Detectron Geiger Counter

[Chris F.]

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?

 

[Sjouke Burry]

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).

 

[N_Cook]

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 ?

 

[Chris F.]

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

 

[N_Cook]

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

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 ?

 

[GregS]

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

 

[N_Cook]

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)

 

[Chris F.]

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&current=detectron.jpg

 

[Chris F.]

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.

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&current=detectron.jpg

 

[Sjouke Burry]

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&current=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.

 

[Chris F.]

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........

 

[N_Cook]

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 [email protected]
# http://www.LearnByDestroying.com AE6KS

Does USA "kludge" = UK "bodge" ?

 

[PeterD]

Does USA "kludge" = UK "bodge" ?

Yes...

 

[Chris F.]

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).

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?

 

[Chris F.]

Did you figure out where the yellow electrolytic in the photo was

connected? Power supply filter or meter integrator?

It's connected across the meter.

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.

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

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.

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

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.

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.

 

[tm]

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:

http://www.dvq.com/geiger/heath_rc1.pdf

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

I did see some replacements available.

 

[David Nebenzahl]

Does USA "kludge" = UK "bodge" ?

Yes, except that I'm going to insist that the proper spelling is
"kluge"[1], which rhymes with "luge", not kludge which would rhyme with
fudge (which would just be wrong!).


[1] I'm fairly certain, though I can't prove it, that this term comes
from a line of printing equipment made by the company called Kluge,
which included a number of very complex Rube-Goldbergian machines, hence
the current usage.

 

[PeterD]

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?

Probably would, but may not for very long. Why not an inverter with an
output of about 90 VDC? The current is not very great, you could
probalby bodge one together easily enough.

 

[Ian Jackson]

I think so. Also sometimes spelled kluge.
<http://en.wikipedia.org/wiki/Kludge>

Be aware that, while "bodge" and "botch" tend to get used
interchangeably, "bodging" and "botching" are not the same thing.

A "bodger" is a skilled, itinerant wood worker.
http://en.wikipedia.org/wiki/Bodging

A "botcher" is someone who fakes doing a proper job of something.

If a kludge provides a functionally adequate (but inelegant) solution,
I'm not sure whether it is really a bodge or a botch.

 

[Chris F.]

At this point, I think you're right - it's probably issue with the B+
supply, probably a combination of poor filtering and inadequate output
current. After tracing out the schematic that's about the only conclusion I
can come to. I'm going to take a break from this for a while, and then try
building a better power supply, and I will post back with the results.
There was something of interest in those links you posted; one specified a
15-volt C battery (for the tube bias) - I was only using about 4.5V. Don't
know if that would have much effect on things, but it's worth a try.
Thanks for the advice.