Products > Test Equipment
Hewlett Packard 740B DC Standard Digital Voltmeter (and 740A)
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Cubdriver:
That would be the critter.  I didn't bother to try to measure mine, though half a millivolt or less wouldn't surprise me...

-Pat
beanflying:
I think the sensible solution is to abandon a battery all together based on the replacement options. A small Daughter board in that space with an additional cal pot hole drilled in the lid and a modern reference fitted with appropriate dividers if needed. Temp Co shouldn't be a major issue as that bay will be heated by the reference and it is at least 30 minutes to get near being stable. So give the new reference an hour or two before tweaking the cal pot and it will be nice and toasty too and well above any ambient variations on temperature.  :-+
Cubdriver:

--- Quote from: beanflying on February 16, 2018, 09:10:18 am ---I think the sensible solution is to abandon a battery all together based on the replacement options. A small Daughter board in that space with an additional cal pot hole drilled in the lid and a modern reference fitted with appropriate dividers if needed. Temp Co shouldn't be a major issue as that bay will be heated by the reference and it is at least 30 minutes to get near being stable. So give the new reference an hour or two before tweaking the cal pot and it will be nice and toasty too and well above any ambient variations on temperature.  :-+

--- End quote ---

I was thinking along those lines too, and did some searching back then and seem to recall finding a couple of methods people had used.  Think I was initially looking for mercury battery substitutes - they're used in old school light meters too, so there's a market for such a thing. I recently got a pair of Chinese voltage reference boards (must have seen them here), adapting one of those into the circuit may be a good option.  The instrument has been sitting on its side in the hall for must be three years now waiting for me to work on it...

-Pat
beanflying:
Rough idea but a 4.096V reference is easy to get a stable one that will drive a few mA without a buffer. Drop the output into 1/3's with the best precision resistors (4-5k total but not important) you can sensibly afford gets you 1.36V with no trimming. Any Pot you add to tweak the last bit can be any sort as it's influence on Tempco will be tiny.

Most of these References will need 10-15V so finding a 12V rail should be simple.
beanflying:
One of the other things I was going to chase up was the old Belden/HP lead. Evilbay pricing shipped to Australia would be $50+ Based on another recent acquisition a 735A from the same era having a factory fitted IEC plug I think a change is sensible.

Maxim reference IC ordered with the few supporting bits for the 1.35V replacement.
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