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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 15th May 2018, 8:18 pm   #1
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Location: Cheltenham, Gloucestershire, UK.
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Default Very high input resistance voltmeter

Bad form to reply to one's own post, but nevertheless...

In my write-up of the 'half restoration' of a Sky Queen chassis, I asked how to measure the voltages around the DAF96: the screen-grid resistor is 4.7 megohm, and the anode resistance 1 megohm. So even many modern digital meters would unacceptably load the circuit and give inaccurate readings.

Solution: a single 100 megohm resistor in series with one of my cheap digital multimeters. I first measured the input-resistance on the DC voltage ranges of the cheap multimeter using another one (). I found the resistance to be 1 megohm on all of 200mV, 2V, 20V, 200V f.s.d. ranges.

Putting a 100 meg resistor in series with one of the leads then reduces the sensitivity to one hundredth (so 2V fsd becomes 200V fsd), and the imput resistance is now 100 meg (or, 101 meg - but the 100 meg resistor was probably 5% tolerance...).

This allowed readings around the DAF96 with far better accuracy.

I guess if you haven't any 100 meg resistors, ten 10meg resistors in series would do the trick.
Dave Teague
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Old 15th May 2018, 8:33 pm   #2
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Default Re: Very high input resistance voltmeter

Another trick I have used is to put one side of the meter on a variable power supply and wind up the low impedance side to approach the measured voltage. The meter then becomes a null detector with impedance approaching infinity at balance.
You always have to be mindful that attaching wires to a high impedance point is like attaching a pick up areal so capacitor filters at the point of measuring if not inherent in the circuit might also be needed. But with your 100 meg that effect is reduced significantly.
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Old 15th May 2018, 10:40 pm   #3
Phil G4SPZ
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Default Re: Very high input resistance voltmeter

Good idea, that. Back in the day when DAF96s needed their voltages measured, most service departments would have still been using Avo Model 7s! Or a proper valve voltmeter with a cathode follower input stage. I have some Heathkit VTVMs, but even they have a resistive input attenuator which fixes the resistance to a disappointing 10 Megohms on all DC ranges.

The Avo EA113 electronic multimeter offers a genuine 1 Megohm-per-volt sensitivity as standard, or 100 Megohms on the 100V range and above. The much older Avo Electronic Testmeter also has a very high input resistance.

“The place where optimism most flourishes is the lunatic asylum” - Havelock Ellis
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Old 16th May 2018, 9:01 am   #4
Craig Sawyers
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Default Re: Very high input resistance voltmeter

The other wonderful technology of yesteryear is the differential voltmeter. Fluke made several types of these, and were the precursor to the DVM. The big advantage is that they are null reading, and when nulled the input resistance is infinite, limited only by leakage paths - so in practice 10^10 to 10^11 ohms.

See for example
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