The photos are of an already disassembled F3P, but there should be no significant deviations from the standard F3.
Unfortunately, overall it is not as easy as hoped.
My F3 has thin shims inserted under the bayonet screws. You have to be careful of their position when removing the bayonet ring.
Spring
The ring with the aperture coupling hangs on a cable that goes inside the camera.
The cable runs over a roller.
The cable is connected to this gear, which is presumably spring-loaded, and is connected to the
FRE element (Functional Resistance Element) to the left below the rewind crank (not visible here).
The aperture and ASA settings are transmitted to the camera's circuitry via the FRE contacts.
Conclusion
- If dirt has accumulated under the aperture coupling ring, preventing it from rotating, you can remove it.
- Be careful not to let the cable pulley come off the roller.
- If the problem is with the internal gear that holds the cable, the F3 will need to be disassembled.
- Never spray oil or WD-40 without visibility, I only say it for the sake of completeness
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All information provided without guarantee and use at your own risk.