| Author | Message | ||
     Glenn Tinkerer Username: Glenn Post Number: 1018 Registered: 07-2006 Rating: N/A |
Thanks Donnie; 1983 is a bloody long way back to remember specs and as the T90 was the only professional T Series camera produced, the T5/50, T70 and T80 thankfully never entered my equipment cases. John, You can forget about trying to release the lens via the rear of the mirror box, the jam is not caused by anything reachable from the rear of the lens. The 'ee pin' is not the location pin and the location pin is a solid, screwed in fitment on Canon produced lenses. Given what you have stated about how the lens is actually now located on the body, it is simple to state why a Breech Ring FD lens has been seated on the body mount in this way. Basically it is failure of one small mechanism, general wear and tear and poor initial construction tolerances. A quick rundown of the FD mount in its Breech form will explain all. When one looks at the chromed mount ring on the body, three segmental 'lugs' can be seen located at 12, 4 and 8 o'clock. The segment at 12 o'clock is longer than the other two (which are of equal length) and has a slot milled in it (dead at 12 o'clock). It is into this slot that the lens barrel location pin fits, orientating the lens and stopping the barrel rotating during focusing etc. If one now examines the rear of the Breech FD lens, it can be seen that the rear face of the rotatable ring has three female cutouts that correspond in shape and size to the 'lugs' on the male body mount. With a correctly working and unworn lens off the camera, one will find that the breech ring is locked in a position that allows the lens to be lifted on and off the body mount. It will also be noticed that the locator pin can be seen plumb centre of the long (12 o'clock) breech ring cutout. So it can be seen that if one tries to mount a correctly working lens radially displaced on the body mount, it is impossible to get the lens to seat flat on the body. Two things prevent this - the location pin has no slot to go into and the small cutout in the rear face of the ring will not fit over the large body 'lug' at 12 o'clock. It is infact possible to to mount the lens and tighten the breech ring if two things occur on the lens. As I have stated, the breech ring is held in the mount/unmount position by an internal latch that is freed as the rear of the lens locates against the front face of the body mount. If this latch fails the ring will be free to rotate between the fully open and fully closed stops - this situation should not lead to a fully mounted and jammed lens if (and it is a big if) the location pin is not worn or the lens not manufactured to poor and thus sloppy tolerances, because the location pin comes up against the front face of the body mount and the lens cannot fully seat. Unfortunately if the pin is worn or there is slop in the mount on the lens, the pin can slip off the front face of the body mount and slide into one of the sectors that were machined out of the body mount to produce the lugs. This explains why the lens barrel can be twisted back and forth by 1/2" or so. The failed breech ring latch allowed the ring to be in such a position to slip over the male mount 'lugs' even though the barrel was radially displaced. Unfortunately the faulty ring latch is now preventing the breech ring being turned back far enough to free the lens so that it can be removed. The only thing is to mount the body solidly on the edge of a bench, hold lens barrel in one hand and breech ring in other hand, then try rotating both back and forth ie jiggle them about, not very technical but you have nothing to loose. I do not hold much hope, but something might free up or give. |