Hi OMer's,
This is a follow up description of performing battery modifications
to OM1s and the related exposure errors. This post is already too long so if
interested in more information check previous posts.
I previously posted results of a number of tests on using various
diode adapters (similar to the MR9 from criscam) for converting OM1's to
silver oxide (357) batteries from mercury batteries. I previously came to the
conclusion that errors using these adapters without recalibrating the camera
would be about 0.6stops maximum with the worst errors generally being at the
higher EV's and with faster lenses. John H. from Camtech posted results after
adaption **and recalibration** using a 1N34A diode. His conclusion (not
tested above EV16) was that errors were negligible after recalibration.
Note: even after recalibration the diode voltages vary with
temperature and will give rise to additional errors for temperatures away
from room temperature. This gives rise to as much as another 0.4stop error
for a temperature change of 20C away from room temperature. (approx 2mV/C
change in voltage)
I had not tested the 1N34A previously so I tested a batch of them
that I purchased from Radio-Shack and found that they performed a little
worse (wider spread in characteristics particularly at low current) than the
Germanium diodes (made from a transistor) that I had previously tested. I
also tested for errors in stops using a 1N34A in an OM1 (with no camera
recalibration). Results for this OM1 shown below gave about 0.5stop maximum
error. I have also included measurements of diode voltage drop vs current on
a number of the 1N34A diodes.
TESTED OM1 ERRORS in stops at approx EV's :
194mV @0.409mA EV16 0 stop
165mV @0.197mA EV8 -0.5 stop
125mV @0.100mA EV6.5 -0.35 stop
100mV @0.062mA EV6 -0.2 stop
70mV @0.023mA EV2 -0.1stop
(63-90mV for different diodes at low current)
Tests on 9 , 1N34A diodes from Radio Shack ( part#276-1123 )
Diode voltage @ current
213mV @ 0.558mA
221mV "
216mV "
218mV "
212mV "
210mV "
204mV "
203mV "
206mV "
------------------------------
173 mV @ 0.27mA
175 mV "
165 mV "
168 mV "
182 mV "
173 mV "
185 mV "
177 mV "
----------------------------------
235mV @1.100mA
Alternative 1N34A type parts from other suppliers:
# ECG109,HEPR9134,SK3087,RE47
** FOR DO-it youselvers here is an updated description*** :
HOW TO MODIFY AN OM1(N) with a Germanium diode:
the 1N34A should be a very easy part to get as it is widely used
by school kids for making "crystal" radios. Radio Shack part#276-1123 sells
for $1.20 and includes 10 diodes.
Before using the diode, trim it's leads shorter 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 (brown on newer
OM's) on its edge (which actually connects to the metal strip spring on the
black plastic) .
Remove the screw that holds the spring strip connected to the wire.
Carefully remove the spring not losing the insulating washer and screw.
Removing the screw and spring prevents damaging the plastic pieces when
soldering wires to the spring strip end. Desolder the brown wire from the
spring strip. Solder the diode in series between the brown wire and the
contact where it had been connected. Insulate the diode with tape or shrink
tubing. The marker line on the diode should be nearest the spring (this is
important as the meter won't work if you get it wrong, although no harm is
done). Reassemble spring strip and insulating washer back into camera. Note:
You may find it easier to remove the complete black plastic battery holder
when performing this modification. Adjust diode position so it does not
interfere with adjacent mechanical parts.
Put a piece of foam in the battery holder with a cutout hole to stop the 357
silver oxide battery rattling around.
Tim Hughes
Hi100@xxxxxxx
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