David:
I bought a Valhalla 2734A in June of 2003 for $270. I soon found out that
it wasn't as stable as Valhalla would like you to believe. It does meet specs
but week to week stability is around 0.5 ppm, up and down. The Fluke 732A is
much more stable on a week to week basis. I found this out after buying a few
732A's.
A few years back I opened the 2734A up and looked inside the ovenized
reference. I remember seeing it using 4 each 1N827 (or something close to
that) zener diodes. Then there was an opamp for each diode to raise the 6.2
volts of the zener up to 7 volts and allow adjustment to exactly 7 volts. Then
these 4 opamp outputs were being summed into another opamp that raises the
total to 10 volts and allows adjustment to exactly 10 volts. All of this was
inside of an oven that was running at around 45 degrees centigrade. It seemed
very room temperature sensitive and there was no insulation around the oven
assembly so I added some insulation around the oven to try to help this problem.
I kept looking for a manual, as Valhalla won't even acknowledge that they
ever made this instrument. I finally found one from Tucker Electronics for
$25. The manual does not show anything about the reference board assembly and
tells you to replace this item as a unit. They do tell you how to adjust the
instrument.
I think that the 732A is a much better instrument. I can scan in my manual
if you need it and email it to you as a pdf. I can't scan "B" size prints so I
would have to break these into 2 pages.
Bill
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