Detectron Geiger Counter
in [Repair]
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From: N_Cook on 23 Apr 2010 11:21 GregS <zekfrivo(a)zekfrivolous.com> wrote in message news:hqsd6m$mqd$1(a)usenet01.srv.cis.pitt.edu... > In article <hqsclg$d0v$1(a)news.eternal-september.org>, "N_Cook" <diverse(a)tcp.co.uk> wrote: > >Chris F. <zappyman(a)hotmail.com> wrote in message > >news:4bd1b19c$0$12419$9a566e8b(a)news.aliant.net... > >> Yes it uses some kind of step-up circuit, and the output to the GM tube is > >> about 300-400 volts. > >> > >> "N_Cook" <diverse(a)tcp.co.uk> wrote in message > >> news:hqrj97$20q$4(a)news.eternal-september.org... > >> > Chris F. <zappyman(a)hotmail.com> wrote in message > >> > news:4bd08a44$0$12419$9a566e8b(a)news.aliant.net... > >> >> I recently bought an old Detectron model DG-7 geiger counter. This is > >> > from > >> >> the early 50's and has three 1U5 tubes inside. Power requirements are > >90 > >> >> volts for the B+, two "D" cells for the tube filaments, and a 4.5 volt > >> > bias > >> >> supply. > >> >> I went over the unit, replacing a selenium rectifier and testing the > >> >> tubes. The unit powers up, but generates a steady stream of pulses on > >> >> it's > >> >> own, even with the probe detached. The frequency of the oscillation > >> >> varies > >> >> with the range selector knob. The probe is working - when brought near > >a > >> >> source (a bunch of vaseline marbles) it will produce extra clicks in > >> >> addition to the stray ones generated by the unit. > >> >> I've checked all the other components - resistors, capacitors, etc. > >The > >> >> only thing I couldn't check is a very small tube, which looks like a > >> >> cold-cathode rectifier of some kind. I've also tried varying the B+ and > >> > bias > >> >> voltages. And I can't find a schematic, which makes matters worse. > >> >> I'm not sure what to do next. Any ideas? > >> >> > >> >> > >> > > >> > > >> > 90V seems low for a GM, is there a step up circuit to 300V or so ? > >> > > >> > > >> > >> > > > >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 ? > > > 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. > > greg 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)
From: Chris F. on 23 Apr 2010 13:34 >> > 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). 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: http://s282.photobucket.com/albums/kk275/packrat79/?action=view¤t=detectron.jpg
From: Chris F. on 23 Apr 2010 13:38 > > Schematic of the DG-9 model: > <http://www.curtcass.com/detectron/dg9-sch.jpg> > Probably quite similar to the DG-7. I came across that already, but it's quite different and therefore not much help. > > Replace it with what? Is silicon, the B+ is probably too high. > 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. >>and testing the tubes. > > 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. Yes, I checked these with a tube tester, and even trying subbing them with new-old-stock tubes. > > Nope. Probably a neon lamp used as a regulator. Photo? > Here's a photo of the circuit: http://s282.photobucket.com/albums/kk275/packrat79/?action=view¤t=detectron.jpg
From: Sjouke Burry on 23 Apr 2010 14:39 Chris F. wrote: >> 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). > > 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: > http://s282.photobucket.com/albums/kk275/packrat79/?action=view¤t=detectron.jpg > > 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.
From: Jeff Liebermann on 22 Apr 2010 20:10
On Thu, 22 Apr 2010 10:41:19 -0700, "Chris F." <zappyman(a)hotmail.com> wrote: > I recently bought an old Detectron model DG-7 geiger counter. Schematic of the DG-9 model: <http://www.curtcass.com/detectron/dg9-sch.jpg> Probably quite similar to the DG-7. > I went over the unit, replacing a selenium rectifier 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. >and testing the tubes. 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. >The unit powers up, but generates a steady stream of pulses on it's >own, even with the probe detached. 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". >The frequency of the oscillation varies >with the range selector knob. The probe is working - when brought near a >source (a bunch of vaseline marbles) it will produce extra clicks in >addition to the stray ones generated by the unit. Good, it's working. <http://www.youtube.com/watch?v=U6x5mSp0PeA> 0.2mr/hr, about the same as background radiation. (I have the same counter and the one in the video seems a bit insensitive). > I've checked all the other components - resistors, capacitors, etc. The >only thing I couldn't check is a very small tube, which looks like a >cold-cathode rectifier of some kind. Nope. Probably a neon lamp used as a regulator. Photo? >I've also tried varying the B+ and bias >voltages. And I can't find a schematic, which makes matters worse. > I'm not sure what to do next. Any ideas? Google for "Detectron Schematic". <http://www.curtcass.com/detectron> <http://www.curtcass.com/detectron/index2.html> -- # Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060 # 831-336-2558 # http://802.11junk.com jeffl(a)cruzio.com # http://www.LearnByDestroying.com AE6KS |