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Old HP 412a, Theory Of Operation

Accurate as hell, very high input resistance. Range that makes conventional VTVMs blush.

They got four light bulbs in there and a wheel that spins. I imagine it is PWM actually, what else ? But how the hell do they get the accuracy ? That old thing agrees with a Fluke 8846 (I think that's the model) which, while not a Keithley electrometer is pretty damn good.

Sometimes I am amazed by old technology. Even things before I was born are seriously accurate and versatile. Have a look at y old 561A Tek scope. Even though the bandwidth is low the thing is accurate as hell and has dual tim ebase. In what, 1957 ?

And this 412a can read 450 megohms accurately, which was proven literally b y 45, precision 10 meg resistors in series, each of which was measured sepa rately before being strung together for the purpose.

Not sure right now exactly when the 412a was made, but I think it predates me.

How did they get it so accurate ?

Reply to
jurb6006
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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.

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It's got a chopper/ synchronous circuit.

Reply to
George Herold

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.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

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

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

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