[volt-nuts] Mechanical choppers for precision instruments

[Dr. Frank Stellmach]

Hi Randall,

I remeber HP and FLUKE meters and calibrators.
First there were the mechanical (coil) choppers, often defective by 
mechanical degradation which led to unsymmetrical duty cycle, i.e. big 
offset.
Later came the opto-choppers, their problem was the defective neon bulb 
for the CdS LDR, which also had to be replaced often. Difficult / 
impossible to find neon bulb today.


FLUKE had both techniques in their 332/335 calibrators, and also in the 
845A/B.
The main coil chopper amp was later replaced with a MOSFET input 
chopper, and a npn transistor at the output rectifier.

Those schematics with both versions you may find on the Fluke site, look 
for service / manuals and search for the model. Download manuals for 
models: 343 (MOSFET) , 332A, 332D, 335A, 335D, 845

The 335A_D manual contains the coil chopper for the calibrator 
circuitry, (page 119/120) and the opto chopper version for the included 
Nullmeter, same circuitry as of the 845A/B. Compare this with the 335_D 
manual,(page 169) where you can find the MOS FET version for the 
calibrator amplifier.

Frank