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 Post subject: HP 6920B Meter Calibrator calibration
PostPosted: Dec Sat 02, 2017 5:39 pm 
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Thanks to Hylaphone here on ARF, I finally have a real meter calibrator.

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The really useful function that I couldn’t otherwise produce in my lab is AC up to 1000 volts. I also suspect the high DC voltages are more stable than my Power Designs model 2K-10 HV power supply.

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Hylaphone did a nice job of recapping it. It checks out to be working fine, but it is a little out of spec according to my (not recently calibrated) HP 3468A,

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and compared against my Geller 10 volt reference (also not recently calibrated).

HP Agilent Keysight still has a manual scan available on their web site. It has a January 1976 revision date.

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There is also an older manual scan at the HP Archive site with an October 1965 revision date, but that scan has less information and the calibration instructions appear to have at least one error.

For example, they tell you to connect a scope between test points 1 and 4, with the scope ground on test point 4. That would short out the 12.6 volt DC reference circuit. I think that is a typo and it should have been test points 1 and 2.

On the other hand, the calibration instructions in the newer manual also sound a bit sketchy. It would have you set the inverter phasing by eyeballing the output on a scope. The older manual has a detailed procedure for balancing the DC offset using a scope.

Does anyone have advice on calibrating the 6920B?

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 Post subject: Re: HP 6920B Meter Calibrator calibration
PostPosted: Dec Sat 02, 2017 6:13 pm 
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Nothing to add, but one comment- the DC reference supply (12.6, 6.2,....) "floats" with no ground return. How does connecting TP4 to a scope ground short out anything? Follow the circuit, and you'll see no connection to either the chassis or AC ground.

With many HP instruments, Ground is rarely a "reference" point - many calibration adjustments are often referenced an internal isolated DC supply, as is the case here.

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 Post subject: Re: HP 6920B Meter Calibrator calibration
PostPosted: Dec Sat 02, 2017 7:13 pm 
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Findm-Keepm wrote:
Nothing to add, but one comment- the DC reference supply (12.6, 6.2,....) "floats" with no ground return. How does connecting TP4 to a scope ground short out anything? Follow the circuit, and you'll see no connection to either the chassis or AC ground.

The "-S" line is the zero volt reference for the DC reference supply, and according to the schematic it is connected to the chassis ground at the selector on S2.

Perhaps I'm not reading the test point call-outs properly. The manual is vague on where they actually are, it just calls out "test point 1". There are circled numbers on the schematic, and I'm guessing they are the test points.

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 Post subject: Re: HP 6920B Meter Calibrator calibration
PostPosted: Dec Sat 02, 2017 8:11 pm 
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Joined: Sep Tue 30, 2014 6:08 am
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Location: The Old Dominion VA 23518
stevebyan wrote:
There are circled numbers on the schematic, and I'm guessing they are the test points.


They are wires that leave the board - look at the board layout diagram. Numbers along the outer edge correspond with the circled numbers on the schematic. You'll see the circled numbers near a component or assembly that is off-board. The manual never calls out the Test Points...... :(

The older manual on Keysight's site has a similar calibration procedure, using the output terminals for oscilloscope connection....

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 Post subject: Re: HP 6920B Meter Calibrator calibration
PostPosted: Dec Sun 03, 2017 4:57 pm 
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The 6920 is a nice addition to the bench, and in addition to its calibration duties, it's also useful as a current and/or voltage source for tests the bench.

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 Post subject: Re: HP 6920B Meter Calibrator calibration
PostPosted: Dec Sun 03, 2017 5:17 pm 
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The HP3468A is a true RMS meter. The AC source to the 6920B would have to have extremely low harmonic distortion for the HP6920B output to be adjusted to match the reading on the HP3468A. An average sensing RMS calibrated meter could be used or an external AC sine wave oscillator could be connected to the terminals on the rear of the HP6920B.


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 Post subject: Re: HP 6920B Meter Calibrator calibration
PostPosted: Dec Mon 04, 2017 12:59 am 
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Thanks Alan. I don't have an accurate average-reading AC voltmeter, but I can drive it externally from a 200CD.

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