James wrote:
>>
I'm hoping Tim Hughes can weigh in on this if he is still on-list. I have
an old Weston Ranger 9 light meter that used mercury 625 batteries that were
1.35 volts. I'd like to conver it to the silver oxide 357 cells our OM's
use.
<<
The easisiest way, is to just add a Ge diode in series with the battery for
each cell. i.e. if it uses two cells add two diodes,3 cells 3diodes etc.
Check my OM conversion notes on Mark's OM website for suitable diodes and
sources. If it uses 3 cells you could most likely add a single Si diode in
series (like a 1N914 or 1N4148)in place of 3 Ge diodes. For four cells you
could add a Si diode and Ge diode in series. Slightly better than a Si diode
would be a Si transistor like a 2N3904 with collector and base tied together
to make a very low leakage diode. Write me off list with your address and I
will send you one.
By far the best way is to add two small parts to the actual meter movement
leads, if you can get hold of the leads, without too much disassembly.
Write me off list if you want to do that, and I will help you work out part
values. I don't do the diode conversions on OM's anymore myself, since the
two part conversion is 100% accurate, whereas the diode adds some modest
non-linearity.
Regards,
Tim Hughes
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