KFF Homebrew Tips

Geiger Counters in the Ham Shack
Many hams are also interested in nuclear radiation detection for a number of reasons. A few of those reasons might be:
A) Collecting radioactive radio tubes
B) Testing high power equipment for X-Ray emissions
C) Testing and cleaning up surplus radio gear (radium paint abounds!)
D) ARRL has approved GAMMA RADIATION as a ham band for contests!!
For many years I never saw any decent radiation detection gear at hamfests. Since the late 1990's
the US Gov't has been surplussing out those Civil Defense units that were stored in basements and
attics all over our country, many of which now wind up for sale cheap.
There are several versions of these yellow, CD placarded hand held detectors. Ones marked
CD V-715, CD V 710, CD V-720 and a few others WITHOUT external sensors or cords are high range
ion-chamber detectors and have little use except near an atomic detonation.
One model is a true Geiger Counter, the CD V-700. This one has a silver handheld probe attached to the
unit by a cord. Sensitivity of this Geiger Counter is very good and has many everyday uses around any
home or ham shack.
Unfortunately the electronics in these 50 year old units is deteriorating, causing intermittents and
down right failures.
Even though all the true Geiger Counters bear the ID CDV-700, there were several different manufacturers,
each with their own circuit preferences. Between makers, even the battery count ( all use D Cells) varies
from 2 to 5 cells!
Also there were different versions within the same maker, such as model 6, 6A and 6B. All versions use
an earphone (no speaker) and have a meter with 3 ranges.
Many regard the Lionel units to have the best circuit design, however the mechanical details are lacking.
Units made by Anton and some others are pretty much collected as display items due to wretched unreliability. One manufacturer,
Electro Neutronics Inc, abbreviated ENi has some really good components and mechanical design, but
their circuit is the worst and most unreliable of all.
In my opinion the physical layout of the ENi is better than that of
the Lionel, so the merger of the Lionel circuit concept into the ENi
mechanical layout really does give the best of both worlds.
What follows is a drastic modification to an ENi, updating it with some modern components, while
changing the circuit to better follow that used by the Lionel. This melding of the two technologies I
dubbed the LENi.
Below each picture is the link to the high resolution version, should you care to download it for details.
Once the basic LENi conversion is finished, additional improvements may be added to further upgrade the usefulness of the device. The Speak2Me speaker module has been the most popular add on. From there it is easy to add, volume control, Sonalert, power LED, Fast/Slow Response switch, reset zero button, pulse LED, variable high voltage, digital readout, internal sensors, etc.


 scan it and stick it in the case bottom over the old schematic.
Fig.3 New cover for instruction manual too:
Except  for this schematic, and ENi board layout, all the other pages in the
manual are from the ENi CD V 700 6B.
Look at the schematics of both the ENi and the Lionel to
observe the "before" and "after" circuits.

By performing the simple and inexpensive K0FF LENi mod, you can
create an Lionel clone from an ENi.

The LENi modification procedure:
Fig. 4 - Parts marked in red will be removed for replacement. Cut one trace on the PCB between the red X's:

1) On the component side of the board, replace Selenium HV diode CR-5 with a new silicon diode, 7 kV @ 100 mA.
Two 1N4007's in series will work fine.

2) Remove R13 and replace it with an 820 Ohm resistor.

3) Add a .0022 uF 50 V capacitor between Base (B) and Collector (C)
of transistor V4. This can be tacked on the solder side of the board.

4) Remove R12
5) Prepare a 900V Zener diode regulator sub board as follows:
Nine 100 Volt Zener never yield 900 Volts, but a bit below, usually 20 Volts or so. You can make up this
shortfall by adding a 10th Zener, I use a 51 V unit for a total of 10
Zeners in series, and 915 Volts:
Fig. 5- Prepare Zener Regulator Sub Board:
Fig. 6 - Wiring the Zener Regulator Sub Board:
Resistors R-17 and R-12 ( on the LENi Schematic) are attached to the Regulator sub-board as flying components. The finished assembly
wires to the main board at 3 points. Note the ground leg needs a small hole drilled in the main board:
Fig. 7 - Placement Detail of Zener Regulator Sub Board:
6)  Replace the input capacitor, C5 (a .01) with a .0025 @ 3kV. The reduced coupling helps prevent overload and
destruction of the input transistor.
7) Cut a portion of the circuit trace between the hole where CR-7
anode was (lead opposite the bar end) and the ground (+) buss. This
connection is now severed for ever. CR-7 is removed and replaced with a jumper
wire, which also proceeds to bridge the gap from the former anode hole to the
switch contact pad immediately adjacent( this is the trace that goes to
transformer Pin 6. Doing so effectively ungrounds the emitters of V2 and V3 and
reroutes them to the negative battery terminal, via the on-off
switch as shown in schematic to correspond exactly to the Lionel circuit.
Fig. 8 - Detail of step (7):
Fig. 9 - Replace CR-7 with jumper ( zero Ohm resistor)
8) Next step will be to change the metering circuit, to improve
operation, hang time, and eliminate premature failures.
Add the resistors
R-14, R-15 and R-16 to the solder side of the board, across the gap created in the foil in
step (7), to points labeled <A>, <B>, <C> in the detail. I make
"Y" out of the 3 resistors and tack solder them on the foil side of the
Fig. 10 - Detail "add resistors":
Optional: Add a BNC connector to the panel in the place where the cable originally entered. Adding a
mating connector to the probe cord allows for interchanging additional probes for special applications.
7) Resolder all the pads for the Large Scale IC Module, the part
where the
CAL pot is located.
These are often loose or bad from vibration, and this is a good time
address this problem. Look closely at all the other pads, especially
transformer connections, and touch up the solder as needed. LENi's
use a
single point ground, a screw located near the transformer. This
*MUST* be
tight and have the star washer in place for a good ground. A loose
here is the number one cause of intermittent operation, and even

That's all the basic electronic changes. Of course I always
encourage a
BNC be
added for probe changing. Please turn off the unit when changing
Fig. 11 - Probe with BNC added:

Lastly, the unit will now work fine from only 2 batteries instead of
the original 4, so you can remove part of the battery box with a
leaving about 1/2" of the cut battery chamber for housing the
option. Save the cut of piece for housing future mods, like the AC
I always do away with the spring contacts that are
supposed to make electrical connection to the board pads, and
replace with hard-wired Teflon wires. The intermittents disappear
Fig. 12 - Battery wiring points (set is POSITIVE GROUND):


Reset Switch = a pushbutton wired across the meter. Manually shorts
meter when driven way off scale by a high countrate.

X2 scale extension = a 10K pot in series with either meter lead,
shorted out
with a panel switch for X1. Adjust pot with a steady source for 1/2

Fast-Slow = Meter averaging capacitor, 100 uF for FAST, with a panel
add 330 for 430 total for SLOW.

Soundcard = (Speake2Me Module) http://www.qsl.net/k0ff/Speak2Me%20Module/

Volume Control = 1K pot in series with the 4 Ohm speaker. Usually
placed in
the vacated single button earphone jack hole. Alternative location
is on
battery case, rear apron, below snap catch.

PULSE LED = LED wired across speaker terminals on SOUNDCARD. No
series resistor but you can use up to 820 Ohms for effect.

POWER LED = LED wired across 3V battery terminals, after on-off
switch. Use
820 Ohm resistor.

Battery Condition = a DPDT momentary pushbutton that turns the meter
into a
voltmeter using a suitable series resistor(68kOhm). Both sides of the meter
need to
be switched.

Headlights = White LED lamps or one White and one UV. Only used on

DRILLING PANEL for SWITCHES = all switch and pushbutton locations
spotted on the bottom of the panel by 1/4" circular mold marks.
Center punch
the desired location and drill from the bottom ( to preserve paint
surface side).
Drill pattern as seen from TOP SIDE:

Nomenclature: I use laser printed clear address type labels, sealed
"decal set".

On certain model ENi's the original panel lettering is painted on
instead of
raised like later versions. The paint is flush, so once the panel is
and nomenclature is added, a single sheet of adhesive backed clear
laminating plastic can be overlaid for a really nice looking and

If the unit fails to calibrate, or some scales are off compared to
always replace V2 and V3 first. Replace them both at once. In many
instances, these parts will test OK but replacing them cures the

Never replace the precision capacitors C-3 and C-4 unless broken or
they are part of the calibration circuit and should be replaced with
parts only.
Fully modified LENi with all add-ons:
Fig. 12A - LENi right side:
Fig. 14 - LENi left side:
Fig. 15 - LENi back side:
Fig. 16 - LENi bottom, shield open:
Fig. 16 - LENi bottom, shield closed:
Fig. 18 -  X-Ray view:
This webapge exists to encourage the hobby of nuclear rad detection:
If you have doubts that an amateur could find radioactive stuff in the ordinary environment, take a peek at some of the "finds" that I have done with my LENi:

Happy Homebrewing, Geo>KFF