Old Geiger counter teardown

[denelec]

I have an old Geiger counter a friend gave me many years ago.
It's a ACR Industrial Services model 661.
Seems to be 1960's vintage. Couldn't find much on the web about it.
It still works.  It indicates about 25 cpm (counts per minutes) with only background radiation.
If I place an old lamp mantle (thorium oxide) next to the tube, the reading goes to about 900 cpm.
It uses a 1B85 Geiger tube which date from the 1950's.
https://www.orau.org/ptp/collection/GMs/victoreen1b85thyrode.htm
The voltage at the tube is only 202Vdc. It is quite low.  I don't know if it's normal. It seems to work anyway.

Does anybody knows anything about this equipment?

[denelec]

The output of the audio jack gives a series of random pulses, each corresponding to a particle detection.

[GEuser]

I like it , real old school and looks straight out of the movie "The Thing" .

Is there a audible clicker in it I could not notice one , but just aside I can see some electrolytics that should be replaced , and what's the current draw using batteries any idea?

Nice pics and thanks for the effort btw ..cheers .

[bench_knob]

Wanna know if the unit is working correctly?

Use Wenzel's Geiger-Counter simulator to synthesize known counts and its useful for calibration:

http://www.techlib.com/science/geiger.html#Geiger%20Tube%20Simulator

His website is chockful of circuit goodies (an authentic design engineer):
http://www.techlib.com/electronics/index.html

bench_knob

[denelec]

The current drain on the battery is about 3mA.  Battery life must have been very good.
The voltage multiplier circuit contains 2 2N1307 PNP Ge transistors for the oscillator. I can hear a very high pitch noise when the counter is on.
There is also 4 old style PH204 silicon diodes.  They tested Ok on my multimeter. With 4 diodes, it's probably a voltage quadrupler at the output of the transformer.
The other board contains 3 2N3392 NPN Si transistors.

The PC boards are very rustic.  No silk screen.  No solder mask. Everything looks hand soldered.

There is no nameplate, no serial number anywhere.
The counter wasn't mass produced, for sure.

So, what we have is a circuit that produce a high voltage for the Geiger tube.
Every time a particle or gamma ray hits the tube, a small current impulse is generated.  The current gets amplified, integrated and displayed on the meter.
With the audio output of the counter and the radioactive mantle, I could check the calibration.  All I need to do is check if the pulse count match what the needle indicates.