Joe:
That was a very interesting teardown of the 732B on eevblog. The one thing
that this answered for me was if Fluke was using their "Reference Amplifier" in
all of the 732B production and Fluke did.
If you look at a paper by David Deaver called "Predictability of Solid
State References" which he wrote, and you can download from the Fluke website
http://us.fluke.com/. you will see on page three of the paper that he talks
about the reference drifts of a number of 732Bs that were checked over a period
of time. Deaver talks about Motorola References and Linear Technology
References and that the Motorola's drift upwards while the Linear Tech's drift
downwards over time. Also look at Fig. 10. I was assuming (wrongly of course
as I see now) that Fluke had switched from the "Reference Amplifier" to the
LTZ1000 in the production of the 732B. Now I see that it is just what company
made the "Reference Amplifiers" for Fluke. Of course the 4 legged device is a
"Reference Amplifier" that they have used for a least 25 years or more as the
voltage reference in a lot of their instruments.
I have never seen a schematic of the super secret "oven assembly" but you
can probably bet that it looks a lot like the 732A but with a lot more "black
magic" built into it. I don't think that Fluke would ever expect someone to
fix this beyond the unit replacement level as I am sure that this would require
factory precedures far beyond anything that we could muster in the field. Part
of the black magic is adjusting the collector current of the reference
amplifier to obtain a zero TC around a very limited range of operating
temperatures. The digital adjustments also add something inside the oven
assembly, probably a DAC, and it would be interesting what they are doing
there, so a schematic would be interesting to see.
Anyway I will quit rambling. So many things to do, so little time.
Bill
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