Recalibrating the meter on a Canon 7S

[Dez]

I have been having a struggle trying to adjust my Canon 7S meter to use modern silver cells. I understand the gear coupling arrangement under the shutter speed dial, so that setting is not an issue. I seem to have an electrical problem and a mechanical one.

First the electrical. The very expensive adapters to allow the use of silver or alcaline cells in place of now-unavailable mercury cells are just a shell with a series Schottky diode and enough room for a small silver button cell. In the 7S, the wire leading from the battery compartment to the metering circuitry is easily accessible, so I just cut it and soldered in a Schottky diode. At full meter deflection, the voltage drop is exactly 0.20V, so that takes a 1.5V silver cell down nicely to 1.3 V where it belongs. At 1/10 meter deflection, about as low as you can comfortably read, the drop is only 0.11V, so the voltage to the meter is 7% high. Since the meter deflection at this point is 1/10 of the scale, the needle position error is only 0.7% of full scale, and that is linear enough for me. I am finding more difference than that over the scale though, as the meter reads a bit low at the high end and high at the low end. It has been a while since I had one of these cameras apart, so I could certainly be mistaken, but I don't recall the presence of any meter adjustments other than the zeroing screw.

The mechanical one is very strange. Changing ranges on this meter only involves moving a mask in front of the CdS cell. There is nothing electrical, so a certain amount of light getting to the cell translates into a certain meter deflection, no matter what. The size of the mask cutout should be exactly what is needed to move between the high and low scales. I am finding though that the meter consistently underexposes on the high range and overexposes on the low range, when I choose light levels giving the same meter deflection. It's as if the hole in the mask is too big at the high range, and that makes no sense at all.:bang:

I would greatly appreciate any advice from anyone familiar with this camera. I don't think this is entirely to do with the new battery, as I never checked the meter thoroughly when using a Wein cell.

Cheers,
Dez

 

[Dez]

Looks like the "mechanical" problem may have been a false alarm- probably some bit of crud was keeping the shutter from closing all the way. It seems to be consistent now. There is still a bit of variation over the length of the scale. I guess the non-linearity of this setup is different from the non-linearity of the original arrangement with the mercury cell, and I'm always within a half-stop or so over the whole scale, so I guess I'll stop messing with it.

Cheers,
Dez

 

[Mackinaw]

I guess I'm fortunate to have a good supply of mercury batteries. Even though it's been more than 10 years since I bought them, they store well in the fridge. I'll keep using these as long as I can.

Jim B.

 

[goamules]

I would look at the battery adapters instead. There are several types that step down the voltage. If you buy one, you could use it in several cameras. I was about to do this for another camera. Have you ever thought of the MILLIONS of cameras and light meters that used the old 1.3v mercury cells that are out there? What a shame a knee-jerk reaction make them all obsolete at the stroke of a pen, when instituting a collection system or such could have worked. Anyway, here is one type: http://www.ebay.com/itm/130646083632?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649

 

[Dez]

The battery adapters work, but all they really do is to provide a Schottky diode in series with a smaller capacity cell, and for a very high price. In the case where a camera gives access to the wiring, it is probably a better solution to do the mod there. One concern for any of these changes- mercury cells were originally used because their voltage was very stable. Adding a diode introduces some variation, as the voltage drop is higher at high currents than lower, but is much less of an effect that adding a resistor. If the camera can be properly recalibrated to use a silver cell, that's the best bet, I think, but most apparently cannot.

Cheers,
Dez

 

[Nokton48]

When I had my 7S body, I used it with the CRIS adapter. I also have good mercury batteries, and did comparisons at low, medium, and bright light levels. By direct experience, results were absolutely identical.