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| Some old school instruments showing how it's done (HP 3325A and Fluke 8506a) |
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| SilverSolder:
--- Quote from: RoGeorge on January 22, 2021, 11:44:40 pm --- --- Quote from: SilverSolder on June 02, 2020, 09:43:00 pm --- Just checked what 1V RMS from the 3325A looked like on the Fluke 8506a thermal RMS voltmeter. No real reason to complain today! (Other than using up my luck for the rest of the year...) --- End quote --- How do you know they are not both off track, but with the same amount? ;D --- End quote --- I very specifically did not say that the signal was 1V RMS, for exactly that reason! :D Both instruments are good, but neither of these instruments can be trusted to that many digits of precision. |
| joeqsmith:
Thank you very much for checking this. If you do decide to try and improve #4, please make a thread for it. I would like to follow along. *** People who complain about changing fuses on meters have never had one of these apart. Crazy 4 HRC fuses and GDT buried deep in the bowels. |
| joeqsmith:
The meter appears in fairly decent shape mechanically for the age. Trying a few different measurements, it's not even close. Shown with 1V from the Fluke 731B and 1K ohm 0.1% resistor attached. It looks like it would need a full alignment to be useful. Have you had to send one out for alignment? Curious who you used and the price. |
| SilverSolder:
It looks in good shape, even has the bails! Did you set the DC zero? Shorting bar, wait a few seconds for the reading to stabilize, and press Zero (the Zero LED should be lit). The zero drifts a lot with time and temperature and needs to be set every time you are looking for max accuracy. Once the zero is set, it can drift up and down by a few counts even on a good unit. If it can't hold zero within about 5 counts, the temperature in the area may not be stable (or there is a problem with the unit). I've never sent one out to be calibrated, but I have performed both the hardware and software calibration methods on some of them, using a 10V reference. It isn't that hard to do (especially software cal, no need to even open it up), the manual is very comprehensive. (I only did cals on some ranges, as I don't have the references needed for all the ranges...) |
| guenthert:
I bought a few older DMMs and all drifted less (or rather the accumulated drift was less) than the original specification would have allowed for (one HP 34401A even seems to have been "spot-on" in the 10DCV range). That might have been survivor's bias or an indication that the manufacturers specification are wider than necessary for instruments living an easy life. In any case, I haven't seen one, which drifted so far as yours. I'd watch it for a while and observe the current drift, which might indicate some fault needing a repair before calibration. |
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