Yokogawa HP 4320A Milliohmmeter

[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 180^o, 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

[RolandK]

Nice unit from the rounded corner case aera. Seems to have 2 modes:


black scale: Works with a constant current source. The voltage across the testee deflects the meter (linear scale, Umeter = R_unknown * I_const). The current source must be calibrated against an internal reference resistor to deflect the meter 100% before use.

red scale: voltage divider mode. One internal resistor in series with the one to measure. The voltage across the internal resistor deflects the meter. Umeter = Rref / (R_unknown + R_ref) * U_const.  For calibration the input must be shorted and the source voltage is set to deflect the meter 100%. The internal resistor is 2 Ohms, as this gives with a testee of 2 Ohms a 2 / (2 + 2) divider and therefore 50% meter deflection.

I forgot: before use, set the zero-point of the meter (the srew in the lower middle of the meter)

It only depends on the reference resistors and the linearity of the amplifier and the meter, not on any voltage or current source.

And all this in AC. Does it humm when in use (then there could be a mechanical chopper)?

Can you make pictures of the inside or make a reengineering of the schematic?

[zepto]

  Yokogawa and HP partnered together in the mid 1960s and sold each other's equipment.

I think you are probably understating the nature of the relationship between these two companies. The project lead for the 4328A(milliohmmeter) and 4329A(high resistance meter) was named Yoshihisa Kameoka. He was a YEW employee who joined YHP after it was formed.

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

Hi David, I have a few items with that badge on the front. As far as I know that badge meant it was destined for the Japanese domestic market. The devices I have with that badge are wired for 100v or 100/200 switchable. US bound YHP devices stripped away the Yokogawa designation, except perhaps on the serial number plate and had 115v or 115/230 switchable. I don't think this 4320a is the only example left of this device but I doubt it was sold in the US.

And all this in AC. Does it humm when in use (then there could be a mechanical chopper)?

Can you make pictures of the inside or make a reengineering of the schematic?

Hi Roland, I do not know how it works. A chopper was exactly what I was thinking of when I said a DC milliohmmeter would not be trivial to design with similar constraints(time period, 6V total battery voltage). I do not think it would be necessary for ac amplification, but I am not an expert.


I didn't expect anybody to show any interest, this is a nice surprise. I took a few pictures and made some observations. Batteries corroded and made the inside dusty even though the batteries are in a nice separate compartment. It takes 4x D batteries, earlier I couldn't remember if it was C or D. A previous owner drew the orientation for the batteries in pencil inside the compartment (thank you!). I haven't put any effort into cleaning it or making these images look nice, sorry.

I don't know what the 4 large metallic cans are. They are sequentially numbered in HP part no. style (04320-xxxxx) there are 8 terminals and 2 larger screw terminals for mounting with what looks like insulating washers. I think I counted 12 transistors, 4 are on one pcb and are Toshiba 2sb200's I believe. There are 8 transistors on the other side, the few I looked up were Toshiba 2SB germanium transistors. Some of the transistors and diodes look like capacitors due to old packaging. There appear to be 2 transformers and a choke all of similar size. There is one potentiometer on a PCB, the only other potentiometer I saw was the front panel current set. There are quite a few 10v/15v electrolytic caps that look fairly anachronistic, but I am not an expert on capacitors. There are several diodes on the PCBs and also on a small detector(?) pcb mounted on the front panel. The detector(?) pcb has 4 diodes that look like the old top hat style as well as a wire-wound resistor. The toggle switch next to the potential cable seems to just open that connection at the cable. It doesn't seem like the alligator clips that are installed are true kelvin, but there are 2 wires going to the base of each alligator clip at least.

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