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From: Chris F. on 22 Apr 2010 13:41
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?


From: Sjouke Burry on 22 Apr 2010 16:20
Chris F. wrote:
> 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?
>
>
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).
From: Chris F. on 23 Apr 2010 10:41
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 ?
>
>


From: N_Cook on 23 Apr 2010 11:00
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 ?


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://diverse.4mg.com/index.htm


From: GregS on 23 Apr 2010 11:09
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
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