In a message dated 4/20/99 2:55:13 PM Pacific Daylight Time, omtech@xxxxxxxxx
writes:
<< Some recent posts questioned the accuracy of the OM-1 with either an MR9
or other meter mod. I've compared the meter accuracy of the last 6
OM-1/1Ns that I've converted to the silver oxide 357 battery (using the
1N34A diode.) Here is what I found: >>
John,
Thanks for the information on the diode you use in your mod. When I
ran my testing I did not recalibrate the meter with the added diode, I just
compared the calibration with a diode and a silver-oxide to the same camera
with a mercury battery and no diode. (i.e. equivalent to a user sticking in a
MR9 with no recalibration). By re-calibrating with the added diode you take
at least some of the error due to the particular diode used (i.e. the diodes
vary from sample to sample and even more from type to type.) It would have
been interesting to see the error at still higher BV for your cameras where
it is likely to be the most, as the meter is most voltage sensitive at high
light levels. Out of interest I intend to rerun a test with a 1N34A to see
how it compares in a no recalibration test. I will report to list when I
finally get round to it.
FOR DO-it youselvers:
the 1N34A should be a very easy part to get as it is widely used
by school kids for making "crystal" radios. It is a very old part and is, as
far as I can remember, a point contact Germanium diode. Radio Shack
apparently sells them in batches of 10 for a dollar or two a pack. Before
using the diode, trim it's leads short and apply solder so they are tinned
and easy to solder into the camera. To make the mod all you need to do is
remove the base plate (4 screws) and find the black plastic part that holds
the negative side of the battery (under the battery plate on base plate).
This has a wire on its edge (which actually connects to the metal strip
spring on the black plastic) . Carefully desolder the wire and then solder
the diode in series between the wire and the contact where it had been
connected. The marker line on the diode should be nearest the contact on the
black plastic (this is important as the meter won't work if you get it wrong,
although no harm is done). Put a piece of foam in the battery holder with a
cutout hole to stop the silver oxide battery rattling around. When soldering
the connection near the black plastic do it quickly and use care to prevent
damaging/melting the plastic. From my tests it seems likely you would need to
recalibrate to get acceptably low errors but I will report what I find with
no recal when I test a few 1N34A's.
Tim Hughes
Hi100@xxxxxxx
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