HP 403A AC Voltmeter batteries

[NOON Design]

I've recently picked up the above voltmeter in very nice condition. It's a battery only model that was designed to take an array of mercury batteries that are of course obsolete.
I'm fairly certain I can assemble some NiMh cells into the right voltages but was wondering if anyone has done this and has any tips.

[Runco990]

You could just do this:  (Pic taken from current ebay auction.)

I did something similar to my 427A.  It runs on 2 9v batteries in series which I hid inside the original battery shell.

The 403B is pretty easy to run on Nimh packs.

[NOON Design]

That's what I was considering doing but with rechargeable batteries

[Greybeard]

The 403B is pretty easy to run on Nimh packs.

Look here:
https://www.barbouri.com/2019/08/17/hewlett-packard-hp-403b-ac-voltmeter-restoration/
https://klamfx.wordpress.com/2014/03/26/restoring-hp-403b/comment-page-1/

[NOON Design]

The B is a very different beast to the A, it came with a mains connection and rechargeable batteries while the A was primary battery only.

[alm]

Mercury cells were sometimes used back then as a semi-stable voltage reference. For example in the Fluke 540b. If that's the case in the 403A, then you might need to add a voltage regulator. For the Fluke there was a mod with an LDO regulator so it could use regular lithium / alkaline batteries as reference supply.

[Gyro]

You could just do this:  (Pic taken from current ebay auction.)

I did something similar to my 427A.  It runs on 2 9v batteries in series which I hid inside the original battery shell.

The 403B is pretty easy to run on Nimh packs.

I think if I was going to put that many DuraHell batteries in a meter, I would enclose the holders in a sturdy plastic bag and tightly sealed around the outgoing wires. I speak from bitter experience of doing the same thing (to replace a PP9) in a Marconi TF2700 bridge. Luckily I caught it before the damage extended further than eating the battery holder springs.

[enut11]

Add a DC socket on the back panel and use a mains plugpak of the correct regulated output.

[Runco990]

I think if I was going to put that many DuraHell batteries in a meter, I would enclose the holders in a sturdy plastic bag and tightly sealed around the outgoing wires.

I totally agree.  Nothing but eneloops in my house!!   

Bitter experience indeed.... I have seen them leak in the package in the stores. 

[trobbins]

I restored a unit in rather poor condition, and used a string of TL431 voltage regulators to replace the internal batteries, and to also allow powering from an external 12+12+6V SLA.   All battery operation certainly needed for low noise response.
https://www.dalmura.com.au/static/HP403B%20AC%20meter.pdf

[TimFox]

To get my 403B running, I also used a suitable string of TL431s, but relied on the charging circuit to power it from the mains.
I need to add an external DC input for lower noise operation.

edit:  corrected from 403A to 403B.

[factory]

Correct me if I'm wrong, but there still seems to be some confusion over the 403A model, as far as I know it originally used single use/disposable batteries (400 hours operation), with no mains power option and no rechargeable battery option.
http://hparchive.com/Manuals/HP-403A-Manual.pdf



The TL431 seems a good solution, for both models, needs a bit of modification, as the 403A model used two 4V batteries and three 1.3V batteries, all obsolete mercury hazmat type.

Or you could fit good quality NiMH cells, three cells to replace each 4V battery and single cells to replace each 1.3V battery.



Can't comment any further, as I only have the later 403B model, one on it's own and another as part of the 3550A test set.

David

[TimFox]

I edited my post to correct from 403A to 403B.