HP419 leaking battery?

Hello Group!

I have a HP419 null voltmeter that I use occasionally. Now it's however been stowed away for a few years due to a major rebuild of my workshop. I recently found that it was leaking something. Taking it apart I found something that looks like a small battery that was leaking some fluid.

Measuring the voltage over it shows nothing, but you can read on one end a + sign, the figure 3 and "U.S.PAT 2712565 AT.AL." The thing is 25mm diameter and about 18mm long. And no, it's not one of the NiCd cells.

Is this some kind of reference cell? Is it a "standard" 3V cell, and can it then be replaced with just about any Lithium or Silver-oxide cell?

/Leif

Reply to
Leif Holmgren
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It is probably a mercury battery. No longer available.

If you would be interested in getting rid of the 419 I am looking for one.

73 Gary K4FMX
Reply to
Gary Schafer

It's a Mercury battery that is used when you use the Null function to buck the input voltage to give you an infinite input impedance. That mercury cell is NLA, so you need to replace it with something else. I removed mine, and its holder, and replaced it with a plastic Radio Shack single AA holder. I put a rechargable alkaline battery in mine, because the don't self discharge, but you could use anything that puts out 1.2-1.5V. Note that it's probably important to use a plastic holder to maintain the necessary isolation from the chassis.

I just attached my holder with a good quality double stick foam tape.

I don't believe I had to change any resistor values to make this work.

The 419 is an interesting little meter.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Reply to
Jim Adney

Interesting, buut it reads "U.S.PAT 2712565 ET.AL." _not_ AT.AL.

Reply to
Robert Baer

Jim Adney wrote: ...

Rechargeable alkaline??? Never heard about these before. Will start looking around.

Are you sure you used one? There is a "big" figure 3 on it, but I guess that could be some size identification since the voltage for a single mercury cell should be 1.35V which makes 3 volts impossible.

How about the voltage drop over time? Have been looking around for data on batteries and they all specify a rather large drop over time compared to what could be expected from a mercury cell. Is it connected so that the exact voltage is irrelevant as long as it does not drop too much when reading the nulling voltage instead of 0.

Regarding isolation, is that a requirement due to the rather high impedances involved, or is one or both of the poles separated from the ground of the rest of the meter?

Yes, and apart from an AVO-6(?) with a stuck needle the only analog instrument around. Sometimes analog is really superior the digital stuff.

/Leif (SM4RPQ)

Reply to
Leif Holmgren

Sure, but late at night the keys on my keyboard start moving around ;-) I actually looked up the patent number and found some very nice scanned pages from 1953. It was too much to read but I guess it was all about the mechanics and very little about the chemistry, even if I saw the words mercury and potassium on my quick look.

/Leif

Reply to
Leif Holmgren

Sorry Gary, not for sale, not even to a ham like myself.

73 SM4RPQ/Leif
Reply to
Leif Holmgren

Ray-O-Vac sold them for a few years, but they are now discontinued. You just need something that has a good shelf life, so you're not opening the meter to replace it all the time. It's not recharged by the meter like the other cells, either, so you have to take it out and replace/recharge it.

Okay, I checked my manual, and the original cell is shown as 1.35V.

Droop with time is irrelevant, because you trim the bucking voltage to get just enough to buck the input signal to zero. Then you measure your adjusted "bucking voltage." The only droop that might matter would be that amount that occurs in the 10 seconds between bucking and measuring. That should be insignificant.

All of the above, I believe. The bucking cell floats in the middle of the circuit, and the meter is so sensitive that you will find that tiny amounts of leakage will show up in the oddest places. I had to carefully take apart the input binding posts and clean off the plastic pieces to get the leakage down. I cleaned the range switch wafers with alcohol and still couldn't get it to calibrate in the lower ranges.

I actually gave up on this meter, thinking that it had some fatal flaw that I hadn't found. When I went back to it a year later, it was FINE! I suspect that I had just touched something and left a fingerprint that finally evaporated enough that it was no longer important.

I couldn't agree more. ;-)

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Reply to
Jim Adney

I would suggest using a silver cell; the voltage is higher and they hold up quite well. One of the good things about a mercury cell, was that its terminal voltage was purported to be a fixed, known value to 4 digits (if unloaded from day one), and that it could be used as a transfer standard for 6 digits (if i remember correctly). Now the environmentalist guys have subverted the constitution and outlawes mercury cell standards (partial dictatorship). Maybe one of the lithium-based cells would be useful; someone suggested to me the LiI cell. Does anyone have any real-world data to support (ore negate) that possibility?

Reply to
Robert Baer

Concerning cleaning with alcohol, that is fine, BUT. One has to dry out the surfaces; if phenolic then some of the bulk as well. To be safe, a whole day for drying seems to suffice. However, be advised that if the cleaning was not good enough (still leakage - but lower than before), then do it again.

If you dunk parts for cleaning, the container itself must be clean - use those "throw-away" single serving cups (or similar) fresh out of the sales container; rinse once with a small amount of alcohol, then fill for dunking. Let part soak for a few hours, dump the alcohol and replace for a

10-15 minute soak, and then dump the alcohol and replace for a short (self) stirred rinse. Shake off the alcohol, do *NOT* attempt to dry with any cloth of any sort (feelthy!); air dry (may warm up to help - but *not* hair drier).

If you cannot dunk, then use a cleaned pump spray bottle filled with alcohol for dousing the part(s); douse & partly dry, repeat 3-4 times then final dry. If the parts are really doity, use a new brush (paint brush or toothbrush as appropiate) for scrubbing nasty areas with alcohol before the above cleaning regimen. Keep the brushes clean the same way...

Reply to
Robert Baer

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