Regarding the 731B:
The E505 is a 1ma current regulator. It is still available at a premium
price but can be replaced by a Vishay J505. Slightly different package,
TO92 vs TO18, but it does work. The J505 is available from multiple
sources for about US$2 including several on the Bay.
The U1 LM308 opamp can occasionally be destroyed if voltage is back-fed
into the 10V output. Don't ask.
Regarding the 750A batteries:
I replaced mine with four ER14250/LS14250 1/2AA 3.6V 1200mAh lithium
cells. Remove the original mercury battery holder and replace it with a
dual AA cell holder. Use the kind that has the batteries side-by-side
because you need to connect to the center tap between the cells. Wiring
is pretty much identical to the way the mercury battery holder was
wired. Install the lithium cells in a series string and you have +/-7.2V
which is a bit higher than the +/- 6.75V of the mercury batteries but
still works. The lithium cells have slightly more mAh capacity than the
mercury cells and a much longer shelf life which works well for the 750A
as the protection circuit draws very little current. Don't disable this
protection circuit, it's too easy to fry the divider string if you're
not paying attention.
Thanks - Steve
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