Fluke 731B Teardown

[VintageNut]

I posted pictures of measuring the output of the 731B in the "Scores" thread. Here are the innerds of the beast.

This instrument is small and light. The back panel was bashed in by a Neanderthal. The aluminum back panel is thin, cheesy aluminum.  I needed to measure the space for a new battery pack and took advantage to shoot some photos.

Removal of the top cover reveals a shield over the top of the reference PCB. Adjustment pots are accessed via holes in the shield.


Bottom view of the reference PCB


Shield removed. Reference PCB top view. 5 hand-wound and selected precision resistors. U1 and U2 cans. The reference is simply a temp compensated Zener that has a transistor for the comp.


Close up of adjustment pots. These pots are on the power supply PCB but live under the reference PCB shield.
 

Power supply circuitry. 110/220 switch. Naked battery connection posts awaiting new battery pack to be connected.


I have to say that this instrument is a minimalist work of art. 3 days cooking on the rack shelf and the pk-pk excursion is 1ppm. My Keithley 2015 reads 9.99999 or 10.00000. Very satisfied.

[Vgkid]

Thanks for the teardown.

[pico61]

Thanks!
A question: but is the Keithley 2015 more stable than the Fluke 731B?

[VintageNut]

I doubt it. The 2105 spec is 35ppm uncertainty measuring 10V on the 10V range. The 2015 is my most recently calibrated DVM at the moment. When I borrowed a 2002, the 2015 was only a few ppm different measuring 10V and it appeared fairly stable. I figure this is the best shot I have at adjusting the 731B closer to correct than it was when I received it. I will not re-adjust the 731B again until I have access to a very good 10V or a very good 1.018 0r 1.019.

The 731B is measuring 9.99998 to 9.99999 today. I suspect the 2015 as much as the 731B and/or temp/humidity.

I will be breaking out a Keithley 150B to start practicing null measurements. The microvolt ranges are higher impedance than a Fluke 845A but there are no high voltage ranges. I bought the 150B for the included cable which are over $400 new.

[VintageNut]

Bought a second 731B for about what I paid for the first one. Warmed it up and adjusted it for as close to zero difference from the first one as possible. They are within 1pmm of each other. Tried to use a null meter to measure the difference between the two and the thermal noise and other wanderings are too large for my Keithley 150B. The needle of the null meter dances. Measuring a short is dead quiet so the 150B is not the problem.

I switched to my 2015 to look at the difference and that works very well with SLOW rate and 10 to 30 digital filter.

After a week of letting them run, they appear to be very stable. pk-pk wanderings of the pair of 731Bs as measured by the 2015 are less than 1ppm. I suspect the 2015 wanders as much or more than the 731Bs.

Switching the cables at the 2015 shows offset microvolts. REL-ing the 2015 with a short removes most of the offset. You can also set both 731Bs to delta-E mode and make them output 0.000000 volts and that also shows offsets.

Two are definitely more fun than one!

Batteries arrived to make new NiMH packs. Maybe next year they can travel to a calibration lab to be measured for absolute. No more adjustments.