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| CDV-700 6B Geiger Counter With Original Phones | LiFePo Battery and Insulating Foam Pad |
CDV-700 6B Geiger Counter Circuit Board Mods, note the remnants of the arced-over trace below the white wire
CDV-700 6B Geiger Counter High Voltage Diode Replacement (1N4007), GV3A Corotron regulator in center
Regulated power supply and speaker driver for the Victoreen CDV-700 6B Geiger Counter
(C) 2020-2023, G. Forrest Cook
This article describes a number of repairs and improvements that can be performed on a Victoreen model CDV-700 6B Geiger counter. The repairs include cleaning the circuit board, replacing the high voltage diode stack and re-routing the high voltage probe line to eliminate common circuit board arcing problems.
Several circuit modifications have also been performed on the Geiger counter. The original 4X D Cell power supply has been replaced with a regulated supply that improves the counter's stability and allows it to be run from a 7.2V lithium battery or a 13.8V automotive power supply. A red LED has been added as a power indicator. The high voltage oscillator has been modified to eliminate a radio frequency ringing problem. A fuse and crow-bar circuit have been added to guard against the application of reverse battery voltage. Finally, a miniature speaker has been added so that the counter clicks can be heard without the use of headphones.
These modifications are based on an earlier article I wrote about the older CDV-700 counter, the speaker driver circuit was borrowed from my Count Accumulator for Radion Levels (CARL) circuit. This version of the regulator produces voltages that are closer to the 3V/6V used in the stock counter. If NiMH cells are used with this regulator, six cells should be used instead of the five used in the older version.
There are several modifications that should be done to a CDV700-6B geiger counter to eliminate common failure modes or repair a defective counter. Beware that the pink-colored circuit board is much more fragile than modern glass-epoxy circuit boards, traces can melt off with just a little soldering iron heat. Proceed carefully and solder quickly.
The high voltage rectifier stack in CDV700-6B counters are often defective, it should be replaced with a 1N4007 1000V piv silicon diode even if it has not yet failed. The original rectifier (CR3) is a skinny brown cardboard tube with metal ends that is located on the bottom edge of the circuit board near the connection to the probe leads. The replacement 1N4007 diode can be mounted in the holes used for the original diode, the cathode (white band) should be closest to the probe wire. Teflon or other insulated tubing should be place over the long diode leads to prevent arcing, see the photo above.
The GV3A-900 “Corotron” corona voltage regulator tube has a finite life span and may need to be replaced. It has a nominal regulation point of 900V and an actual regulation point at a slightly higher voltage. The tube has a small amount of radioactive material, possibly radium, that helps to start the corona discharge. This material decays and becomes less radioactive as it ages, making the tube less likely to start the regulating corona discharge. Lack of regulation can cause damage to pulse transformer T1, pulse detector transistor Q1 and possibly the 6993 geiger tube. Unregulated high voltage will also cause the counter's calibration to drift.
The regulation of the GV3A tube can be checked with a DMM and a high impedance 100:1 high voltage probe. Rotating the R5A 8K oscillator bias adjustment pot will cause the high voltage across the 6993 Geiger tube to rise and fall, if the GV3A tube is operating properly, the voltage will not rise above the regulation voltage. The regulated voltage was measured at around 940V on two different CDV700-6B counters.
A defective GV3A-900 tube can be replaced by a series stack of solid-state zener diodes which should be built to regulate just above 900 Volts. Zener assemblys for the CDV700-6B can be found on eBay. Typical zener diode stacks use three 300V zener diodes and a fourth padder zener diode to fine-tune the regulation voltage. The cathode (white band) side of all of the zeners should point toward the connection with the 1M resistor R9, .001uF capacitor C5 and transformer T1 pin 4. Many 300V zeners could be used to build your own 900V regulator, type 1EZ300-DC should work. Combinations of zener diodes with different voltage ratings can be used as long as the stack adds up to around 900V. The recommended regulated voltage should be in the range of 910V to 920V after the padder zener has been installed.
Another common failure with the CDV700-6B is shorting of the high voltage probe trace to the board mounting screws and nearby traces. Remnants of this trace are visible in the photo of the circuit board, below the white wire. The trace snakes around two of the board mounting screws. A black arc path can be seen between the middle of the original trace and the adjacent trace. The old trace should be lifted from the circuit board and replaced with a short piece of insulated wire, see the white wire in the above photos. The circuit board pad for the probe center wire came off of the board, the hole was enlarged and a small 2-56 machine screw was used to mount a solder lug to the board. The new white wire was connected to the probe center wire at this point.
The two high voltage disk capacitors (C4, 0.001uF and C5, 0.01uF) in the high voltage rectifier circuit can sometimes short out. It is a good idea to repace both of these parts, be sure to use capacitors rated for at or above 1000V.
The 400uF electrolytic capacitor (C1) that is located across the meter should be replaced, a 470uF 16V capacitor can be substituted. Old electrolytic capacitors are a common failure point in many types of electronics.
The original solder flux on the circuit board can become conductive and lead to high voltage arcing, it should be cleaned off. Use an X-acto knife or similar sharp tool to scrape the solder flux from around all of the circuit board pads. Pay particular attention to the high voltage traces that connect to the probe, transformers and high voltage rectifier. Avoid making scratches in the board material if possible.
Once the majority of the flux has been scraped off, apply rubbing alcohol to the board and scrub it down with an old toothbrush to dissolve the residue. Dab up the excess liquid with a tissue. Repeat the alcohol cleaning treatment until the board is clean.
If your counter works after performing the above repairs, you can stop here and put everything back together. Be sure to perform the oscillator current adjustment before using the counter (see below).
A number of additional problems can occur with the CDV700-6B, these include open or shorted probe wiring (see article), failure of one or both of the adjustment pots, failure of either or both transistors and resistors that are out of tolerance. Dirty switch contacts and corrosion caused by leaking batteries can also cause problems. The oscillator transformer can have arcing problems on the high voltage winding. In some cases it may be possible to repair the transformer by cleaning the area where the shorting has occurred, then coat the area with silicone. Be sure to let the silicone fully cure in a warm area for several days before applying power, uncured silicone is electrically conductive. Electronic grade RTV silicone is recommended.
The following modifications allow the CDV700-6B to be run on a variety of DC power sources while providing regulated power to the counter. Running the counter on regulated power produces more repeatable readings.
Remove the eight 6-32 screws that hold the original platic battery holders to the back of the main circuit board. Mark where the 0V (black), 3V (blue) and 6V (red) wires connect to the board or take a photo of the original wiring for your reference.
Replace four of the six board mounting screws with 1/4" 6-32 screws. The screw that is closest to the Phone jack should have an insulating plastic washer to prevent shorting the circuit board trace to the case. Note that this screw connects one of the transformer legs to the case and acts as the ground return between the Geiger probe and the circuit board. A small green wire was added between the probe shield wire and the main circuit board's ground, this keeps the probe grounded when the board is removed from the mounts.
If the new screws don't seat fully, don't over-torque them or you may break the mounts. Use a 6-32 tap or self-tapping screw and some cutting oil to deepen the mount threads if necessary.
The stock rotary switch turns on both the 0V and 6V lines. Adding the regulator circuit requires isolating the 6V wiring from the switch so that it can be used to control the input power to the regulator. The 0V side of the power switch can be left alone, it will be bypassed.
To isolate the power switch, cut 1/4" sections of copper trace around both sides of the switch contact and re-route the 6V line around the switch using tinned copper wire and teflon insulation. This can be seen in the above photo where the yellow/black wire connects between the blue regulator board and the main counter board.
The positive 7-20V input voltage to the 6V regulator connects to two adjacent switch pins via another yellow/black wire. The negative side of the input voltage comes from the green/white wire and is wired directly to the ground trace on the board.
Use a 1"x2" or larger piece of perforated circuit board for the 6V regulator circuit. Mark and drill two mounting holes in the regulator board so that the holes align with two of the counter's original battery holder mounting holes. The regulator board will be mounted on top of the main circuit board using two 1/2" 6-32 screws and 1/4" nylon spacers. The blue regulator board shown in the above photo is small and is only mounted with one screw, a longer board that uses two mounting screws is recommended.
Assemble the voltage regulator IC and associated resistors, capacitors, fuse and diode to the regulator circuit board. If the supply voltage will be above 10V, the regulator should be attached to a small heat sink. In the board shown above, the regulator tab was (quickly) soldered to a small copper trace on the perf board for heat sinking. The regulator board was hand-wired on the back side with tinned copper wire and teflon insulation.
Stranded wire was used to link the regulator board to the counter board. Three wires attach the board to the counter: green/white for ground, red for regulated 6V and yellow/black for switched 7-20V. The 6V line connects to the main board via a 2 pin header and through a blue rectangular jumper. The jumper can be removed and a current meter can be inserted across the header to monitor the current through the 6V line when calibrating the oscillator current.
The original CDV-700 circuit has a split battery power supply that draws less current in the lower 0-3V line and more current in the upper 3-6V line. To compensate for this when using a single voltage supply, a 3.1V diode string regulator was wired between the 3V line and ground. The red power LED is run from the 0-3V line, it's drive current is essentially free since it would otherwise be wasted in the diode regulator. The diode regulator is mounted on the counter's main circuit board. It can be seen inside of some translucent spaghetti tubing in the circuit board photo. A 470uF electrolytic capacitor is attached between the 3V line and ground, it is used to filter the oscillator signal from the 3V bus and boost the oscillator's signal strength. The capacitor was secured to the circuit board with hot-melt glue.
A pair of 3.7V, 2.6AH LiFePo (Lithium-Iron Polymer) batteries power the modified Geiger counter. The cells are installed in a dual 18650 size battery holder and should be surrounded by closed-cell foam to keep them from moving around the case and shorting out. LiFePo batteries can release a large amount of energy if they are shorted out, follow the manufacturer's guidelines carefully. The two wires that connect the battery to the circuit board should be installed with mating connectors so that the battery holder and cells can be easily removed and recharged in an external charger.
LiFePo cells should be charged according to the manufacturer's specifications. A typical charge regime is to charge the cells with constant current that is about half of the battery's mA-hour rating up to a specified voltage, then top the battery off at a constant voltage for a specified amount of time. The cells can also be trickle-charged at a lower current, but they should not be left on a charger continously. An inexpensive TP4056-based USB charger board or similar is recommended for charging the individual LiFePo cells.
A matched pair of 4 pin molex connectors were used for the battery connectors, they were recycled from an old desktop PC. The two black wires are used for the negative lead and the red and yellow wires are used for the positive lead. The female side of the connector pair should be installed on the battery side. It is a good idea to install an extra fuse between the positive wire on the LiFePo battery holder and the wire leading to the circuit board. All of the battery connections should be covered with heat-shrink tubing to prevent shorts.
The red power LED is attached to the bottom of the meter with hot-melt glue, the LED power leads are the red and black wires that attach to the back of the counter's main circuit board. The black LED- ground wire connects to the ground bus and the red LED+ wire connects to the regulated 3V line through the gray 100 ohm resistor that can be seen on the back of the circuit board. The resistor is held in place with hot-melt glue. A high intensity red LED should be used for the best visibility when the LED is mounted behind the meter.
The stock high voltage oscillator circuit produces small radio frequency oscillations every time it switches. These are sometimes called snivets. This RF can cause interference with radios and puts unnecessary stress on the oscillator transistor. The snivets can be eliminated by adding a small capacitor to the circuit. Solder a 2.2nF, 50V capacitor across the base and collector of the oscillator transistor. This can be seen as the yellow capacitor that is near the lower right corner of the blue regulator circuit board in the circuit board photo above. A small loop of wire was soldered to one lead of the capacitor, this allows an oscilloscope probe to be clipped to the oscillator for monitoring purposes.
The 8K variable resistor (R5A) in the high voltage oscillator circuit should be adjusted so that the current between the 6V regulator and the circuit board is set to 33mA. Remove the blue jumper between the 6V line (red wire) and the main circuit board and connect a milliamp meter across the two jumper pins to perform the adjustment. After adjusting the current, remove the current meter and re-install the blue jumper. If you have access to a 100:1 high voltage probe, adjust R5A slightly above the point where the voltage across the geiger probe stabilizes, see the Corotron regulator section above.
The audible clicking sound that the CDV-700-6B Geiger counter produces is not very loud and the stock headphones can be uncomfortable to wear. It is fairly easy to add an amplified miniature speaker to the unit so that the clicking can be heard without headphones. The speaker driver circuit shown in the schematic can be used to drive a small permanent magnet speaker with an impedance of around 32 ohms.
The speaker driver circuit uses a CMOS LMC555CN IC wired as a one-shot pulse generator that drives a 2N3904 transistor. The transistor amplifies the pulse current enough to drive a miniature speaker. The trigger input of the LMC555CN timer uses a 5.1V Zener diode to clamp the negative going pulses from the counter to a minimum of around 1V, this protects the IC's input from being damaged by negative voltages.
The volume of the speaker can be adjusted by changing the value of the 22 ohm series restor, the resistor should be 10 ohms or greater to limit the speaker current. The 1N4001 diode across the speaker clamps negative inductive pulses that form when the transistor turns off, this protects the transistor and also improves the sound of the clicks coming from the speaker. The original phone jack should be removed and the miniature speaker should be cemented into the connector hole with epoxy, hot-melt glue or silicone. A short piece of 1/2" ID PVC pipe or PEX tubing can be cut and glued around the speaker to focus the sound toward the operator.
CDV-700 6B Geiger Counter Miniature Speaker addition
1X LM2941CT low dropout adjustable voltage regulator 1X LMC555N or TLC555CP CMOS timer IC 2X 3.6V LiFePo cells, 18650 size, 2600mAH capacity 1X dual 18650 battery holder (Jameco 2146231) 1X 1/2 amp pico-fuse 1X 2N3904 NPN transistor 6X 1N4001 Silicon Diodes 1X 1N4007 1KV Silicon Diode 1X 1N5818 Schottky Diode 1X 1N5231 or 1N751 5V, 500mA zener diode 1X 22 ohm, 1/4W resistor 1X 100 ohm, 1/4W resistor 1X 1K ohm, 1/4W resistor 2X 10K ohm, 1/4W resistors 2X 100K ohm, 1/4W resistors 1X 3.3K ohm, 1/4W resistor 1X 500 ohm multi-turn trimmer potentiometer 1X red high intensity LED 1X 10nF, 25V capacitor 2X 100nF, 25V capacitors 1X 2.2nF, 50V capacitor 1X 470nF, 25V capacitor 1X 22uF, 16V electrolytic capacitor 2X 470uF, 16V electrolytic capacitors 1X miniature permanent magnet speaker, 8-32 ohms Assorted pieces of insulated/stranded and tinned copper wire Assorted pieces of teflon or plastic insulation tubing One piece of 3/16" spaghetti tubing One 4-pin male PC power supply connector with wires One 4-pin female PC power supply connector with wires 2X 6-32x1/4" nylon insulated spacers 1x 6-32 nylon insulated washer 4x 6-32x1/4" machine screws 2x 6-32x1/2" machine screws 1x 1"x2" or larger perforated circuit board material
Here is the original schematic for the CDV700-6B counter.
Back to FC's Geiger Counter Circuits page.
