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Forum: Pentax SLR Lens Discussion
05-20-2017, 04:55 AM
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DrawsACircle. Thanks again for your last suggestion. Looking at this information it is possible that the 7th (digital) connector (which was added to allow digital communication of lens parameters to the body) is possibly being used to pass information from the body to the lens which would then allow the lens to know whether the body was set to Liveview or normal autofocus. I will do a further strip-down next week and check the optocoupler, ribbon cable and connectors since these seem the most likely source of trouble.
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Forum: Pentax SLR Lens Discussion
05-16-2017, 08:53 AM
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I have seen comments in various forum about auto-focus problems with this lens at particular focal lengths.
My own one has a problem above 50mm focal length. The motor operates but not into focus. Beneath 50mm it operates correctly using auto-focus. It can however be focused manually over the full range of focal length and it does give a green LED when reaching focus. Interestingly in Liveview it does auto-focus correctly over the full Zoom range.
I have tried this lens on two different bodies with the same result so it does seem that the lens is the problem. I also have a second identical lens which does not have this problem when used on either of my camera bodies.
My lens was bought cheaply with this fault present and since it is still usable, albeit manually in the upper range or in Liveview, it would not make economic sense to send it to a repairer so I have decided to make a DIY attempt.
I have undertaken basic repairs and cleaning on Pentax lenses so was happy to partially dismantle it to see if anything obvious was amiss such as bad contacts etc., but nothing has been identified so far. I have attached some photographs in case anyone more knowledgeable on electronics can see any possible problems. (In order to relate the PCB photos to the lens mount the seven large round gold contact points sit against the lens contact springs and the two large rectangular ones are the DC power contacts).
My first thought was that the contacts in the lens barrel (sorry no photo) which feed back focal length position may have suffered damage but they seem OK. I have cleaned them and increased the spring pressure to ensure good contact in future. The ribbon cable connector connecting these contacts to the underside of the PCB was not checked because I was unsure how the locking system worked on this connector and did not wish to risk damage by forcing it. (shown in p5130077 beside my thumb). (Anyone actually know this particular connector and able to tell me how to unlock it? No guesses please.) The cable appears to be firmly attached but it remains an area that I wish to check, clean and re-fit to ensure good contact. The other ribbon cable between the manual focus ring and top side of the PCB (shown in P4240080a) was checked and contacts at both ends seem OK. After these checks the lens was re-assembled but the fault remains therefore I am about to have another go at it and would appreciate any advice.
In addition perhaps someone could explain (or provide a reference) concerning the technical operation of this auto-focus system. I assume the measurement of focus takes place in the camera body and it then feeds power through the two power contacts at the mount to drive the lens focus motor but some more detail would help to understand this process. In particular I am struggling to understand why the lens PCB is so complex with two IC modules etc., particularly since the auto-focus measurement is determined inside the camera.
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Forum: Troubleshooting and Beginner Help
04-24-2017, 04:49 PM
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Jeff Lopez. Thanks for your suggestion. I think the lens elements are OK but will try to check this further. The lens seems optically satisfactory with very sharp images and when operated in manual the focus system recognises a correctly focused image with the usual green LED. The problem is that in auto-focus the motor attempts to focus but drives the lens to an out of focus position. This only occurs above 50mm focal length. Beneath that it is usually OK.
I have now stripped the lens partially and the attached photo shows the pcb above the motor. There does not seem to be any obvious mechanical problem. Focus ring and zoom barrels move smoothly over the whole range. I intend removing the pcb and the casing ring on which it is mounted. This will allow access to the axially mounted "tracks" on the barrel and spring contacts which signal the zoom position back to the pcb. I need to check whether a bad contact in this system is causing my problem. Usually the electronic part of most mechanisms is reliable and it is the mechanical bits that fail so at this stage I do not suspect the pcb or it's components. The fact that this issue only occurs above 50mm also makes me suspect a zoom position feedback problem.
If anyone has had experience of a similar repair or some information about the principle of operation of the focus mechanics/electronics inside the lens their input would be welcome.
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