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, [email protected] 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 -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. > _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
