Another reason I like the FX-3...built in voltage regulator 1.25V +/- 0.15V in the meter circuit.

I'm sure there are a lot of busted FX-3s out there with intact voltage regulators. Perhaps it could be used in the OM1 circuit ("Vreg" on the diagram).

Too much bother for me; I use zinc cells in all my equipment that requires mercury cells.