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 ?


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