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03-24-2016, 04:07 AM   #1
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SMC Pentax 300mm F4. Broken!

Hi all,

I've got a SMC Pentax 300mm F4, but sadly it is broken .
the focus ring always turned very heavy. Now turns light, but does not focus anymore.

Is it still worth, fixing this lens?

Ideas appriciated...

thanks.

03-24-2016, 06:10 AM   #2
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the earlier lens designs are "fixable" with a good set of tools, good light, and lots of care. Likely you will find some broken part inside (perhaps just a spring). You will need to figure out how to fix the broken part and then get it all back together! Not for the feint of heart, but not rocket science either (unlike modern lenses). There are a handful of people around the country who would do a CLA of that lens for not too much money -- a repair would just be an incremental fee on top of that.

M
03-24-2016, 06:48 AM   #3
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Sounds like a dirty helical the forcing of which eventually resulted in the loosening of the screws attaching the outer focus ring. A standard CLA should resolve all these problems.
03-24-2016, 11:47 AM   #4
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QuoteOriginally posted by dcshooter Quote
Sounds like a dirty helical the forcing of which eventually resulted in the loosening of the screws attaching the outer focus ring. A standard CLA should resolve all these problems.
The front of this lens unscrews so the repair is not that involved (I think). Focus to 4 meters. Hold the focus ring in one hand. Grab the barrel between the lens hood and focus ring with the other hand, and turn counter-clockwise. The thread is fine-pitch, so turn many times. Then remove the front assembly. You can see the three screws that hold on the focus ring, deep in a recess just inside the ring. You'll need a JIS #00 screwdriver with a long shaft, about 40mm long, to get to these screws. They are probably not very tight right now.

Probably from here, the lens is similar to other K series lenses, only bigger.

03-24-2016, 12:05 PM   #5
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Problem is, without a helical cleaning/relube, the screws are apt to loosen again. That'll require removal of the mount and focus synchronizing rails and complete teardown of the helical assembly. Of course, all of those things are standard components of a CLA, so basically, the screw and stiffness repairs would be no additional work.

QuoteOriginally posted by Just1MoreDave Quote
The front of this lens unscrews so the repair is not that involved (I think). Focus to 4 meters. Hold the focus ring in one hand. Grab the barrel between the lens hood and focus ring with the other hand, and turn counter-clockwise. The thread is fine-pitch, so turn many times. Then remove the front assembly. You can see the three screws that hold on the focus ring, deep in a recess just inside the ring. You'll need a JIS #00 screwdriver with a long shaft, about 40mm long, to get to these screws. They are probably not very tight right now.

Probably from here, the lens is similar to other K series lenses, only bigger.
03-28-2016, 01:02 PM   #6
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Be aware that helical focussing probably involves a multistart thread. Before taking the lens apart, measure its length at infinity and or closest focus. It must be the same dimension after reassembly.
03-28-2016, 01:43 PM   #7
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Simply marking the thread exit point and counting the number of turns (then fine tuning infinity before tightening the screws on reassembly) is a much easier and more accurate way of maintaining calibration than trying to get accurate lengths with a caliper, particularly on such a long lens.
03-28-2016, 01:51 PM   #8
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You guys know way to much about this.
You take the mystery out of everything.

I always thought there were little elves in there moving the lens forward and backward to focus.

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