[volt-nuts] Update on 720A

[Chuck Harris]

I really doubt that contamination of the oil is an issue.

My top candidates are:

1) there is a unexpected (by David) parallel resistance
   across the affected resistors.  It could be caused by
   a strap, or connection on the output terminals, or a
   missed circuit when reading the schematic.  It could
   even be caused by putting the 3458A in grounded "LOW"
   terminal mode, and having a grounded wire on the output
   terminals... scope probe, something...
2) there is a failure causing the insulator to have a
   significant resistance to the can.
3) a short between any two turns on the wire wound resistors.

I consider #3 highly unlikely.

And, lastly, my favorite thought is, each resistor in the
oil can contains the bulk of the resistance needed for that
stage, with the remaining resistance being from an external
series resistor, and a trimmer pot.  What if Fluke ran out
of the proper value for the in can resistor, and decided it
would be expedient to use what they had on hand, and change
the value of the external resistor to make it work?

Or, perhaps not Fluke, but some technician that opened the
can, and saved the day by making the swap with resistors
salvaged from another Fluke product?

Not much physical abuse you can do to resistors submerged
in a full can of mineral oil.

-Chuck Harris

Dan Kemppainen wrote:
> On 8/9/2017 12:00 PM, volt-nuts-request at febo.com wrote:
> 
>> I have trouble believing that the precision WW resistors
>> in the oil bath did go low.  I wouldn't presume to argue
>> that you didn't measure them right, but there has to be
>> something other than their value drifting low. WW
>> resistors usually go up and become erratic on their way
>> to becoming open circuit.
>>
> 
> That's a great observation. What possible causes are there?
> 
> Contamination in the oil? (Water, other fluids)
> 
> Loose/shorted turn of the resistor wire?
> 
> Not much else that could make resistance drop. If the unit suffered physical or
> environmental abuse, these may be possible explanations.
> 
> Dan
> 
> _______________________________________________
> volt-nuts mailing list -- volt-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
> and follow the instructions there.
>