T3D Dead FED cell

[john bercovitz <bercov@xxxxxxxxxxx>]

I went on a field trip with the camera club last weekend and did 
my usual FED thing.  I got out in the field (away from my camera 
bag) and _then_ tried to fire the FED and found the cell was dead.  
Again.  I don't know how many times I've done this.  I am a very 
slow learner.  8-(  Fortunately I also had my N70 (yes, I own a 
few flatties) and the loaner RBT with me so the only enduring 
annoyance was the extra weight.  I'm FED UP with dead cells and 
I've been swearing I would install a switch in the bottom panel 
but now I mean it!  8-)  Pursuant to this, I checked my old notes 
and made some new ones but was unsuccessful in finding the right 
switch, today.  I will persevere this time.

***Old notes:

original FED cell:
0.455" (11.56 mm) diameter
0.211" (5.36 mm) height
test @75 degrees Fahrenheit:
load, ohms             voltage, V            current, mA
No Load  (NL)            1.562                    0
510                      1.528                    3.0
NL                       1.558                    0
270                      1.510                    5.6 
NL                       1.556                    0
100                      1.459                   14.6
NL                       1.555                    0
NL (later)               1.559                    0
NL (much later)          1.563                    0

The original appears to be like a silver oxide #76 cell which 
is a Radio Shack SR44, a Burgess 76SO, an Eveready S76, an 
RCA VS1776, a NEDA 1107SO, a Ray-O-Vac RS76, a Mallory MS76.
Duracell says these are .455 diam x .210 high, 180 mAh
and Kodak says they are .457 diameter x .213 high, 160 mAh.
According to the Burgess catalog, the cell should read 
1.56 V for a few hours under load and then drop to 1.5.  
Burgess diam. is .455 +0/-.010, height .200 +/- .008.

MS076 Duracell silver-oxide cell:

load, ohms             voltage, V            current, mA
No Load  (NL)            1.566                    0
510                      1.500                    3.0
NL                       1.546                    0
270                      1.468                    5.6 
NL                       1.547                    0
100                      1.459                   14.6
NL                       1.553                    0

***Today's notes:

Energizer (Eveready) #303 silver oxide
load, ohms             voltage, V, rounded off
NL                      1.57
1K                      1.55
510                     1.52

With the Energizer, I did load tests at various light levels.
Wide open with a flood a foot away from the sensor, I got 1 ma.
With the ASA dial turned so that there was no hole in front of 
the sensor, in a dimly lit room, I got 0.1 ma.  With the sensor 
covered, I got .03 ma.  I imagine this last resembles the current 
when the camera is in the camera bag.  So cell life would be 
170/.03 = 5700 hours  This is less than a year and probably is 
a pretty good approximation of how long these cells are lasting 
me.  Sensor dark impedance must be 1.57 V /.03 mA = 52K.  So if 
I install a switch to improve cell life, the switch must have an 
open impedance which is >>> 52K.  This should be trivial.

I took the bottom cover off the camera (this doesn't hurt
anything) and found that I have a space about 1/2" on a side in 
which to install a switch.  So there's plenty of space; the only 
problem will be finding a good gold plated slide switch.  The 
wires to the cell holder are long so it will be easy to install 
a switch without even adding any wire.

John B


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End of TECH-3D Digest 313
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