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Fluke 731B Voltage Standard Restoration and Mods?
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Edwin G. Pettis:
My Fluke 731B came from the Washington Naval Yard, Washington, D.C., Eastern Standards Laboratory, vintage 1976.  As received, the battery pack had been removed, the unit operated without batteries for some years as I had not opened the unit.  A few years ago I opened the unit, with the exception of the batteries, it was in excellent condition.  I replaced the battery pack with NiMh, 3.3AH batteries, made a slight change in the series resistor charge circuit and also made some minor changes to the op amp circuit to reduce the noise a little bit at the output.  The noise specification in the manual appears to be quite conservative.

It is important to note that the battery circuit aids in reducing noise in the power supply and voltage outputs.  Without batteries, significantly more noise is passed on to the voltage reference circuits.  If you don’t require battery operation (output noise is slightly better on battery power), a zener diode of the correct voltage paralleled with an electrolytic of moderate size will produce similar results to the batteries on line operation.

My unit has undergone several calibrations in the past few years to determine long term stability as my unit came without any cal data unfortunately.  Initial calibrations were done at 6 month intervals and then at one year intervals.  There has been very little drift over these measurements, the 1V output was adjusted once the last time, it was 100µV low, it appears that R19 may have developed a weak spot where the wiper was sitting, it was noted there was some slight variability at that setting (probably worn spot).  It will be rechecked at the next calibration and replaced if needed.

These are the last set of readings for last year’s calibration; note that the 1.018V and 1.019V outputs were not readjusted from the previous calibration.

20C ==> 9.9999940 / 1.0000005 / 1.01806359 / 1.01910223 VDC

23C ==> 10.0000065 / 1.0000027 / 1.01807099 / 1.01910386 VDC

You will note that there is a small variation in the 10V output with temperature of 1.25PPM equaling a TCV of 0.41667PPM/°C, not too shabby for an unregulated temperature device.  Al voltages were stable over time while at calibration.  It was also noted that noise in the readouts were limited to 2-4 counts.

The 731B is also pretty much immune to being cycled on and off (for hours), it has repeatedly returned to the same value after being on for an hour or so, the cal lab did this at my request and those are the findings, any changes were too small to be significant, hidden in the noise.
TERRA Operative:
What were the changes you made in the opamp circuit?


I have the results in on the 1V scale and it's looking good, so it seems I had a temporary PEBKAC error... :D

Next step is to try at a higher and lower temperature in each range to see what the drift is like.
dietert1:
A 1.25 ppm/K TC is low enough to be difficult to measure. From my own LTFLU DIY experiments i know that to get below 1 ppm/K one should put one of the devices into a thermal chamber and carefully measure the temperature of the other device. How big of a mod is it to mount a quality thermistor close to the reference inside the 731B? Then you can always subtract the TC effect in case it matters.

Regards, Dieter
TERRA Operative:
It wouldn't be too hard to stick a thermistor in there somewhere. I have it working well enough for now, and I have other projects staring at me from the shelves in my room, so I'll call this one as done for now until I get back to it. :)

Uploaded a video to youtube about it too.

Conrad Hoffman:
I've got two 731B and one 731A. I never realized that the design of the 731A is quite inferior. It has no voltage regulator and is just a single board with no floating shield. In spite of that, it was possible to optimize the tempco and the unit was my favorite of the three, save for a lot of ripple on the output. Surprisingly, that doesn't seem to bother most measurements and null setups. I'm talking a few hundred uV of ripple, even with a large cap and zener replacing the batteries. I'm presently installing a small circuit board where the batteries were, with an separate bridge, filter and LM317 regulator. Hopefully by getting all AC current loops out of common with all DC loops, the ripple can be reduced.

Anyway, nice job on the 731B!
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