CDV-457 Models 2 & 3 110 Volt Geiger Counters, Nucleonic Corp. of Am..
- Hi Folks!!! I've been blessed with not one, but TWO CDV-457 110
volt AC type GM counters.... They are both made by the Nucleonic Corp
of America (Brooklyn 31, NY)...
These units are hybrid, i.e. some vacuum tube and solid state
components. The unit is primarily VT, so those of us that appreciate
glassFET's won't be disappointed!
Both models are very similar in appearance: They're housed in a
typical period enclosure similar to the HP 400-D VTVM.. Color is a
Robin's egg blue... Both units use the CDV-700 series 6993 GM tube in
a CDV-700 type probe arrangement... They have a very large, well
illuminated meter dually used for 0-1500 counts/min or DC volts..
Three controls exist: Off/HV Check, X1, and X10; HV adjust; and audio
volume... There is also a nice NE-2 style neon tube that fires when a
pulse is detected.. Both units also have an external output on the
back of the instrument for a plotting recorder.
The unit is capable of supplying a variable DC voltage to the GM
tube of approx 450-1200 volts DC... This should immediately clue you
in to the fact that this can become an extremely versatile piece of
By either of two routes, the CDV-457 can be used to test GM tubes,
or be outfitted to use other, possibly more sensitive probes. The
standard corded probes could be done away with, and replaced by a
chassis mount connector of the BNC/MHV/SHV type, and run to a test
cable to allow for the testing and use of other probes, or an
Amphenol three conductor plug can be inserted into the unscrewed
probe, allowing the purist to use the meter for testing/other probes
without damaging or modifying the unit in any way, allowing it to be
converted back to stock configuration in no time (I can appreciate
the beauty of this approach, having seen all too much classic
electronics equipment permanently "improved" in one way or another..).
The Model 2 uses an earlier style narrow probe that was fashioned
out of several pieces of metal, while the Model 3 uses the familiar,
solid machined 700 6B style probe. The cable on #2 is just that...
The older, very inflexible coax type resembling RG-59 cable. The
Model 3 uses the standard thin 700 6A/B type flexible cable.
The Model 3 came from an East coast source, and suffered from the
usual amount of corrosion: Aluminum parts were oxidized, but
everything basically worked fine with the exception of a broken
external recorder switch. The cheap, pressure applied lettering gives
both units a home made look, and flaking of it was a real problem
inside the #3 unit..
#2 came from a West Coast source, and had an arid type of "dry"
corrosion: There was paint and dirt buildup under the Dakaware knobs,
but the unit was in better shape and cleaner than the Eastern unit..
Both units have a schematic inside the case, making it easier to
figure out when things go amiss...
The tube lineup is: 5Y3, 6V6, 6CM7, and 12AT7...
The 5Y3 is a full wave rectifier providing the basic B+ to run the
unit.. Both halves of the 12AT7 are used as pulse amplifiers from the
GM tube. One half of the 6CM7 runs the audio output into the onboard
speaker. The other half of it, in combination with the 6V6, provides
probably the neatest function of the whole unit: Rather than engineer
a costly, built like a brick outhouse HV circuit, the engineers
cleverly used a principle of the old 50's tube type Geiger counters
to provide for the high B+ needed for operation of the GM tube:
Namely, they resorted to using the other half of the 6CM7 as a
relaxation oscillator, essentially feeding voltage pulses to the 6V6,
which then amplified them, charging and discharging an inductor in
the plate lead circuit.. As a coil's magnetic field collapses, huge
voltages are created... If any of you have ever held onto the
terminals of a starter solenoid or other relay as it kicked out, you
know EXACTLY what I'm talking about!!! Anyways, the 6V6 charges the
choke, and as the choke discharges between cycles, it releases a HV
pulse that is then half wave rectified by an early solid state
diode.. This then is capacitively filtered, and applied to the GM
As the frequency applied to the 6V6 changes, so does the voltage on
it.. In my Model 3, you can hear the high pitched mosquito like sound
of the relaxation oscillator quite well!!!
Bottom line: If you can pick up one of these units, they are
historically significant, work well, and they will make a NICE
addition to a working collection!!! Two thumbs up!!! -Tom