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  1. #21
    titrisol's Avatar
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    The meter system does not rely on the voltage of the battery by itself, but compares voltages between two parts of a circuit. The first part has a set resistance and the second part has the CdS cell (photo-resistor). Once this has been calibrated for the resitnace of the CdS cell, the input volatge is not so important (within certain limits).
    The Spotmatics (and the K1000) used a 0-meter or a needle that goes to the 0 position when balanced.

    I have used silver batteries in the spotmatic for ages, in fact my dad did so in the mid 80s when mercury batteries became expensive. The only think I've found is that fresh Silver oxide batteries tend to make the needle overreact to changes, after a month or so the movement is a lot smoother and works like that for years. (a 395 cell should last 2+ years)

    After all this hoollaballo of replacing with zinc-air batteries started about a couple of years ago, I bit the hook and spent a lot of money on those batteries... they last a month or two. So I checked with a Minolta SpotMeter and a 395 battery... it was within 1/3 stop for bright sky, neutral wall and shady spots so I decided to save me some money and let the camera run as it should.

    I'd reccomend joining the Spotmatic list (http://groups.yahoo.com/group/Spotmatic/), where there are documents on replacing batteries as well. Diagrams of the metering circuits can be found there as well


    Quote Originally Posted by cao
    I am having trouble convincing myself. I think there will be 15% greater current through the meter with the Ag cells. So the meter should read slightly higher. Maybe setting the meter's hairspring tension to center at 3.4ua would work. In short, why is the meter current not linearly dependent on the supply voltage?
    Mama took my APX away.....

  2. #22
    cao
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    If you read the Asahi or Nat Cam service manuals, you'll see that in some cases the Spotmatics didn't use a balanced bridge, and there was a specific non-zero meter curent for a centered needle. Would someone please show me the network analysis proving the current doesn't change at all? While everyone may be right in practice (the change is small), I want to be convinced of the theoretic soundness (the change is zero). Every fiber of my being says current flow is linearly dependent on voltage in a resistor net. Prove me wrong please!

  3. #23
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    I don't have an answer to our question then,.... all i can say is that it worked or me :-|
    Mama took my APX away.....

  4. #24

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    I have 2 bodies, one works fine with standard battery, the other was 2 stops high and the needle seem to bounce around a lot. Hearing aid batteries worked, but I bought an adapter for that body and all of the ills seem to resovle, it may that over the years the metering system evolved.

  5. #25
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    Or it may be that the one with the bouncy needle had a problem in the speed dial resistor or one of the meter cells. The CdS cells in the meter, as semiconductor devices, could potentially fail in a manner that would make their conductance voltage dependent, like a zener diode (though more commonly they fail as simple open circuit or high conductance, insensitive to light). A noisy (i.e. dirty) resistance strip or wiper in the speed dial could do the same thing, with increasing voltage amplifying an effect that's still there, but less noticeable, at lower voltages.
    Photography has always fascinated me -- as a child, simply for the magic of capturing an image onto glossy paper with a little box, but as an adult because of the unique juxtaposition of science and art -- the physics of optics, the mechanics of the camera, the chemistry of film and developer, alongside the art in seeing, composing, exposing, processing and printing.

  6. #26

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    Quote Originally Posted by cao
    Every fiber of my being says current flow is linearly dependent on voltage in a resistor net. Prove me wrong please!
    Hi,
    The point of a simple balanced bridge circuit is that at the null point (ie correct exposure indication) the two arms of the bridge across which the meter is connected are at the same potential, regardless of what that is. Consequently no current flows and the meter gives a null or centre indication. Whether the Spotmatic meter system is in fact a balanced bridge circuit is another matter (I've not seen a circuit diagram), but the fact that the needle defaults to the underexposed end of its travel when the meter is switched off and doesn't naturally fall to the centre suggests that it does need current flowing and that although it is a bridge, it isn't a balanced bridge but an unbalanced one, indicating a null when a current of 3uA flows through the meter winding.
    My feeling is that a different battery voltage will theoretically give a different null point but that the difference of 0.15 volts between the original EPX400 and a 1.5v cell is sod-all (to use a technical term) in the overall run of things and within the tolerances of the mechanical shutter, fim latitude, etc. won't make a blind bit of difference.
    All that theory and pontification aside, I still have a Spotmatic working happily with an EPX400 cell (don't tell the Green Police) so will buy a silver oxide cell, stick it in and see what transpires (ie whether it blows all the above out of the water!)
    Good fun, this analogue photography!
    Best wishes,
    Steve

  7. #27

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    Quote Originally Posted by cao
    Now you're standing exactly where I am with this. The Sp F does default to a centered position, whereas the previous Spoties do not. Since the net consists entirely of resistors whether variable or not with the exception of the meter coil, it seems that recalibrating for silver simply means changing the hairspring arm so that the meter is centered which 3.4uA flows (15% increase) through the winding.

    For diagram see page 23 (pdf page 24) in: http://www.pentax-manuals.com/markro...t_cam_spii.pdf
    Warning: 11mb file
    Yes, tweaking the hairspring arm would seem to be the way to do it, although, as I say, I would doubt whether 15% would be a significant difference. Apart from the mechanical innaccuracies of the camera when new, there are also the effects of 30/40 years age to consider! If the shutter speeds have all become a bit slower over the years and a slightly high meter needle means you tend to stop down a little more, the two might counter one another to give the right result! Another option that wouldn't even need the top taking off the camera might be to fudge the ASA setting. Probably the only acid test is that of suction and observation, ie suck it and see!
    Best wishes,
    Steve

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