While we do not have actual metrology-grade Fluke 720A Kelvin-Varley Divider (yet?), I manage to get some parts from parts unit to check.
Here’s everythink in bulk. This is basically all decades except first “A” decade (which is located in oil tank) and some of trim resistors from calibration board.
Some switching armature, circular labels and gear for switches included as well. I think teal switch missing wiper part and shaft, also top and bottom covers.
Here are decades “B”, “C” (with teal resistors) and “D”, “E” and “F” with wirewound 1K+2.5K resistors on mica substrate. 10 resistors of same value + one higher to form Kelvin-Varley ratio divider arrangement.
One of decades is just as is, with desoldered resistors.
This is Kelvin-Varley trimpot, not a usual trimmer. It manufactured by Precision Control MFG. from Bellevue, Washington. Nominal resistance is 5KΩ, tolerance ±3% and rated power up to 3W. This resistor is used in bridge balance adjustment circuit.
TECH LAB’s switch, with 720A-801 Rev.A marking on it. It manufactured around same date as resistors, 1985. All this stuff is already 31 year old.
Bonus COTO reed relay, model number 3206-12-71. I could not find any datasheet for this series.
Just check that wire thickness on 1KΩ wirewound mica.
Not everyone survive shipping and life… Two more resistors in two decades are dead too (broken wire).
Closeup on Fluke-made nice teal-colored resistors. N1.5, P1.5, N3 labels near date code probably meaning TCR rating and direction? N1.5 could mean negative TCR 1.5ppm/°C when P1.5 positive TCR +1.5ppm/°C? Further testing will reveal this.
Decade switch ring and resistor arrangement.
Overview, decades “B” and “C”
Resistors on them.
Rest of decades with 1K+2.5K setup.
I took some measurements using Keithley 2001 in 4W-mode with NPLC10:
Building DIY Hammon divider
Now since we have plenty of high-precision stable resistors, what we can do with them? One of things I wanted to try, is DIY Hammon divider for calibration needs. I had implemented simple online calculator for this clever resistor network, now it’s time to try it on practice.
IET KVD-700 is substitute of 720A
Modified: Aug. 9, 2016, 4:36 p.m.