I originally replied to Chip off list not realising he posted this query
generally.
Here is a slightly edited version of the reply:
Chip,
I have a document (attached) with some sketchs I made on this, I sent
to all the people who asked me about this when we had a long discussion on
the list before. I actually do a different conversion on my own OM1's that is
much better in that the meter linearity is not changed in anyway and you
don't need to recalibrate at all. (sometimes the diode conversion is a little
off near max EV) The dowside is you have to remove the top cover unlike the
diode conversion where it is easy to remove the baseplate. The document has a
lot of stuff to do with the other conversions and OM2 circuit theory but at
the end is the part about diode conversion.
I looked at a variety of diodes, schottkys and Ge. The large schotkys
are a bit leaky while the smaller schottky diodes like the 1N5700 series from
HP are a little too high in forward drop. The common point contact 1n34A
available from Radio Shack for about $1.20 for 5 seems close and that is what
John Hermanson mentioned previously he had used, to the list. If I were using
a diode I might try to select a small Ge transistor connected as a diode
since they have better leakage characteristics than point contact diodes like
the 1N34A. Unfortunately these are hard for amateurs to get particularly in a
small enough package to fit. Similarly Ge junction diodes are also not so
easy to find.
You can improve accuracy if you have a DVM and electronic skills: Buy a
couple of sets of diodes and select a diode by connecting it in series with
the camera (AgO) battery and the camera and measuring the diode forward drop
at maximum allowed EV (something around 18EV or so from memory) using your
fastst lens (at least f1.8). A slightly less accurate way would be to measure
the forward drop with about 0.5mA through the diode (approximate typical max
EV current of meter with fast lens) . The mercury cells initial voltage is
around 1.35V but drops to about 1.3V if left on for a long time similarly
with the silver battery it starts out around 1.55V-1.57V and drops a little
with age.I have a tedious long discussion of these voltages in the archives
from some yeas back. Bottom line you need something of the order of
0.2V-0.21V drop at around 0.5mA. The forward drop varies very approximately
20mV/octave or 60mV/decade of current leading to some modest meter
non-linearity with the diode.The drop also varies with temperature leading to
a slightly reduced accuracy over temperature. A rough approximation is
%voltage erors translate directly into 0f EV value. that is why errors are
only really significant at higher EV.
Battery spacer : I just used a piece of carefully cut foam to stop the
battery rattling. You can (carefully) bend the spring to increase the battery
holding force a little too.
Regards,
Tim Hughes
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