Today I’ve got a very important message from my friend Heikki Repo (Director based in Tampere, Finland) for creating a sub-page related to the issue causing headaches.
A small hole on the lens mount that leaks light, esp. noticeable on the back-lit scenes mostly, would cause the partial flicker thus affecting the images undesirable and makes them almost useless!
The oscillating reflex-mirror (don’t confuse with the exposing focal-plane shutter!) will cut that stray-light when exposing every next frame, assuming the hole is on the right side, so as every next frame won’t be exposed by the light-leak when the shutter moves to the right, because of blocking the stray-light as it will be in-between with the exposing frame, but every other will be affected as the mirror moves to the left, thus causing the flicker phenomenon mentioned.
The above photos and video courtesy of Mr. Heikki Repo, Myrskykallio-Filmi
This is absolutely not related to the camera motor instability, because most of the ACL motors are precisely regulated by the electronic governor circuits (the later motors are piezo crystal-controlled closed-loop feedback circuits), as well as early ACL motors have hunting between poles because drive coils are not overlapped at all as stated by my friend Mark Gulbrandsen (Service technician based in Salt Lake City, UT).
The cause behind this issue is that ACLs have the 2-in-1 ports (e.g., a fixed C-mount thread with an adjacent outer robust mount adapter-thread on camera body) for accepting the lenses having the C-mount and robust CA-1 or TS/Arriflex mounts. Screwing the C-mount lenses requires removal of the robust mount adapters, thus the outer area remains unprotected to the light.
If, somehow, these holes are remained uncovered, then you are in trouble!