To check the ASA dial calibration:
To check for proper calibration of the Bolex reflex series ASA dial:
1) Open ASA dial lock
2) Turn the lock tab full counterclockwise (CCW)
3) The red colored 18 FPS mark should be opposite (between) the letters S&A of the ASA marking.
4) Turn the ASA control full clockwise (CW)
5) The red 18 should be opposite the first A of ASA
If the above settings don't match, the outer dial may have moved relating to its correct position or the galvanometer's internal gears may have moved relative to each other. Here's how to check:
1) Orient the camera so you are sighting from the lens towards the ASA dial.
2) If you tilt the camera whilst looking at the dial, you can see slightly under the top of the dial.
3) In a direct line with the center of the battery cap and the center of the ASA dial, you will see a series of 'cogs'.
4) Almost, but not quite directly on center, you will see a pin between two cogs.
5) Note where the pin is and look on the top of the dial.
6) The pin should be aligned between the 20 and 25 ASA settings.
The outer ring is glued down. The inner ring is attached to the galvanometer's shaft. The shaft end opposite the dial has a small gear that turns another gear at a right angle. Sometimes if the glued outer dial's bond is broken, the outer dial's cogs can 'jump' the pin. If it only jumps 1 pin, one way or the other, you can compensate.
Example: If pin has jumped one cog to the right, it will now shows the pin split between the 16 and 20 ASA markings. It is now reading and calculating the ASA incorrectly and the true ASA will now be somewhat higher than that read off the dial. Not a problem as the difference is only 4 ASA.
However, when you rotate the dial to ASA 100, the same offset of the pin jumping 1 position is now off by 25 ASA. You can make a mental note of how far the offset is allow for it. Typically, you will be using the same film and once the offset is set, you can just place the ASA vs. FPS markings normally and the meter will read correct.
If the meter is off by more than a couple of cogs, or the ASA dial seems to jump around, the meter should to be overhauled and recalibrated. This involves removing the ASA dial. The center screw of the dial is cemented in place. On about half of the cameras I recalibrate, the dial screw snaps. Not sure why - rust perhaps. On the other half of the cameras, the screw comes out normally.
If the screw breaks, the corresponding shaft of the meter assembly needs to be replaced. That in turn requires a complete camera disassembly down to the internal meter and the reassembly and recalibration of the meter and dial assemblies. This is one of those repairs that only the experienced can do.