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Author Message   Brcamera Tinkerer Username: Brcamera Post Number: 243 Registered: 08-2010 Rating: N/A Votes: 0 Posted on Tuesday, February 12, 2013 - 10:16 am:    The optical encoder consists of a phototransistor and a LED. As the diaphragm of the lens closes, the optical encoder sends a signal to the CPU. A spinning disk passes and cuts off light between the phototransistor and the LED. When the count to the CPU is correct (matches the value that is needed for correct exposure) a signal is sent to the aperture magnet which releases and stops the lens diaphragm. So you can see that there are several points here that can cause problems: Dirty encoder, wiring between encoder and CPU, wiring between aperture magnet and CPU, aperture magnet sticking or its coil is open. When I was actively repairing these cameras (not really feasible for customers to pay me to service these anymore) I would look at the encoder signal with an O scope to check the pulses. Also, I would check resistance of the aperture magnet for an open circuit. Bill