Does your mirror lens by chance have a non-conductive mount (black vs bare metal or with pins)? Lenses with non-conducting mounts on a K-50 will cause the green button not to work properly. If the lens does not have a conductive base a quick and dirty test would be to place a bit of metal foil over the "data" pin (near 6 o'clock on the body mount) and trying the green button.
Are you experiencing dark frames when using say DA lenses without aperture rings or A, F, or FA lenses set to "A"?
Does the fixed aperture mirror lens have exposure problems in Av mode?
The aperture control failure usually occurs with fully automatic lenses where the stop down actuator does not position properly to keep the iris at the proper opening. The lens stops down completely to say f22 rather than f8. Lens with aperture rings continue to work properly with the green button because it is the ring setting that stops the iris at the set opening not the camera body. Since there fixed aperture lenses have no stop down levers on them the failure of the stop down actuator is irrelevant.
And in the past no one who has worked on the solenoid and checked has reported the parts being magnetized - i.e. the filings from horse shoe core are not magnetized. You may cause more harm than good exposing electromagnetic components of your camera to strong degaussing fields. You may even demagnetize the permanent magnet of the solenoid which will cause it to fail too.
See also:
https://www.pentaxforums.com/forums/151-pentax-k-30-k-50/315725-dark-exposur...-me-first.html
Last edited by Not a Number; 07-10-2018 at 04:41 AM.
Reason: reorganization, add link