In a message dated 9/14/00 10:49:37 PM Pacific Daylight Time,
afildes@xxxxxxxxxxxxxx writes:
<<
Depends how they've been stored, I think. I understand that most cells that
depend on light stimulation of a metal like selenium or germanium will lose
their sensitivity with age. The generation of cameras with large selenium
cells would be in trouble by now if stored uncovered in a light place -
most would be 25-40 years old by now. I've been advised that the same thing
could happen to my lttle Leica CL which has a 'cell on a stick' behind the
lens (as does the M5) - they have to be replaced in as little as 10 years
depending on use and storage.
I'm not the expert on this - I'm relying on occasional comments in general
research - but older cameras with CDS meters seem to be more reliable (if
you can find batteries!)
Andrew
>>
I believe the Selenium cell's transparent conductive metalization
(probably tin oxide) degrades with humidity.
Most cells that I remember seemed to be sealed with a laquer over the
metalization. I have seen defective old cells with very discolored stained
looking surfaces. If this is the failure mode, then the active material would
just get electrically disconnected as the transparent conductive surface is
slowly damaged.
Cadmium sulphide photo-conductive cells last much better if they
are hermetically sealed in a metal package with a glass window. Unfortunately
Many cameras just use low cost plastic packaged or laquered devices. Some old
SLR's added an extra cell looking back toward the eyepiece to make a first
order correction for extraneous light entering the viewfinder . OM's did not
do this, hence some recent and old threads on metering errors with light
entering the eyepiece.
The Leica CL mentioned uses a photoconductive cell. Some cameras use
CdSe instead of CdS cells. These are meant to be more stable but they need a
filter to match the films spectral response unlike CdS cells that have
approximately the right curve intrinsically. Power generating solar cells are
damaged by very long time exposure to high light levels but it seems unlikely
most camera storage conditions are anything like harsh enough. High
temperature is meant to be particularly bad for the CdS cells, this is
probably related to the fact that these are screen printed devices. Many
electronic components have failure rates that approximately double every 10C
temperature rise, so don't leave your cameras in the trunk ("boot" for Brits)
for exteneded periods!
The Quantum exposure meters and flash meters use silicon photo-diodes
with a suitable blue enhancing filter but the package is poorly sealed
(despite being partly ceramic) and they seem to fail easily. In the case of
the Quantum's, if you are lucky, the flash trigger sensor (left hand cell )
fails and this is easy to replace with a non-filtered conventional cell with
no effect on performance.
Tim Hughes
>>Hi100@xxxxxxx<<
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