[volt-nuts] Fluke 720a self calibration repair

[Charles Black]

Hi David,

I had the same problem with my Fluke 720A, among other problems. The 
problem was the plastic for the adjustment pots had disintegrated with 
age and a couple of them had quit working. I ended up fixing the unit 
but removal of the bad pots made it obvious that the other "working" 
pots were just hanging on. As I removed the bad pots they kind of fell 
apart. I have been there before so I found a good price on 30 new pots 
and replaced them all at once. It really made a new instrument out of my 
720A. I was amazed how well it agreed with my Agilent 3458A, to better 
than the spec! It was nice to have confirmation of the linearity my 
3458A since it has not been out for calibration since it was almost new.

Charlie

On 9/24/2015 9:38 AM, David Garrido wrote:
> Hello All,
>
> I am trying to track down what I think is a small issue in the calibration of my new to me 720a.  I am dreadfully paranoid of jacking something up in the process and need a little hand holding.
>
> I was going through the Self Calibration steps and every thing was going well up to the adjustment of the "A 1.0" decade switch position.  All of the variable resistors up to that point needed to be turned CW in order to null the appropriate switch position.  The “A 1.0” adjustment pot does not null the switch position and is not getting any closer than about 2.5uV.  I verified the meter balance between every other step.  I have checked, in circuit, R302 and R303 and I am getting a VERY SOLID reading of 9.896 on all of those.  Of course, checking R1006 and R1010 give me significantly different readings than the stipulated 8.45k.
>
> I cannot seem to be able to find a CCW or CW limit to this resistor and I think I read somewhere that there are supposed to be run from end to end several times if an issue arises.  Do these var resistors have limits?  I need a little help moving forward.
>
> Cheers,
>
> David
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