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"GREAT" Meter! The HP 419A DC Null Voltmeter, [RESTORATION]

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johnlsenchak:


Service manual  for those playing along  at home  !
http://literature.cdn.keysight.com/litweb/pdf/00419-90004.pdf


]"GREAT" Meter! The HP 419A DC Null Voltmeter, [RESTORATION]
•Feb 27, 2020
Mr Carlson's Lab
Lets make this Hewlett Packard, HP 419A DC Null Voltmeter work again! Very useful information regarding the chopper circuit, and how to make it dependable. Also in this video, Mr. C shares his custom designed circuit to replace the original neon chopper board.


Posted  on  02/28/2019

rastro:
I'm not sure why Mr. Carlson doesn't mount the LED's on the other side of the PCB. (edited)

Just realized he did do that on the installed version...

oPossum:
He explained that in the video. The side of the board with the LEDs is painted black so there will be not reflections from the PCB.

Dr. Frank:
A pity, that he as the "Tube Guy", has absolutely no clue about what he's doing with all the neons.. he obviously never heard about the Dark Effect of neons, and he did not understand, nor did he reverse engineer this neon circuit, i.e. the A3 board.
He did not measure any characteristics of all of his neons, either, like break-down and maintaining voltage. That circuit is similar or identical to a neon ring counter, so it divides the 330Hz by two, which makes these neons illuminate alternatively. 

If he would have understood, how this circuit works, he probably would have selected A3R1, A3R2 and A3C1 and maybe the driving voltage fitting to his neons, as this is the real reason, why hp at that time sold complete boards only.

Btw.: Any critical or negative comments on his YT video will be 'moderated' by him, i.e deleted.

Frank

johnlsenchak:

--- Quote from: Dr. Frank on March 01, 2020, 06:38:31 pm ---A pity, that he as the "Tube Guy", has absolutely no clue about what he's doing with all the neons.. he obviously never heard about the Dark Effect of neons, and he did not understand, nor did he reverse engineer this neon circuit, i.e. the A3 board.
He did not measure any characteristics of all of his neons, either, like break-down and maintaining voltage. That circuit is similar or identical to a neon ring counter, so it divides the 330Hz by two, which makes these neons illuminate alternatively. 

If he would have understood, how this circuit works, he probably would have selected A3R1, A3R2 and A3C1 and maybe the driving voltage fitting to his neons, as this is the real reason, why hp at that time sold complete boards only.


Frank

--- End quote ---


I agree  with you  Mr Carson  real  didn't explain  why neon  bulbs   where chosen to   create  the oscillator circuit verses  using  germanium transistors which  where more common  when  that unit  was built 

We need  Outback  Dave to explain this  one  8)

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