurical lab at Chicago where the first pile was built and part of the Manhattan project. Later it morphed into Argonne National Lab.

One of the meters I have contains two scales and is range selectable. This involves pulling a rod out for the high range. This moves the fixed plate a bit farther from the moving plate's plane.

As for "B" batteries, (true B batteries), are always higher voltage like 45-90 volts or more. "C" batteries were bias batteries and ranged from 6 volts to 30 volts. "A" batteries are traditionally high current drain, low voltage batteries (1.5 volt to 6 volts). With the advent of resistor cathode biasing in the 1920's, C batteries stopped being used. With the avent of transistor circuitry, the "B" battery fell to the wayside. This A,B,C nomenclature were strictly vacuum tube technology terms. A huge fraction of all counters ever made were vacuum tube based. Virtually 100% of the GM, ion and PMT collector market is vacuum tube based.

The person looking for a usable counter will invest or choose, wisely, a modern semiconductor based GM or ion chamber instrument. This series of posts will cover these solid state counters as well; so, fear not! A massive number of older silly-con based counters also exist as surplus and often fall, defective, into amateur hands. They will not be forgotten.

"B" batteries are best tested under load! Ion chambers do not load their 22.5-30 volt C batteries significantly, however even these should be checked under light loads. The average digital multimeter has 100 megohms input impedance. This will not load any B or C battery. Thus, your digital meter might say a B or C battery is Ok when it is not. One should strive to pull at least 3-5ma from a B battery and at least 1 ma from a C battery. In this fashion a true account of the battery's value is obtained. This means a shunt resistor of about 0.5K to1k ohm per rated battery volt is needed to give a good indication of performance. To test a 67.5 volt battery you might chuck a 47kohm resistor across it and you should see over 60 volts on your meter. If not, it is weak or shot. An ion chamber 22.5 volt battery should have a 27k ohm resistor across it to see it it has any energy left in it.

Now, setting the high voltage for a GM tube or a PMT absolutely must use an electrostatic voltmeter, especially if the unit is portable or a field survey type unit as the HV supplies in these are crappy and very weak. An old bench PMT supply or a large bench ratemeter or scaler can be set with a common digital DVM with a 100meg input resistance as these larger units are wall powered and have rather beefy HV supplies in them. Example, the voltage set on the 1613A Nuclear Chicago above is done internally on its 100ua meter movement with a simple single resistor of only 15 megohms. Thus the HV supply is loaded with about 15 megohms when the voltage is set. This is a much heavier loading than the average digital meter would produce yet the voltage setting is good. The supply would be happy to feed 2-5 ma to the load, instead it is tasked at about 75 microamps. for a 1 kv GM tube setting.

The key is in knowing what you have to measure and what you actually have in hand to measure with versus what you need to measure with.

The average, portable, older GM counter with B batteries will drain them at about 2-5 ma rate while the A batteries are hit at about 300ma. The resulting HV for the GM tube will have great difficulty supplying 25 ua of stable current Thus, if the voltage is adjustable, you will need an electrostatic voltmeter to set or read the HV. The above is also true of portable Scintillators. Here the requirements for accurate voltage setting are much more critical.

A crude work around is possible if you are blessed with a large number of high ohm resistors. You may take a 5% 1 megohm resistor and put it in series with a made up string of (10) 100meg ohm 5% resistors and place this entire string across your HV supply. Now, within limits, You can drop your DVM across the 1 meg resistor and a 1 volt reading on the DVM will be 1000 volts on the HV supply. You have upped the resistance of your meter to 1 gigohm, loading the 1 kv supply to 1ua. This might still be too heavy a load on the whimpiest of systems. You are back to the electrostatic meter requirment.

In summary:

1. All batteries must be fully loaded to determine if they are truly good. No voltmeter will load them properly and an external load will be required to determine their true status.
2. All GM and PMT supplies in portable instruments should be set using very light loadings. Most all voltmeters will overload these weak supplies due to the instrument's internal resistance. A special electrostatic voltmeter will usually be required or a meter that has an input impedance greater than 1 gigohm.

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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