Pentax Spotmatics, any fans out there?

[Steve Roberts]

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

 

[Steve Roberts]

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/markroberts/nat_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