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

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My HP 419A is basically unmodified except for the new batteries I put in some time ago. I bought four of the 5 cell AA NiCd battery packs on eBay designed for phone use and they fit in nicely and work well. Here is a Youtube link of the meter comparing two 10.00000 volt LTZ1000ACH references with the null meter set for 3uV full scale. Also added a photo of the replacement batteries in place.


Ill join the fun here;

Made a battery pack for mine using 20x 300ma 2/3 AAA NiCad's.  Found a bunch on eBay for cheep with tabs, total $15.  Center tapped it since the meter needs + and - 13v from the pack.  Fits great and works great.  The charger circuit provides about 10mA in trickle, about 18mA in fast charge mode.  The meter draws about 5mA in operation leaving 5Aa from the charger circuit to trickle charge.   With a bit of hot glue and some foam laying around it made a nice pack.  Lots of clearance with the AC fuse and well protected now.

The last problem to fix on mine, replacing the 1.35v mercury Buck supply battery.  After some testing I decided to use a CR123a lithium cell with a 2.7k series resistor to bring the battery back to within range and show proper full scale.  The current draw is about 650uA, so a 1.5Ah CR123a should run for well over 2000 hours.  Easy to find, cheep and good long life stability.  Holders are available, I'm going to try to mount this near the original mercury battery location.

Replaced the buck supply battery with a 3v CR123a lithium cell, a 2.7k series resistor brought down the current to match the original 1.35v mercury cell.  At full lock, the meter will just peg at 300mv per the battery test in the manual. 

Just a quick story about my HP 419a. I bought it a week ago on ebay. Marked as defective because of leaked batteries.
That was true. Quite a mess. Most of the time went into cleaning everything up with gloves and so on.
I found replacement cells that fit with just minor adjustments to the plastic holder. They also have similar electrical specifications:

Instead of the mercury cell, I just used a sr44 cell, it has a flat discharge curve. I mounted it with a holder to a pcb and glued that in.
The drop to 1.35V, I did with a simple schottky diode. Seems to work.

After installing the new batteries, the meter worked instantly. As far as I can tell on all ranges.


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