The Pentax light meter calls for a mercury battery 1.3 volt. Of course mercury batteries are no longer used so what can be used instead?
There are a few approaches commonly used for this general problem:
1. Adjust the unit for proper readings with a different chemistry.
Alkalines are 1.5V, silver oxide 1.55, for example. Alkaline chemistry
has a sloping discharge voltage tail that's less than ideal for photo
use. I don't see a silver oxide size equivalent for the E640N / MR52
type batteries. 640 family alkaline cells are easily available. I've
seen people gin up some sort of hacky homebrew spacer arrangement that
allows use of a smaller size of battery -- e.g. rubber gasket rings to
keep the thing centered, and some sort of conductive shim to make up
missing height.
2. Find an adapter that includes a voltage regulator. This plus a
silver oxide battery, which has a good voltage discharge curve, is the
best solution, if you can find such an adapter. I haven't managed to
spot one for this type of battery.
3. Find a physical adapter that allows use of zinc-air (aka Wein)
cells. This provides a good voltage discharge curve, but the zinc-air
cells have a limited life once you pull the seal tab off of them. Small Battery has an adapter. I think you may be able to find their stuff
elsewhere too.
https://www.smallbattery.company.org.uk/sbc_mr52_plus_mr44_adapter-combination.htm
The 640 family batteries are about 15.9mm in diameter, and 11.2mm high.
De
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