How to check how bad a suspect 720A is?

[perdrix]

I have a 720A which has most likely been abused at some stage.

When I got it R302 read 9.711k and R311 read 9.793k).   To work round that I added resistors in series with R1008 and R1044 to bring the resistance for those parts of the A divider to a value that it would calibrate OK.  I also had to remove five turns (about 1.3 Ohms)  from R1051 as the combined value of R313 and R314 was too high to allow me to calibrate S2.   I also replaced R1041 as it was worn out or corroded (almost impossible to get a null).   Once I'd made those changes, I was able to complete the "Self-Calibration Procedure" on pages 2-4 to 2-7 of the downloadable manual.

Charles Steinmetz is of the view that it will never work properly (volt-nuts mailing list) without being sent back to Fluke for a rebuild at astronomical cost.   I do accept that it may not fully meet specification ever again, but I'd like to get a feel for how good/bad it actually is.

Section 4-14 describes a linearity check for the instrument but that requires the I have another "Known Good" 720A, and I'm sad to say that I don't.

So are there any other ways in which I can get a feel for how good or bad this beast is.

I have a 3458A which was last week aligned against another 3458A that had just come back from Keysight calibration (only a CAL 0 and a CAL 10 were needed to get agreement).  I also have voltage sources in the form of a 732A and a Datron 4808.

PS I've had it dinned into me the "Thou shalt draw NO CURRENT (not even a miserly 1pA) from the output terminals of a 720A".

Thanks
Dave

[Echo88]

Connect the Datron 4808 in parallel to the 3458A (3458A shows the real calibrator values and has better linearity than Datron 4808) and use a Nullmeter (for example Keithley 155 from ebay) in series to the output of your 720A, which is supplied by your 732A.
This way you can measure your 720A without compromising its linearity via output loading errors.

[MisterDiodes]

Perdix,
As long as you can null out all the pots, you're unit -should- be back to operating condition and in calibration accurate to +-0.1ppm.  Unable to null S2 is a common drift problem but if that even gets to within 5uV of null you've got it very close to dialed in.  You should get a realistic sanity check on your 3458a - just keep your 3458a locked in to 10V range (if you're testing your 720a by dividing a 732a/b) and remember that any error you see will be relative to 10V - or whatever your input voltage is.  The left 3 dials will be most important, the other 4 are usually close to perfect anyway if the switches are clean - unless the unit has been overloaded.  Remember you are checking for a divided ratio out of the 720a, so keep cross-checking your 3458a between the 720a actual input vs. actual output.  Low thermals are a must here also.

Also check extended settings such as 0.5000000 vs 0.499999X - They should be very close.

The problem with sending a 720a back to Fluke for repair is hardly anyone at Fluke knows how to repair one of these.  Not efficiently anyway.  They tend to drop in whole switch assemblies when a single resistor might have drifted.

Anyway, good luck!.  These are a fantastic tool to own if you're interested in real accuracy.  If you get it to complete the calibration procedure you've just saved yourself $46,255 over the cost of a new unit.  So it pays to spend some time going through the cal sequence carefully and completely - and keep it in tip-top condition.