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Old Fluke Multimeters

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Dave Wise:
D.R. Taylor, if you're listening, there's text missing from your Hidden Features paper.  At the end of page 1, you write, "The accuracy loss is due to a".  At the beginning of page 2 you write, "out in the normal 10 megohm volts mode".  I'd love to know what went in between.

And it seems to me that one can only get hi-Z by having the MAC disconnect the OREF- pin, because otherwise there's a sneak path to ground through R2, RT1, Z1.2-4, Z5.7-6, OREF-, OREF+, and Z5.8-10.

UPDATE: In normal 10-Meg mode, DC goes through a voltage divider with R2 (1K) on top and 10 Meg on the bottom.  Hi-Z mode disconnects the 10 Meg so we read 0.01% high.  But I can't figure out how to word that to fill in the missing text.

blue_lateral:

--- Quote from: Dave Wise on April 12, 2024, 06:08:44 pm ---I don't see what I left out, except possibly the well-known trick of using a voltmeter with known resistance to measure small currents.  I put the meter in series with power supply and DUT and let the meter's input resistance be the ammeter shunt.  With a 10 Meg meter, 10mV is 1nA.  Since I had three DUTs in play, I used three meters, a Fluke 8000A, a Fluke 8100B, and a Monsanto 2000.  After measuring the leakage, I shorted the meters.  Then I turned off the power supply, which has a few kilohms of bleed at the terminals, and after 10 seconds I pulled off the meter shorts and watched the readings go negative, level off, and fall.

--- End quote ---

That did occur to me. Years ago I often used that trick to measure leakage with a HVPS and a VTVM in tube based equipment. After posting I tried it with the power supply and DMM (Tektronix TX-3). I wondered at the time if you were timing self-discharge somehow, but now I see. It never occurred to me to also check for dielectric absorption until I read your post. Now I probably will.

It seems to have stabilized at around 3.4 mv, give or take a few counts as it drifts. It's been there most of the day. It does get a little worse if you heat it up with your fingers, but we are down where it shouldn't matter for C36.

I just got an email from Mouser that the Kyocera caps ( RPF0509220M035K ) have finally shipped. I'll post a comparison when I get them.


--- Quote from: Dave Wise on April 12, 2024, 06:08:44 pm ---It was quite a surprise to see the ceramic cap perform worse than the electrolytics!

--- End quote ---

I bought some of those same series Muratas in 22uf as a possible C36, but they have a huge capacitance rolloff with voltage according to the datasheet, about half of it at 10 volts as I recall. I've not tested it. It occurred to me that maybe 47uf would be closer to a 22uf electrolytic when connected to a 9V battery, but I sort of gave up on that idea. I too am surprised they were the worst performers of the 3 you tested. Can you see anything about them (other than cost) that is bad enough to kick them out as a possibility for C19?

Dave Wise:
If my instrument is representative, C19 sees only a few millivolts in normal operation and did not have to be a special low-leakage part.  Although its leakage and DA is worse than basic aluminum electrolytics, a Type II ceramic should do fine, and outlive your great-grandkids besides.  I won't bother; a 10000-hour electrolytic would outlive me and that's plenty.

Dave Wise:
I finished my 8060A beep frequency mod.  Works great.  See topic "Low Frequency Continuity Beep for Fluke 8060A".

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