Joe:
The 731A and 731B were run at room temp and therefore were subject to room
variations. They both use the same "Reference Amplifier" as the 732A & B use.
I have one 731B and it is amazingly stable for the room temp variations I
experience at home. When I was working we had a 731B that was also very good.
I had sent it for calibration over a 10 year period and the drift was equal to,
or better than the 732 series. At the end of the 10 years it was absolutely
stable at less than 1 ppm. This was all being done at a Fluke Service Center
that was located in Milpitas California, and their 732As were being routinely
sent to Fluke in Everett WA.
The difference between the 731 & 732 is that starting with the 732A all of
the temperature sensitive components were enclosed in a oven assembly that had
heaters surrounding the oven enclosure which is running at 45 degrees C. This
includes all of the resistors and adjustment pots. I have torn down a 732A to
the oven assembly and gone inside the assembly to replace a component. It is a
real trick to do this, but by being careful it can be done.
The fact that the 732As have been in operation for a much longer period of
time probably explains the lower drift rates than the 732Bs. The 732As have
been "aged" much longer. I know people who prefer the 732A over the 732B
because of this. The only problem with the 732A is that the heaters use more
power than the 732Bs. Early manuals spec 24 hour battery operation, but later
manuals have changed that to 12 hours. All of the 732As I have observed will
only run for about 12 to 15 hours before the "in cal" light goes out meaning
that the batteries have dropped below about 22 volts. The 732B is speced at 72
hours battery operation but I have no experience if this is true.
I guess I am a real "Volt-Nut" in that I have 6 732As at home right now. I
inter-compare them once a week. The last time I was able to verify what the
official "volt" is was in 2005. Right now I need to find a good Cal Lab in the
South SF Bay Area where I can have one of them "certified" against their
traceable standards. The Fluke service center was closed years ago.
Bill
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