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Yokogawa HP 4320A Milliohmmeter

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zepto:
I got one of these about a year ago, but I only tested it briefly before putting it away. It is strictly battery operated, there is no line cord. It takes 4 D (or C?) batteries. There is a .100 ohm resistor on the back. It has a current cable and a potential cable coming out the front, they split/merge into positive and negative alligator clips. It has a blue case similar to some old HP tube cases, but the depth is only about 5 inches and it is non-vented. The serial prefix is 524 (1965?).

Given the battery operation, I figured it was a DC device. I recently pulled it out and decided to learn more about it. After struggling to measure current flow, I connected it to an oscilloscope and saw it was actually AC. An accurate DC milliohmmeter from that era would not be trivial to design using said batteries, either.

I haven't been able to find any information on it. I have some YHP branded and logo'ed test equipment, but I've never seen a YEW/HP logo like on the meter face. It appears to be a predecessor to the famous hp 4328a. A previous owner left a note on the side of the device, which would have been helpful if I could read! If anyone can translate, that would be highly appreciated. The device seems to work, but I'm not confident I am using it correctly -- specifically the current adjustment.

TizianoHV:
Using google translator text recognition:

"How to use the MILLIOHM METER
① Make sure the snap is OFF and set the desired range with the RANGE dial. 
②.Do not short the red-black clip of the cord ・Press the PRESS button.  At this time, the pointer swings to near OJ.  Apply this deflection to the CURRENT SET knob and extend the needle to the CUREENTSET position.  Replace CCUREET. 
③ Short the red-black lip and turn on the sampler.  Around this swing is the resistance value of the lead clip.  (Do not press PRESS BOTION.)
④ Place the object on the red-black flip and turn the snapple.  Read the deflection at this time, and check the resistance value of the lead bream of ③: the clip.  Caution ( 1 ) Always call Snap 10,000 Yen except when measuring 731 ( 2 ) Always dial POWER OFF after use SIT"

AC test currents are normally used to measure earth resistance to avoid electrochemical reactions.
Using AC also remove the effects of offsets voltages.

Stray Electron:
  Yokogawa and HP partnered together in the mid 1960s and sold each other's equipment. If you turn the hp badge upside down it says yd and that was supposed to stand for Yokagawa something-or-other.   I used have an HP catalog printed in Japanese. 

  Your date code would indicate that your meter was made in the 24th week of 1965. If you'll send the rest of the SN I can probably tell you where it was made. 

  PS:  The YEW on the meter face is the same logo that I have on my analog Yokogawa power meters.

TimFox:
If you rotate the -hp- logo 180o, it reads -dy-, so Hewlett-Packard named a line of industrial modules "Dymec" in 1958.
I don't know if the current Dymec company is the continuation of that division.

factory:
I've seen this YHP badge on a few Yokogawa designed/made HP products, the boards are usually labelled YHP too.



Given that this might be the only 4320A out there, could you please add some internal pictures.

David

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