I finally got this working, at least to the point where it's now usable. The
trick was to filter the B+ supply between the three tubes, with a
filter+inductor+filter combination, to prevent stray oscillation. It still
generates about 1 stray pulse every 5 seconds or so (not counting normal
background readings), but this may straighten out with some tweaking. Even
if not, the detector is now suitable for measuring above-background levels
of radiation, up to about 20 mr/hr. Should come in handy if the nearby
nuclear power plant ever blows....
Thanks to all for the advice.
"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?