HP Clock

Sunday, January 13, 2013

Cold War Artifact

When my friend Curtis visited me last week he surprised me with this cold war artifact possibly from the late 50s early 60s.  He picked it up at the museum in Los Alamos, New Mexico.  Initially, none of us recognized the device or knew what its purpose was.  I sent this picture to my daughter and she suggested this was worthy of blog post.

Opening up the box only deepened the mystery when I was greeted with a circuit board containing flash light bulbs, resistors, diodes, one transistor and a transformer.  There was also a place for a single D Cell battery. There was a tantalizing clue to the devices purpose when I discovered a sticker on the inside with a schematic.

Here is a closer view of the interior of the case and the schematic.

Using Google and the number CD V-750 that appeared on both the outside of the box and the interior sticker the manual for the device was located.

Using Photoshop I was able to create a better image of the schematic.

So what is it?

The operating and maintenance manual explained everything, including the purpose of the device, how it worked and how to fix it.  The yellow box is a Radiological Dosimeter Charger.  It’s used to charge, or ‘zero’ a quartz fiber dosimeter.  This style of dosimeter is essentially a small electroscope, and the quartz fiber is a delicate gold plated indicator.  This quartz fiber indicator is inside a small airtight ionization chamber.  The ends of this chamber are transparent so that the fiber can be viewed with a built in microscope, and compared to the built in reticule to determine the charge on the fiber.  To reset a dosimeter of this type requires 150 to 200 volts.

With the manual in hand I wondered if this charger was still functional.  The circuit is very straightforward and is actually a simple switching supply used to generate the high voltage from a D Cell battery.  Transistor Q1, capacitor C1, and transformer T1 primary windings create an oscillator.  The output of that oscillator is stepped up through the transformer where it is rectified by CR1 and filtered by C2 to create the high DC potential.  Potentiometer R2 and resistor R3 create a voltage divider.  The wiper of R2 creates an adjustable output voltage to reset the dosimeter.  In this picture you can see the printout of the manual showing the waveform at the anode of CR1, the D Cell, and the jumper used to bypass S1 to activate the circuit. 

What about the light bulbs?  The light bulb that is lit is used to read the dosimeter.  The other bulb is simply a spare held in a rubber grommet.

What could be better than to test the charger with a vintage instrument from the same era?  I used my Tektronix 535A tube oscilloscope to view the waveform at the anode of CR1.  You can see that the waveform is nearly identical to the waveform shown in figure three of the manual.  Each large division on the oscilloscope reticule is 10 microseconds. 
A more exact reading was taken with a digital scope.  The period of the transistor oscillator is 36.26 microseconds, or about 27.6 KHz.  The ringing between each major pulse had a period of 5.8 microseconds, or about 172.4 KHz.  So it does work!

One last observation...

Opening the cover let out the unique aroma of 50s Science and Science fiction movies.