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Thursday, May 28, 2009

Mercury battery

Recently, I received a Zenit 12, bought by my brother Christophe. He asked the seller a little history of her camera and sent me the answer. When I read that she had to stop using it, because her photographer had told her that the lightmeter battery was no more available, I leapt up!

Not too long, the leap, the "Phot'Argus" test of Zenit TTL remained on the table behind me, and reading it confirmed my old school memories:
The lightmeter of many cameras of 1970-1980 years was powered by a mercury battery, very often a PX625-MR9(1.35v). Some examples: Canon FTb, Minolta SRT101, Pentax Spotmatic F and K1000, and of course Zenit TTL. Mercury batteries having been prohibited, the 1.35v MR9 disappeared, but the PX625 still exists, in 1.5v alkaline.
Okay, you will say, but the readings of my meter will be distorted with 1.5v instead of 1.35v!
Well nope: in many of these cameras, with the help of a particular wiring diagram, the metering is done by "electrical zero": the galvanometer needle indicates correct exposure when it measures zero volts, by balancing resistance values related to shutter-speed selector (and aperture), and the resistance of the CdS sensor depending on the amount of light which reaches it.
How to know what are the relevant cameras: display uses a needle, of course, and that needle is in rest position (approximately) in front of a fixed reference point, the one on which you'll have to align it for a correct exposure. It works like this for Pentax Spotmatic and K1000, and Zenit TTL.
On the other side, for the Canon FTb and Minolta SRT101, there is no fixed mark, but a moving one, mechanically controlled (racks and pinions for the Canon, thread and pulleys (!) For the Minolta) by the diaphragm ring and the shutter speed selector. The galvanometer needle goes up proportionally to the light falling on the sensors, and the battery voltage... unless the manufacturer has taken the precaution of inserting a component (Zener diode) to regulate the voltage (eg to 1.2V). That I'd like to know, if you have a Phot'Argus test of one of these cameras and would like to send me the wiring diagram...
About that, I recently found a very nice site (in French) about the Minolta SRT101 and as the author obviously has the famous Phot'Argus test, I asked him, but I did noy have the honor of a reply... This is not the first time I'm ignored this way, so no link to other collectors sites on my website or on this blog. On the other side, I would like to know if sometimes someone else than me or my little brother visits this blog, so if you happen to come towards here, let me know, thank you!

No need to say, I'm still waiting for the comments!
The wiring for the "zero volt" metering, it's called a Wheatstone bridge.
In fact, the only film that you can still easily find, is 200ISO color negative; exposure tolerance: -2 to +3 diaphragms, compare this with the inaccuracy of a meter fed by 1.5v instead of 1.35v...