720A Voltage Divider Cal

[cobychuck]

I am having issues with the calibration of the 720A.  We are using
the 721A lead compensator as we do not have the LC875B.  There is a note in
the procedure (33K8-4-529-1, page 5) that states "Circuit adjustments may be
necessary to ensure the Null Detector deflects in the appropriate
direction".  So far we have not been able to determine what those
adjustments need to be, and our reading are all the opposite polarity of
what we need.  If there is anyone that can shed some light on what we need to do in order to get the correct null meter reading, I would greatly appreciate it.

[John Treekiller]

Hello;
I am the writer assigned to work this CTO, but I'm at NCSLI right now and am unable to access any of my documents.  If you are using the Fluke 721 you might be able to find some answers in their 720A manual, I believe they do call out the 721 for their procedure.  If you're unable to get any other insight, I'll be back at my desk the 29th and I'll research the problem for you then.
John Treekiller

[John Treekiller]

I am having issues with the calibration of the 720A.  We are using
the 721A lead compensator as we do not have the LC875B.  There is a note in
the procedure (33K8-4-529-1, page 5) that states "Circuit adjustments may be
necessary to ensure the Null Detector deflects in the appropriate
direction".  So far we have not been able to determine what those
adjustments need to be, and our reading are all the opposite polarity of
what we need.  If there is anyone that can shed some light on what we need to do in order to get the correct null meter reading, I would greatly appreciate it.

Sorry for the long delay in getting back to you.  Hopefully you were able to find some information out to help you.  There wasn't much in any of the history folders.  It looks like that NOTE went in 2006 as a direct result of an AFMC 252 Form from the TCM.  I couldn't find an AFTO 22 regarding it, so I assume it was as a result of a phone call requesting an explanation of differences with the 721A.  I did find one handwritten Note on the front of an old AFMC 252 from 1999;  "When using the Fluke Lead Compensator in place of the LC875B, reverse Input B."  Not sure how much help that is, but again there was no AFTO 22 or other record of the change.  Many times a NOTE that reads similarly to this, is written to allow the technician some leeway for test connections when substituting test equipment or for changing connections to avoid ground loops and such.
Sorry I couldn't find out more.  John Treekiller

[cobychuck]

    We ended up getting a reply back from Ken Eddy.  We were able to reverse the leads on the null detector to help get the correct polarity readings.  We just weren't sure if that was one of the "circuit adjustments" we could use in order to correct our problem.  We were also able to get some clarification regarding the R Std<R Test/R Std>R Test knob as well.  Thanks for looking into it.  I appreciate the help!

[measure]

With all due respect to Ken Eddy, reversing the leads is not the correct solution. When using the ESI LC875B, it was necessary to swap the leads at some point in the verification to null out the lead resistance. However, when Fluke designed the 721A Lead Compensator (many moons ago), they recognized this problem - that's why the inclusion of the MODE (R STD > R TEST —— R STD < R TEST) switch. If everything is connected correctly, when you get to the point where you can't null out the leads, swap the position of the aforementioned switch (not the leads!) and try again. Note that you may have to make significant adjustments in the LOW BALANCE and HIGH BALANCE switches and pots to re-establish the null after changing the mode switch position. I've done this many times. (As an aid for connecting, you may want to take a look at the Fluke commercial manual for the 720A, pp. 4-4 thru 4-6. It is available online at www.flukecal.com.)

Hope you find it works for you too - it should!