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Old HP 412a, Theory Of Operation

Started by Unknown September 21, 2016
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 timebase. In what, 1957 ? 

And this 412a can read 450 megohms accurately, which was proven literally by 45, precision 10 meg resistors in series, each of which was measured separately 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 ?
On Wednesday, September 21, 2016 at 2:39:57 PM UTC-4, jurb...@gmail.com wrote:
> 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 timebase. In what, 1957 ? > > And this 412a can read 450 megohms accurately, which was proven literally by 45, precision 10 meg resistors in series, each of which was measured separately 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 ?
http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1959-07.pdf It's got a chopper/ synchronous circuit.
On Wed, 21 Sep 2016 11:39:53 -0700 (PDT), jurb6006@gmail.com wrote:

>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 timebase. In what, 1957 ?
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.
> >And this 412a can read 450 megohms accurately, which was proven literally by 45, precision 10 meg resistors in series, each of which was measured separately 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 ?
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
On 09/21/2016 02:51 PM, George Herold wrote:
> On Wednesday, September 21, 2016 at 2:39:57 PM UTC-4, > jurb...@gmail.com wrote: >> 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 timebase. In what, 1957 ? >> >> And this 412a can read 450 megohms accurately, which was proven >> literally by 45, precision 10 meg resistors in series, each of >> which was measured separately 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 ? > > http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1959-07.pdf > > It's got a chopper/ synchronous circuit. >
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