How to adjust/calibrate the Canon F-1 meter?

[BHuij]

I have done diode modifications to my Olympus OM-1 and Canon FTb so I could use SR-44 batteries in them. The Olympus had a cam inside that I could adjust until the meter matched my "source of truth" spotmeter at all relevant EVs. The FTb had a set of 3 potentiometers under the top plate that served the same purpose. Both are now within 1/3 stop of my spot meter, and have produced several rolls with fantastic results relying on those meters.

I recently purchased my grail SLR, a Canon F-1n, in really fantastic condition. While I'm undecided on a diode modification there, and am currently just using 675 zinc air batteries, I find its meter doesn't agree very closely with my spot meter in most cases. My Google-fu has not turned up much of anything helpful. Anyone know what the procedure is for adjusting the meter on the F-1n (not the New F-1, to be crystal clear).

 

[lumoStoria]

If the deviation of the light meter is only caused by the voltage difference of the battery, why not just use SR44 with an MR-9 adapter? This would not require any changes/adjustments inside the camera. Also, our Nikon specialist, @Andreas Thaler , should know what to de.

 

[BHuij]

I should be more clear - I have not tried using a 1.55v battery in this camera yet, because I don't have a good way to reduce its voltage (no MR-9 adapter), so I have no reason to expect it would be accurate with an SR44. So far I have only used 1.4v 675 zinc air batteries in it, and with those, it still seems to recommend overexposure in the neighborhood of 1-2 stops compared to my spot meter, in standard open shade outdoor conditions where I do most of my shooting. I can usually get reasonably close by offsetting the camera's ASA setting, but the "error" isn't perfectly linear, so I'm hoping it has multiple adjustment points like the FTb.

 

[Laurent]

As far as I know the F1 meter is supposed to be calibrated at the factory, and the galvanometer/cds cell/resistances set was provided as a single unit. The potentiometer is supposed to allow calibration of the test circuit.

So I'm all is left is try to adjust the resistances?

Maybe a dumb idea: according to.my expérience on other devices (from cameraq to motor drives to electric scooters to bicycle speedometer) négative wires tend to go bad during years of use, with résistance increasing (I measured 4 ohms for 50cm of wire recently) so it may be worth trying new wires. Low risk and potential for fix.

 

[4season]

I found a service manual that you can freely browse here.

Of particular interest are sections 1.5 (disassembly of meter unit), and 2.11 (calibration).

Although I have performed some service on my own F-1, my meter seemed to be working OK so I didn't attempt to recalibrate it. Manual refers to a single potentiometer but does not illustrate it: Apparently contained in the same block as the CdS cell itself.

Be mindful of the little bits of clear celluloid, such as the wheel which provides the viewfinder shutter speed display, as they are easily flaked off and misplaced. Original adhesive used was "Diabond" which appears to have been a clear cement offering modest adhesion (but the latter may be due to age).

 

[Laurent]

I found a service manual that you can freely browse here.

Of particular interest are sections 1.5 (disassembly of meter unit), and 2.11 (calibration).

Although I have performed some service on my own F-1, my meter seemed to be working OK so I didn't attempt to recalibrate it. Manual refers to a single potentiometer but does not illustrate it: Apparently contained in the same block as the CdS cell itself.

Be mindful of the little bits of clear celluloid, such as the wheel which provides the viewfinder shutter speed display, as they are easily flaked off and misplaced. Original adhesive used was "Diabond" which appears to have been a clear cement offering modest adhesion (but the latter may be due to age).

Sorry, my bad... although the meter is a unit, it can indeed be calibrated, but it's via mechanical ways (chapter 2.11 in the repair manual) and normally a scribed line indicates the proper place for the galvanometer.

The electronic circuit can't be adjusted, save for the battery test circuit, which holds the only potentiometer. The image below is an excerpt from the Camera Craftsman of nov/dec 1974

Benoit Suaudeau's site, although factually incorrect about the meter calibration for 1.55V, shows this potentiometer:
Canon_F1_Old

 

[BHuij]

I translated that page with Google, it seems he's saying the potentiometer needs to be re-zeroed even with a zinc-air 1.4v cell, and once you get the zero right, he just assumes the rest is also right?

A little surprising to me that the FTb has such easily accessible controls for calibrating the meter (zero as well as 2 different EV levels all have their own potentiometers), but the F-1 can only have the meter truly adjusted at the factory. Still, I'll give this a shot and see how consistently I can get it to match my spot meter with a zinc air cell.

 

[BHuij]

Seems like the potentiometer is used to set zero in "battery test" mode with the camera set to ASA 100 and 1/2000th of a second, and then "eccentric screw A" on the meter can be used as a single point of adjustment. Guess I just have to hope that it gives me a correct reading at EV10, 12, and 15 at a single adjustment point.

 

[Laurent]

I translated that page with Google, it seems he's saying the potentiometer needs to be re-zeroed even with a zinc-air 1.4v cell, and once you get the zero right, he just assumes the rest is also right?

A little surprising to me that the FTb has such easily accessible controls for calibrating the meter (zero as well as 2 different EV levels all have their own potentiometers), but the F-1 can only have the meter truly adjusted at the factory. Still, I'll give this a shot and see how consistently I can get it to match my spot meter with a zinc air cell.

Yes, this is what he says, but Canon says otherwise. The potentiomer does not belong to the metering circuit.

My understanding is they adjusted the circuit at the factory, so that it worked properly out of the box. The final adjustment is done by adjusting the rods and galvanometer position (hence the scribed line, I think I pictured it in my thread on the "F one kenstein" camera)