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05-16-2017, 08:53 AM - 1 Like   #1
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PENTAX DA 18-135mm ED AL IF DC WR Zoom Lens Focus problems

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.

Attached Images
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05-16-2017, 10:14 AM - 1 Like   #2
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If the lens is able to focus using liveview and not when using the optical viewfinder, the root cause is burried in the difference in operating the lens when using LV/OVF.
Have you checked the optocoupler (no dust in the encoder disc/optocoupler gap) at 8 o'clock in picture #1?
That's usually to provide position/speed feedback to the controller circuit, Could be used when using the OVF - the lens might be doing some of the work the camera does when using LV.
Could be a bad connection in the small ribbon cable connector - flip the black locking mechanism using a nail/small screw driver.
05-16-2017, 10:41 AM   #3
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Usually that ribbon cable is held by the clip (very tiny, like mentioned, need to use finger nails to open). If that ribbon is not expensive you can try replacing it.

I work on computers, in some of the older Dell laptops this ribbon cable connects to the HDD and my laptop was acting weird. Replaced the cable and it's all good.

Hopefully it's a simple fix.
05-16-2017, 11:24 AM   #4
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I have been using this lens with a newly acquired K-3 with excellent results throughout from 18-135.

05-17-2017, 02:05 AM   #5
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Many thanks for the replies to my post.

DrawsACircle. Your suggestion was very helpful and perhaps I can provide some further information about the optocoupler. The disc on top of the optocoupler does not rotate when the manual focus ring is operated and while it is within the operational range of the manual focus ring. When the manual focus ring is moved beyond itís operational range at either end of that range the disc rotates. I assume it is part of the manual focus over-ride system and perhaps if itís signal is corrupted in some way it would affect the auto-focus function. Do you have a view on this? Do you have any advice on the best method of checking/cleaning the gap and can you advise what type of optocoupler this is (looking at component catalogues optocouplers all seem encapsulated items without a rotating disc) ?

Suraswami. Thanks for your helpful input. I will check continuity on the ribbon tracks and open the connector on the PCB and clean/re-seat the cable.

I am still puzzled that Liveview operates without problem while the view finder system does not. I appreciate it is a different system using different measuring components but both systems control the lens through the same two power contacts. As far as I can determine the other six contacts provide aperture and lens focal length information from the lens to the camera body.

Any further suggestions for identifying and resolving this problem would be much appreciated.
05-17-2017, 06:58 AM   #6
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It seems strange if the disc is not rotating when the lens is trying to focus (motor running), but since I don't know how it's supposed to operate, it might be how it's meant to be. Could be the disc has become loose.
I have the lens myself, but since it's still under warranty, I do not want to open it to check.
I would use a dust blower to clean the optocoupler, just be carefull not to destroy anything.
05-19-2017, 02:13 AM   #7
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DrawsACircle. Thanks again for your input. I will try to understand the whole system a bit better before trying a further strip down. In particular I still don't understand how the camera signals the lens that it is in Liveview mode (when it works correctly) and how the lens knows it is in normal autofocus mode (and has the fault). I suppose that sort of research it is all part of the challenge in taking on a repair project like this!
05-19-2017, 07:55 AM   #8
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There might be some usefull information here: K-Mount digital pin protocol, power supply - PentaxForums.com

05-20-2017, 04:55 AM - 1 Like   #9
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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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