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Old 2nd Dec 2015, 9:43 am   #88
wireful3
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Join Date: Sep 2003
Location: Bristol, UK.
Posts: 808
Default Re: Test equipment for valve radio repair

Many years ago we were working on power supplies with a very high internal resistance. At that time AVOs were the general test meter but a valve voltmeter had been produced with an input impedance that was so high we were assured it was almost infinite. A colleague using this was quite surprised to see very wide variations in his voltage measurements even though he made no changes to his set up.

It turned out that a visitor had come in and was walking about the room. He was wearing a trendy nylon or similar suit and generating an electric field, our meter was showing the variations of this. I have not seen the effect since, even though DMMs are claimed to have very high input impedance. It does though make me think when using one, especially when checking circuit voltages against data sheets produced in the 1930s
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