Detectron Geiger Counter
in [Repair]
|
From: N_Cook on 23 Apr 2010 03:46 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 ?
From: Chris F. on 23 Apr 2010 19:04 >> > 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........
From: Jeff Liebermann on 23 Apr 2010 20:47 On Fri, 23 Apr 2010 10:38:23 -0700, "Chris F." <zappyman(a)hotmail.com> wrote: >> 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. Oh, I was hoping they were similar. Sorry(tm). >> 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. 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. <http://wiki.answers.com/Q/Can_you_replace_a_selenium_rectifier_with_a_silicon_rectifier> >Yes, I checked these with a tube tester, and even trying subbing them with >new-old-stock tubes. The tubes are probably fine since it does go click on your vasalene marbles. >Here's a photo of the circuit: >http://s282.photobucket.com/albums/kk275/packrat79/?action=view¤t=detectron.jpg 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(a)cruzio.com # http://www.LearnByDestroying.com AE6KS
From: N_Cook on 24 Apr 2010 04:02 Jeff Liebermann <jeffl(a)cruzio.com> wrote in message news:kcf4t5hlkdkjj4h04vns4qf7t8d3dhcsob(a)4ax.com... > On Fri, 23 Apr 2010 10:38:23 -0700, "Chris F." <zappyman(a)hotmail.com> > wrote: > > >> Schematic of the DG-9 model: > >> <http://www.curtcass.com/detectron/dg9-sch.jpg> > >> Probably quite similar to the DG-7. > > > 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(a)cruzio.com > # http://www.LearnByDestroying.com AE6KS Does USA "kludge" = UK "bodge" ?
From: PeterD on 24 Apr 2010 08:12
On Sat, 24 Apr 2010 09:02:32 +0100, "N_Cook" <diverse(a)tcp.co.uk> wrote: > > > >Jeff Liebermann <jeffl(a)cruzio.com> wrote in message >news:kcf4t5hlkdkjj4h04vns4qf7t8d3dhcsob(a)4ax.com... >> On Fri, 23 Apr 2010 10:38:23 -0700, "Chris F." <zappyman(a)hotmail.com> >> wrote: >> >> >> Schematic of the DG-9 model: >> >> <http://www.curtcass.com/detectron/dg9-sch.jpg> >> >> Probably quite similar to the DG-7. >> > >> 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(a)cruzio.com >> # http://www.LearnByDestroying.com AE6KS > > >Does USA "kludge" = UK "bodge" ? > Yes... |