Detectron Geiger Counter

When the b+ becomes to high, the gas-discharge tubes start oscillating, producing a steady stream of pulses.

So start with lowering b+ to zero, and then increase, until either back at 90 volts, or the pulses re-appear. At that point lower the voltage a bit, and re-check with your radiation source. Also there should be a capacitor across the tubes somewhere, If so, check their quality(leakage and value).

Schematic of the DG-9 model:

Probably quite similar to the DG-7.

Replace it with what? Is silicon, the B+ is probably too high.

At that age, 99.999999% of the electrolytic cazapitors have high ESR. However, looking at the schematic, there are none. Weird.

How? With a tube tester? I sometimes work on ancient Motorola radios with pencil tubes. I have to make my own adapter to work in the tube tester.

Ok, something is oscillating. I have some guesses based on the schematic, but the right way is to get a scope, with a high voltage probe (i.e. 100x) and sniff around for which section is oscillating. My best guess is that since the beast doesn't have any power supply filter capacitors on any of the batteries, and you've probably assembled a mess of power supplies, batteries, and junk to simulate the batteries, you have a fairly high impedance on the power supply lines. Rather than fixing it with a proper battery, find a big fat capacitor (with a suitable high voltage rating), and put it across the power supply leads as close to the circuit (not as close to the battery) as possible. The magic buzzword is "motorboating".

Good, it's working.

0.2mr/hr, about the same as background radiation. (I have the same counter and the one in the video seems a bit insensitive).

Nope. Probably a neon lamp used as a regulator. Photo?

Google for "Detectron Schematic".

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com               jeffl@cruzio.com
# http://www.LearnByDestroying.com               AE6KS

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90V seems low for a GM, is there a step up circuit to 300V or so ?

Yes it uses some kind of step-up circuit, and the output to the GM tube is about 300-400 volts.

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I use an ex military GM tube with ditched 1960s electronics (some strange battery required although transistor) and my own inverter to sweep the workshop every now and then . I forget what voltage drives it but I remember that if it was set too high it would self discharge internally causing regular clicks . Now there is no thorium loaded gas mantles around these days , nor radium dials , what is commonly around to give an above background check that they are working ?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

If one has elevated radon levels, there is a rise near your furnace air filter, if its electrostatic material. Still, you have to integrate the meter to see average increases. I still have an old bedside clock around, as well as my Pentax lens, 50mm vintage early 70's. You can buy Uranium stuff. On one meter I use there is a calibration spot on the side. I don't know how many smoke detectors have it, but it seems hard to measure through the case.

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ordinary domestic smoke detectors use alpha emitters so not much use for GM checking should anyone be unwise to break into the shielding (shielding probably more to keep people out, than the alpha in, as almost anything blocks it)

Already tried that, and I replaced all the ceramic capacitors in the circuit. By gas-discharge tube, are you referring to the neon lamp inside? There's another tube, with just two leads, that I can't identify or test. See this photo:

I came across that already, but it's quite different and therefore not much help.

I tried putting a decade resistor in series with the diode, but even at

100k-ohms it still didn't make a difference. It still oscillates even with the diode disconnected.

Yes, I checked these with a tube tester, and even trying subbing them with new-old-stock tubes.

Here's a photo of the circuit:

No, I was referring to a detector tube filled with neon, where the voltage on the tube is just below the sustained ignition level, and where the charged particle causes a conducting ion trail, which discharges the parallel capacitor a little bit, causing the click signal you can pick off. The feed resistor slowly restores the voltage accross the tube/cap. The amplitude of the click shows the energy level of the detected particle. Long ago I built a cosmic ray detector/display using these tubes. Name is geiger muller tube I think.

OK I understand. But that's still not the problem, because the unit produces its own clicks whether the probe is connected or not........

Oh, I was hoping they were similar. Sorry(tm).

I don't know exactly what series resistor is going to work. 100K is certainly too large. When I was doing the same thing on much higher power ancient Motorola radios, about 10 ohms 25 watts was about right. I'll probably get yelled at for this suggestion, but use an ohmmeter to mearsure the selenium recifiers forward resistance and use a resistor with a slightly lower value. I dug around with Google and found recommendations varying from 30 to 90 ohms for low power devices.

The tubes are probably fine since it does go click on your vasalene marbles.

Yech. Note the big paper electrolytic cazapitor. I can't tell if it's a power supply filter cap or across the meter to smooth the output. If across the power supply, replace it as judging by the age and type, it's almost certainly dead. If you have an ESR tester, use it. (If you don't have one, buy or build one).

I'll stand on my comments that your clip lead and external power supply kludge is probably causing problems. Find some suitable electrolytics and place them across where the battery would normally connect. The idea is to reduce the impedance that the counter sees in the direction of the power supplies. That should reduce the motorboating (oscillation).

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com               jeffl@cruzio.com
# http://www.LearnByDestroying.com               AE6KS

Does USA "kludge" = UK "bodge" ?

Yes...

I tried putting a 100uF cap in that area, the result was that it greatly increased the frequency of the stray clicks.... I wonder if a row of 10 9-volt batteries would work?

Did you figure out where the yellow electrolytic in the photo was connected? Power supply filter or meter integrator?

Find an oscilloscope that can tolerate some high voltage (i.e. 100x probe). Look at the power supply line. Got pulses? If so, get rid of them with a filter cap.

Old carbon composition resistors tend to change value with age (and moisture). With the power off, you might walk though the pile and see if there are any that are radically off value. I don't think this is causing the problem, but it might be contributory.

Probably, but rather expensive. It should work with your external power supplies, but due to the apparent lack of power supply filtering, it might be oscillating.

If all else fails, trace out the schematic. It doesn't look all that messy and you can probably use the DG-9 schematic as a guide. The author of the web site where we found the DG-9 schematic is the founders son. Send him email asking if he has a DG-7 schematic or if he knowns anyone that can help.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

I think so. Also sometimes spelled kluge.

-- Jeff Liebermann snipped-for-privacy@cruzio.com

150 Felker St #D Santa Cruz CA 95060 Skype: JeffLiebermann AE6KS 831-336-2558

It's connected across the meter.

Already tried that.... or are you talking about the AC line feeding my transformer/rectifier/cap power supply?

Checked them all, and all are well within their specified tolerances.

I might do that. Can't think of anything else it might be, as it's really a rather basic circuit. BTW that small tube with the 2 leads I mentioned is a

5841 (voltage regulator), and I don't have a sub for it.

Check this out:

Googleing says it is a 900 volt regulator. maybe a stack of 150 volt zeners would work.

I did see some replacements available.