Bipolar Disc Caps

[pentium]

....I swear I made this thread already...

Anyways, I bought a CDV-700 geiger counter off ebay and ended up getting it free because it has a problem.
When turned on it works fine and all for a few minutes, then the high voltage dies and the tube loses power. Turn it off, wait a minute and switch it back on. You get it briefly.
Anyways, something seems to be not happy that it's warmed up. I'm immediately suspecting the capacitors.


Aside from C2 which is a 3v 400uf axial for the meter and C3 which is part of the clicker circuit I'm looking at C1, C4 and C5 which are ceramic disc capacitors. So I replaced those and now I get no high voltage at all. Put the old caps back in....no high voltage with those now too, wtf?


(my replacement caps are the ones at the bottom)

Looking at the schematic these capacitors are polarized...but they're discs. They shouldn't be polarized....are they?

[budm]

Re: Bipolar Disc Caps

They are non-polar high voltage ceramic disc. The high voltage power supply is very high Z circuit, so did you clean the board real well for any residue, etc.? HV is generated by T2, CR5, Q2 free-running Osc circuit. Is the transistor Germanium type? R7B has any affect when adjusted?

[redwire]

Re: Bipolar Disc Caps

Does the HV oscillator run? I would suspect the transistors, they are probably old PNP germanium types 2N404? R7B sets the HV oscillator drive, so twiddle it to see if the oscillator starts up. I'm not sure how to calibrate it, perhaps measuring current drain or the HV.
V2 is a shunt regulator gas-tube, but the GV3A comes in different voltages i.e. GV3A-900 is 900VDC.

Found more info on the CD V-700 here:
http://www.eevblog.com/forum/project...the-waveforms/
http://www.anythingradioactive.com/partsstore.htm

[pentium]

Re: Bipolar Disc Caps

God dammit I fell for it again!

After marking the position of R7B I gave it a few twists back and fourth with contact cleaner and High Voltage returned and stayed.
Everything is good now. Also, thanks for confirming that they were non-polar.

[budm]

Re: Bipolar Disc Caps

Glad to hear it is simple fix.

[Longbow]

Re: Bipolar Disc Caps

A few comments: it was the convention to draw any type of capacitor using one straight line and one curved line. Usually the curved line represented the most negative point. On paper or film capacitors, the curved line represented the outside foil, so the capacitor can be positioned safely. Those capacitors still use a black line near one end to indicate outside foil. This drawing uses the earlier convention, except gets it wrong on the 400 uF capacitor.

You say you immediately suspected capacitors as the problem. Aside from one electrolytic, there would be no reason to think that a handful of ceramic capacitors are your most likely failures. Only one type of capacitor is polarized. Electrolytics are in effect small batteries with battery acid between the plates. That's why polarity matters with electrolytics.

A unit from this era shows the common use of PNP germanium transistors. Many designs mounted the transistors in tiny sockets, since the designers didn't know what the expected life span would be. They also thought that soldering the transistor would ruin it. There weren't many pencil soldering irons at that time, so a 300 watt Weller would do some damage. But there are lots of units out there with the original transistors that still work fine. Germanium still has its uses. Just look at the 1N34 diode which is commonly used in RF detectors.

[PeteS in CA]

Re: Bipolar Disc Caps

Electrolytics are in effect small batteries with battery acid between the plates. That's why polarity matters with electrolytics.

Batteries use the chemical action of an electrolyte to allow ions to conduct between its plates. Electrons do not move internally between the plates of a capacitor. There is an insulator between the plates. There are five "parts" to an electrolytic capacitor: the anode foil, which is pure aluminum, and has been etched to increase its surface area; the cathode foil, which is pure aluminum and not etched; the aluminum oxide layer that has been formed on the anode foil, and is the dielectric; the electrolyte, which is conductive (highly!) and can maintain conformal contact with the etched and formed anode foil and with the cathode foil; the paper, which keeps the electrolyte in contact with the full length and breadth of the anode and cathode foils. Thus, the purpose of the electrolyte is to be a conductor, not to act chemically on the plates; the electrolyte is, effectively, part of the cathode of an electrolytic capacitor.

[redwire]

Re: Bipolar Disc Caps

Longbow, I find 1960's germanium transistors not very reliable, due to their construction. Example: the 2N404 is an alloy-junction beast.
I find leakage current and noise go up, beta drops and then they are dead. A bunch have died sitting in my parts drawers.

[Longbow]

Re: Bipolar Disc Caps

Batteries use the chemical action of an electrolyte to allow ions to conduct between its plates. Electrons do not move internally between the plates of a capacitor.

Yes, thank you for the complete explanation of how electrolytics work. I like to refer to them as small batteries for humor's sake, and the fact that they are similar in some ways to batteries.