Hello,
according to the different documents for the 335A and the 335D, the 335A
had a mechanical chopper for the main error / chopper amplifier, and the
voltage reference consisted of a discrete (separate) zener and
transistor in an oven.
The 335D already had the FET chopper and the TI or Motorola 'reference
amplifier', i.e. zener and transistor on-one-chip.
Both versions obviously have the opto-chopper for the differential
voltmeter amplifier.
This is the same design as in the 845AR/AB.
The 332D therefore is preferred over the 335A, and over the 332B, if the
differential voltmeter is needed.
Maybe it's easily possible to increase its sensitivity and damping to
1µV instead of 10µV.
I own a defective 332B/AF version (possibly AF = Air Force), which is
built the same as the 335D, only without the additional differential
voltmeter.
Its big problem is the high voltage pre regulator, as the supressor
Zener (CR14) and the switching transistor (Q1) are not available any
more. Therefore, a 5440/5442 was the best choice, especially as it has
Autocal; all the others need additional equipment for linearity and
range calibration.
The HP 740B is also nice unit, but is has several neon bulb based
opto-choppers, which may be a big pain if they go defect.
Frank
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