Introduction: CDV-700 ENI Geiger Counter Essential Upgrades
The ENi model 6b was the last CDV-700 commissioned by the US government for civil defense. It uses 2 or 4 batteries (more on that later) and is generally the least well regarded CDV-700 made because it lacks proper voltage regulation and is therefore prone to fail without warning. The ENi 6b also has a battery pack with loose clips. The battery clips simply insert in holes in the circuit board and are held in place by compression when the board is screwed down. Disassembly of a ENi 6b is therefore quite easy, but reassembly isn't. The meter is not connected by wires to the circuit board like all other CDVs. The meter posts go through the circuit board and is screwed down with the board.
Step 1: First Upgrade
The battery holder removes with 6 screws to reveal the main board held by the two meter posts. Note from the circuit diagram included with the instrument that there are 4 batteries, but they are wired as 2x2 series and parallel. No idea why they did that, except that maybe the carbon-zinc cells of the day didn't have much capacity. Lionels get by just fine with 2 cells.
So the first fix is to remove and discard the battery pack and get a Radio Shack 2700408 2 "AA" holder and solder the leads as shown. Two modern AA alkaline cells have about the same capacity as the original carbon-zinc cells originally had. The battery holder is glued [Gorilla glue works fine] in place upright between the battery post screws and the cover left off to make it easier to insert and remove cells. A wire tie is needed to keep the cells from falling out. With 2 AA cells instead of 4 D cells, the ENi doesn't feel like a brick anymore, either.
After discarding the battery holder shorter screws have to be used, or the original screws cut down. Only 3 of the original screw are needed for this. Two in the upper corner above the meter and one on the corner near the phone jack. The phone jack screw grounds the probe cable by means of a solder ring. The other screw (hex head) grounds the circuit board to a post next to the main power transformer and does not need to be cut down.
And yes, the CDV is positive ground as was common practice in the day.
Step 2: Second Upgrade
The ENi uses two zener diodes back to back to limit the voltage the flyback transformer sees. However, small changes in the value of the zeners make large changes in the flyback voltage, so the high voltage applied to the probe is not adjustable easily or precise.
Using a high value resistor of known value [I use a 1 gig ohm resistor] grounding the probe hot lead through a micro amp meter allows calculating the voltage without loading down the circuit. I measured about 980 volts, which is a little bit higher than it should be. To correct this I unsoldered one end of the anode resistor of the high voltage circuit and inserted a 47v zener diode backwards. You don't have to remove the board to do this as the resistor has one leg right on the edge of the board. I used a 1 watt 47v zener made by Freescale.
1N4756a or 1N4656
http://www.ebay.com/itm/25-x-1N4756A-1N4756-Zener-... Different manufacturer, but only $5 for 25 on eBay from a US supplier.
After inserting the diode I rechecked the voltage and found it had dropped more than expected, but the instrument works correctly, so I left it that way. I don't know it the zener was off-spec or the addition of the zener interacted with the original two. The final voltage I measured was 890. There are plenty of other zener diode values to choose from. If you aren't sure in advance you can get an assortment from eBay and just pick the one that works best.
http://www.ebay.com/itm/300Pcs-2V-39V-30-Values-1-... Here is an assortment of 1/2 watt zeners. 300 pieces with values up to 39 volts. You can make a series string of zeners backwards to drop more voltage.
The other picture at the top shows the circuit board unscrewed and flipped out for service.