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