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07-11-2013, 02:36 PM   #1
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Repairing Pentax M 28mm F2.8

HI

I bought a Pentax 28mm F2.8 off ebay slightly damaged (filter thread dented) but the distance scale ring is also loose and it feels like some grit has got under it also. It was so cheap it wasn't even worth sending back so I figured I will have a go at repairing it.

Does anyone know of any instructions available on how to take it apart? I searched online (youtube, google etc.) and couldn't find any. I did try taking the screws off the lens mount but it seems they are very tightly screwed on.

Thanks for any suggestions you can give

Joe

(P.S. hope this is in the right forum, wasn't too sure where to ask)

07-11-2013, 02:55 PM   #2
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Well, if the the distance scale ring is the one that bothers you (never heard of that problem in M-series lenses), I think access from both sides is possible in my broken M28 copy, but I could only open it from the rear, because there was a lens hood stuck on its filter ring also. I think your best bet would be to send the lens to a jeweler or a lens technician, to straighten that dent out. It should go with a couple of clever hammerings.
Sorry, I can't remember exactly how one is supposed to remove the distance scale ring. But if the rear screws must be removed, be careful that they are EASY to strip. Do use a proper screwdriver (correct head - JIS and size - no idea, #00?), and also apply enough pressure on the screw, so you don't strip its head.
07-11-2013, 04:11 PM - 1 Like   #3
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The ring with the focus distance scale is the actual focusing ring. There are three screws that hold it in place. To access the screws, you first need to remove the dress/vanity ring (the ring that has "smc Pentax-M 1:2.8 28mm" etc,) written on it. However, this will be very difficult to do in your case as the dent on the filter thread will prevent the ring from unscrewing. As the poster above has said, you'll need to get the dent straightened out first, without destroying the thread. Normally, the dress ring can be removed using a rubber bung.

Trying to straighten out the filter thread ring is nearly impossible without causing damage to the actual thread. If you're not worried about the external looks of the lens and just want to use it, you can use a small power tool such as Dremel, to cut out the affected area. After completion of the repair, you can always attach a filter to the lens and the cutout will be less noticeable. You can also go further by filling the affected area with epoxy or putty, then sand and paint.

There are few tricks to removing a stuck screw. The first rule is to use a CORRECTLY fitting driver and never use more force than necessary. If you feel the head starting to strip, STOP. Secondly, you can try dissolving the thread lock glue with acetone. Use a cotton swab to carefully seep the solvent into the thread, wait a few minutes and then try unscrewing. Thirdly, use some heat to weaken the glue. You can use either a soldering iron or a small gas blow torch for this purpose. BE VERY CAREFUL not to get the solvent or heat anywhere near plastic parts. For instance, the rear lens baffle is made from plastic, so you'll want to remove this part first before attempting to remove the screws holdling the lens mounting flange. Another WARNING. Acetone is extremely flammable. Do not use acetone when you have any heat source, such as soldering iron or source of open flame near by.

***DISCLAIMER***
I will not be responsible for any damage or injury that may occur by you attempting to follow the above instructions.

Last edited by tom1803; 07-11-2013 at 04:40 PM.
07-11-2013, 06:06 PM   #4
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I think the smaller M primes are of much the same basic construction. There are at least a couple of practical guides out there for M 50mm lenses which I have used as instruction to disassemble the rear portion of a M 28/2.8 (the aperture spring had come off ifs retainer pin). I've unscrewed the front decorative ring of the lens and the insides look consistent with the guides. One day in some free time I need to re-calibrate the focus on my copy - the loose spring and non-calibrated infinity mark suggests my lens was used by a Canon person at some stage of its life.

See here: Pentax 50mm repair 1

and here:

Depending how damaged the filter ring is, you may not get the decorative front ring off (although if the thread is only very lightly damaged, running the ring off may just be enough to re-cut and re-align the thread).

Good luck. It will be a good learning experience if nothing else.

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