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[OM] Locked diaphragm on 75-150

Subject: [OM] Locked diaphragm on 75-150
From: Chuck Norcutt <norcutt@xxxxxxxxxxxxx>
Date: Sun, 27 Feb 2000 12:02:59 -0500
Tom Hoefer said:
I just bought a beautiful 75-150 Zuiko on ebay, and darned if it hasn't
=
got a locked diaphragm. No problem with the seller refunding, but I'd =
like to fix it if I could (closet engineer!). Both the depth of field =
button and the diaphragm actuator arm are locked. The aperture ring =
turns freely. Any thoughts on how to fix this? What happens if I pull =
out the three screws holding the lens mounting ring ... a nightmare? Any
=
idea of where I could see a blueprint of the lens to help me?
-------------------------------------

I think I'd return it unless you bought it for a song and just have to
have a peek inside.
If you insist, however, I'll quote you from Book 2, page 89 of Tomosy's
Camera Maintenance & Repair concerning the Zuiko 75-150 f/4.

"...If the diaphragm mechanism needs work, you can access it from the
rear without taking the front apart (but you can't pull it out).  After
removing the bayonet mount (just three screws), remove the aperture
indexing tab and the setting and activating levers.  All threee of these
are fastened to the rim of the apreture ring by two screws.

Pull off the rubber grip from the zoom ring and you uncover eight holes
in the ring.  Turn the ring until you see four screws through the
holes.  Remove those four screws, and the rear optical group will come
sliding out.  Mark its exact position first; otherwise, you will have to
readjust it.  Now you can see the diaphragm mechanism and can do some
minor work on it without pulling it out."

I'll stop there since actually getting the diaphragm mechanism out
requires removing the front group, cover sleeve, dealing with helical
threads, the pulley mechanism, removing zoom rollers, you need an
appropriate spanner wrench, etc, etc.  If you don't understand what
you're doing here you will likely get it apart but never again back
together such that it works and focuses properly.  Best send it to
somebody or at least buy the book I've mentioned which will teach you
the general caveats about working with lenses along with the specific
details of this lens. 

Chuck Norcutt
Woburn, Massachusetts, USA

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