Hi Oly Repairers,
Still more comments relating to my previous post
and
some follow up comments on the thread in general.
John lind wrote:
>>
There are three off-the-shelf solutions I know about. The following URL's
go straight to their sites: ...
<<
Unfortunately John, the 1.4V of the zinc-air cells still leads to errors as
the charts at the end of my last post indicated. Also the MR9 adapter will
give
errors as **estimated** in my post (since I don't own one)
and apparently verified by Gary Reese (see Below.)
For zinc-air, I have reformatted data from my last post and
interpolated to 1.4V in a few cases:
approx error:
EV3 0 stop
EV6 -0.1 stop
EV 7 -0.6 stop
EV 16 -0.6 stop
This is *much* better than nothing but still a bit too loose for my liking
===============
Gary Reese wrote:
>>
I just tested 4 OM-1 's on an exposure meter tester and I got non-liner
errors in all four using a MR-9 adapter in each. The situation is personally
disgusting since the errors are major. I refuse to sell a camera that I know
is out of specs (ex. for shutters at 1/1000 and above, which is to be
expected in an aged camera). To me, it makes no financial sense to have a
commercial overhaul of an OM-1 since overhaul costs eliminate any reasonable
profit. I guess I'll have to learn to adjust them myself.
I'm planning on retesting the meters with a mercury cell just to see what
error is attributable to the MR-9 adapter.
<<
Gary,
were the errors mainly at high EV as my test suggests is likely
with an MR9 type adapter? What was the largest error?
A general comment: if the errors are at low EV's it is likely that it is the
photocell as the Cadmium sulphide or cadmium selenide cells used
in most cameras of that vintage do drift over time and the camera is
generally insensitive to battery voltage at low EV. Interestingly the meter
response really gets sluggish at low light when going quickly from light to
dark conditions. Be careful when you test.
=======
Wayne Shumaker commented about using a Linear tech
i.c ,switched capacitor voltage regultar which draws 18uA and would use
a pair of thin Lithium batteries. At 6V input this would have a
much higher power consumption than a simple linear design
with a similar parts count. On the other hand the batteries will probably
have more energy to start with, as the Lithium batteries have higher
energy density.
Wayne reposted later saying he might find a single cell to fit (3V)
but that in rereading the i.c.spec sheet he found it had a standing
current of 150uA rather than 18uA. Better than
a shunt regulator but still rather high for forgetful photographers.
===========
John Maynard commented:
>>Tim ... I'd like to try this. What do you plan to use, specifically?
Perhaps a PCB
can be designed with surface mount components and made inexpensively; while,
as you say, this might be beyond the average home repairman, it might be
simple enough to be duplicated by folks with experience in the area.
<<
John,
I have a "back of the envelope" 3 transistor design using 5
resistors.
I may need a cap for stability as the parts get rather slow at low current.
The current drain should be about 11uA. The static current is determined
largely by the requirements (min gain) of the series pass transistor
near saturation. I may have to tweak this depending on my junk parts
availability. I will keep you informed if this works out well in test for
me, when I get round to building it!
Surface Mount ?? There is no reason that this should not be built with
sot23's
and chip resistors for surface mount but I am hoping I can find enough space
in an used corner for regular parts,1/8W resitors and perf. Or even
(horror) a spider in epoxy!
Maybe I am too optimistic on size.
Tim Hughes
Hi100@xxxxxxx
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