CD-V700 radiation survey meter is a sensitive low-range instrument that can be used to measure gamma radiation and detect beta radiation. I own a functional vintage CDV700 unit, that I’ve presented here. Functional, at least until it decided to stop working for no apparent reason.
Checking everything step by step, I saw the following:
1. The oscillator works
2. Only 100V output, after the rectifier diode
Here is the circuit diagram:
I changed the two caps right after the diode. The issue remains. Disconnecting the diode’s positive end, shows 900+ volts, but when connected in the circuit, I barely can get 200V, and the funny thing is the unit was operational last time I’ve used it. I then changed the second transistor (the one handling the headset and the dial) (PNP) to a generic PNP and also changed the selenium diode to a 14KV 10mA modern one. No luck.
Thanks to the CDV700CLUB group, I got some excellent suggestions on what to try next, but most of that I checked already to no avail: Bad regulator, leaky hv cap, bad probe cable, bad geiger tube.
As disklog suggested, the problem might be failure of the T1 transformer, resulting in the defects that I have observed/measured. So I would either need to rewind the transformer or find a replacement (which is a bit complicated for such old designs).
Doug was so kind to help me on this matter, and sent me not one but two CDV700 circuit boards and some additional components. Thanks Doug!
Given this nice donation, getting my CDV700 operational again was so easy. I replaced the entire board with one received from him:
It worked from the first try!