GregS wrote in message
...
In article , "N_Cook"
wrote:
Chris F. wrote in message
. ..
Yes it uses some kind of step-up circuit, and the output to the GM tube
is
about 300-400 volts.

"N_Cook" wrote in message
...
Chris F. wrote in message
...
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)