On Thu, 22 Apr 2010 10:41:19 -0700, "Chris F."
wrote:

I recently bought an old Detectron model DG-7 geiger counter.

Schematic of the DG-9 model:
http://www.curtcass.com/detectron/dg9-sch.jpg
Probably quite similar to the DG-7.

I went over the unit, replacing a selenium rectifier

Replace it with what? Is silicon, the B+ is probably too high.

At that age, 99.999999% of the electrolytic cazapitors have high ESR.
However, looking at the schematic, there are none. Weird.

and testing the tubes.

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.

The unit powers up, but generates a steady stream of pulses on it's
own, even with the probe detached.

Ok, something is oscillating. I have some guesses based on the
schematic, but the right way is to get a scope, with a high voltage
probe (i.e. 100x) and sniff around for which section is oscillating.
My best guess is that since the beast doesn't have any power supply
filter capacitors on any of the batteries, and you've probably
assembled a mess of power supplies, batteries, and junk to simulate
the batteries, you have a fairly high impedance on the power supply
lines. Rather than fixing it with a proper battery, find a big fat
capacitor (with a suitable high voltage rating), and put it across the
power supply leads as close to the circuit (not as close to the
battery) as possible. The magic buzzword is "motorboating".

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.

Good, it's working.
http://www.youtube.com/watch?v=U6x5mSp0PeA
0.2mr/hr, about the same as background radiation. (I have the same
counter and the one in the video seems a bit insensitive).

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.

Nope. Probably a neon lamp used as a regulator. Photo?

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?

Google for "Detectron Schematic".
http://www.curtcass.com/detectron
http://www.curtcass.com/detectron/index2.html

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