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Measuring V high impedance


I had an interesting time troubleshooting a faulty ionised particle detector a while back. It is an old unit from the early seventies. Eberline E-120

After checking various circuit operation,
I suspected the 900V supply to the GM tube but this was hard to measure as it was 23M impedance, more than twice the DMM impedance.
It is a regulated supply with a tube "zener" diode to sense the voltage.

I needed to load the circuit minimal to detect if there was a problem with the Vsense loading the circuit.
I ended up placing 136M in series with the DMM & 100K in parallel with the DMM to give reasonable voltage ? to measure.

Attached spreadsheet was used to do the hard work.
The measured result indicated the VSupply was ok, I had already checked the cable to the GM tube.
The problem turned out to be an open contact inside the hand held tube holder, cleaned off the corrosion & it came to life.

The job was complicated somewhat as I didn't have a source, the smoke detector does'nt emit any more than background. Now I know how many particle per minute from background I know next time.

Tis funny how sometimes corrosion is conductive & other times it is an insulator. Maybe the impressed voltage changes its characteristic ?



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