Reviving a Dead Geiger Counter

See: http://www.solorb.com/elect/geiger/cdv700/
for more info.

I purchased a Victoreen CDV700 geiger counter on EBay, the unit
looked good in the photos, but had a number of electrical and mechanical
problems.  Here's what I had to do to get a functioning device:

The counter appeared to have been exposed to water while powered up, this
caused the high voltage traces on the circuit board to turn green with
corrosion, the dual trimmer potentiometer was badly corroded, and the
driver transistor was fried.

Also, the meter movement was broken.  Luckily, the transformers, coils,
and geiger tube were in good shape, those parts are difficult or impossible
to replace.  I had a similar meter movement in my junk pile, a wooden spacer
had to be cut and painted to allow the replacement meter to fit in the case.
The replacement meter has no calibration marks, it is not very important
on this device, thanks to the addition of the numerical counter (CARL) box.

The trimmers were replaced, I soldered modern trimmers to the bottom of the
circuit board.  One of the original trimmers had the unusual value of 8K
ohms, that was replaced with a common 10K pot.  The other trimmer had a
common 100K value, a direct replacement was made.  The original dual trimmer
assembly was likely a custom part, it would be difficult to find a
direct replacement.

The corroded circuit board was scraped clean using a wire brush and an Xacto
knife.  The AC high voltage connection between the oscillator transformer
and the high voltage rectifier continued to have arcing problems on the PC
board.  I unsoldered the transformer lead and wired it directly to the diode,
bypassing the problem circuit board area entirely.

The oscillator transistor was changed to a common 2N3906, a TIP42 transistor
can also be used, it has a higher power rating than the 2N3906.
Due to the higher gain of the 2N3906, it was necessary to add a 1K resistor
in series with the 10K oscillator adjustment trimmer to optimize the
adjustment range.

After reviving the high voltage supply, the 0.01uF, 1000V high voltage
filter capacitor developed a short, the capacitor had to be replaced.

The geiger probe wire had seen a lot of use, a short circuit had developed
where the wire exited the probe assembly.  The probe assembly had to
be rebuilt.  A BNC connector was added to the end of the probe wire
and to the counter case.

This experience shows why a $100 "working" counter may be a better deal
than a $50 "as is" counter.

-FC