Old HP 412a, Theory Of Operation

On Wednesday, September 21, 2016 at 2:39:57 PM UTC-4, snipped-for-privacy@gmail.com wro te:

al VTVMs blush.

s PWM actually, what else ? But how the hell do they get the accuracy ? Tha t old thing agrees with a Fluke 8846 (I think that's the model) which, whil e not a Keithley electrometer is pretty damn good.

e seriously accurate and versatile. Have a look at y old 561A Tek scope. Ev en though the bandwidth is low the thing is accurate as hell and has dual t imebase. In what, 1957 ?

by 45, precision 10 meg resistors in series, each of which was measured se parately before being strung together for the purpose.

s me.

It's got a chopper/ synchronous circuit.

Maybe a little later. I never liked the 560 series; they were the low-end scopes, and ran hot. The 547 was a thing of beauty, electronics as art.

The input signal is chopped into AC by CdSe photoresistors. That's AC amplified and synchronously detected by more photoresistors. A synchronous (clock) motor spins a shutter disc that lets the light from a bulb hit the photocells and times everything.

It's cute but obsolete. A good modern DVM is a lot better.

I think it's mid-1960s technology. I used one when I was in high school, 1964-ish.

IIRC it's a chopper made of CdS/CdSe photoconductors. The HP guys did a lot of stuff with those--the Nixie drivers for the 5245 counter were neon bulbs shining on CdSe cells. (I have some in a box someplace.)

Cheers

Phil Hobbs