Cutting edge GM counter (1953)

[Richard Hull]

Like Bob, I obtained a gutted "Tower" brand work alike111 (Sears and Roebuck's house brand made by El Tronics). El Tronics made a near carbon copy of the precision 111 scintillator. Sears got into all the fads and created house rebranding of normal products. They sold Henry J autos for a few years as their "Allstate Six" Those who know Sears products will remember that all their automotive products, batteries, tires, etc., are all "Allstate" house brands.

I took just the ray gun and tossed the battery pack and electronics and turned it into a GM counter over 25 years ago. I kept the meter and on-off, range switches and placed a 5979 GM tube in the snout, boring a 1" hole in the end to expose the mica window. It still functions and uses a small 9 volt transistor battery and a little 1" speaker. All circuitry is stuffed into the large round rear chamber of the gun. I find it difficult to believe the battery I put in the counter still working after 7 years in the gun as I just took these two images after pulling it out of storage.

[Mark Rowley]

The ElTronics PR31! I just restored one a year ago August. Those are fun to work on. Unlike the subminiature laden PRI’s, the PR31 uses a 1U4 and three 1U5’s. Performance wise, the PRI seem to be better and more stable. On the bench are also three Detectron DS234 Scintillator guns. I’ve yet to restore them but I may do so next year. If they perform anything like the lunchbox DS-222 or DS-277 scints, the PRI’s may have a contender.

A Facebook friend builds small modules which incorporate a HV driver and a pulse amplifier. It’s even compatible with an MCA and easily powered by 5v. I’ve been planning on gutting one of the more distressed 111B’s and modernizing it with one of these modules. Who knows when I’ll get to doing it, but it’d be a fun project similar to yours.

Mark Rowley

[Roberto Ferrari]

In my small collection, I have a Isoto, built in Argentina during the 70's. Copy of a French design.
Unfortunately I have no schematics to attempt to rebuild it.

[Mark Rowley]

Hi Roberto,
I have two of the French SPP2 versions and have noticed they are more or less modern versions of the PRI 111 detectors except that they have the audio squealer function (similar to how a classic metal detector sounds when getting close to a metallic object). Based upon your picture, the main box has some differences which makes me think the circuity may be different than the SPP2.

Hopefully the design of yours follows the modular arrangement found inside the French version. It makes for very easy disassembly as well as easier tech work. Fixing any malfunctions should be fairly straightforward.

Somewhere in my files I have the schematic for the SPP2. Let me know if you think it'd help. I can forward you a copy.

Mark Rowley

[Richard Hull]

My repairs/rebuilds do not have to save the patient's internals or respect the original purity of circuitry contained within. If the patient is more dead than alive, I tend to gut the circuitry and update all circuitry to a more modern standard. What is important is to at least attempt to maintain the original outward appearance and calibration. This is easily accomplished in 99% of what I do. I make every attempt to save the vacuum tube identity of the device. But, sometime the old circuitry just will not respond especially if the circuitry is too tightly packed in or the 25-50 henry boost HV choke is shot. Most of the time when this key component is bad, it is due to battery juices and vapors getting in or settling on the 2000 turns of #56 or #68 AWG windings in the choke and corroding many opens in the turns. 90 percent of the GM and scintillator issues are HV related and or B battery elimination issues. Filaments can remain lit as always, ("D" cells). I tend to check the old tube circuitry by putting the usual 120 volts on the HV oscillator by just clipping in a variable 0-150 volt DC supply. If the counter comes to life, it is just a matter of installing a good, tiny, home made B battery eliminator board and a new lower voltage battery to power this add on. Usually there is a ton of room ready made in the old hollow for the two B batteries.

From my standpoint I desire a look-alike, work-alike restoration via the easiest and most reliable path. By gutting 100% of the tube circuitry and replacing it with the latest circuitry, I find you have enough room in the typical old counter to build an entire rad lab inside. Each job is unique for me.

It is rare for me to be approached by someone wishing a full flawless restoration with no alterations in an old counter. It does happen, but they need to check their pacemaker as it will not be cheap. One recent restoration had me creating a small bass wood frame with beveled corners and pasting on the old original battery printed labels with a small battery and miniature boost board inserted within which looks just like the original eveready or burgess B battery which snaps into the original battery clips. Got $450.00 for that one. The counter needed no work other than cosmetics. The customer knew what he wanted and had the cash to pay for it.

I have very tiny battery eliminators that I have designed, but I attach, here, an image of my combo install board normally inserted in many tube systems. Not only does it eliminate the B battery, it also supplies GM tube voltages variable from 400 volts to about 900 volts, as well as outputs a positive or negative prefect square wave pulse from the tube of 50us width.

[Mark Rowley]

Impressive! Doubt I'll progress to making my own circuit boards but who knows, retirement has been an interesting road so far.
I'm far from a purist when it comes to restorations, but I just cant gut a detector unless its a totally corroded/rusted mess. Like to the point where identification of components is impossible coupled with supporting hardware reduced to an oxide. And I do have a couple in that condition....hence the MCA module.

I'm with ya on the restoration price. I've done quite a few (radios included) with most over $300. Shipping can add $60+ to that as well.

Mark Rowley

[Roberto Ferrari]

Hi Mark

Electronics inside the main box is splitted, as you can see in this picture.
I am not in the mood to track the wires, etc.

Hi Richard
Your approach is great, but I stick to the conservative way, not to modify too much the original object.

[Mark Rowley]

Very similar Roberto. High probability the modules are the same.

Mark Rowley

[Roberto Ferrari]

Hi Mark

Being similar the models, I accept gladly your offer.
Please get in touch by PM.

Thanks!

[Mark Rowley]

-QSL-

Mark Rowley

[Roberto Ferrari]

Hi Mark

Thanks a lot for that file.
Very similar design.
Now I will struggle for devoting some time to the restoration.

[Mark Rowley]

Just for fun I used a 6.5 decade old PRI 111 Scintillator to detect x-rays coming off the fusor. The 111 goes to full deflection on the 2.5mR scale as I ramp it up from 22 to 25.5kV.

https://www.youtube.com/watch?v=w1cHW6TrhTU

Mark Rowley

[Richard Hull]

Quite normal near a fusor. I use a victoreen ion chamber detector and, as most here know, my view window points down to the concrete floor. I get about the same reading, (2-3 mr/hr) here when about 12 inches from the fusor body at 38kv. However, if I get the nose pointed in the narrow cone almost touching the window,1-inch away, I get 40mr/hr. At the controller work station, ~ 6 feet, I get about 0.5 mr/hr. from the fusor body and nothing from the cone. My permanently affixed camera, of course, takes the full 40mr/hr and some pixels are lost in the CCD from the 15 years fusor IV has been around.

I have some 100 mren dosimeters and three chargers and always wear one, but to date, they never move off zero as the blast is only present during 3 or 4, 5-minute runs over the two hours it takes to start and take data from the fusor. Based on the .5 mrem/hr ion meter reading with 20 minute total exposure at 6 feet that would register a total dose of 0.5/3 or less than 0.2mrem which is virtually no dose at all.

I took data for over 12 daily runs in a row prior to the HEAS event for a probable total absorbed dose of 12 X 0.2 or 2.4 mrem I rarely run the fusor more than twice yearly. The rest of the time it just collects dust and takes up space.

Put you 111's snout right at the naked window and take a reading and you will see why I point my window straight down into the floor. Will the 111 even read that high??

Richard Hull