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Free at last - Pantel -- Sigma-Z 300mm mount dismount
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PostPosted: Sat Jun 22, 2013 8:52 am    Post subject: Free at last - Pantel -- Sigma-Z 300mm mount dismount Reply with quote
I have moved this discussion

http://forum.mflenses.com/sigma-z-300mm-f5-6-t59479.html

over here, since this epilogue is likely of interest only to those who enjoy working on MF lenses.

As the OP indicates, the lens arrived needing repair. It is a beautiful lens, BTW, virtually new and in better condition than the photos depict. Just as the seller had suggested, the aperture is sluggish (though not frozen). But it's only oil!! I had thought there might be a mechanical problem. Nope. It's only oil and easily fixed once the aperture is exposed. With the lens being in such pristine condition, I have to assume that at some point in the last (almost) forty years it was exposed to warmth too great.

Anyway, so far I am beaten. Looking at the mount photo in the auction ad, you can see the three Phillips screws which hold the mount together. As best I can tell, nobody had attempted to extract them prior to myself. I was able to remove only one of the three screws. It is perhaps one mm in diameter and roughly 5 mm in length. The one screw I was able to remove did not show evidence of thread lock or glue.

The other two screws are frozen in place! I have tried everything: WD-40, acetone, extreme torque. Nothing works! I don't know how to get the two screws out.

My alternative is to attempt going in from the front, but that's not an approach I relish on what is, after all, virtually a new lens.

So for now I'm stumped and beaten. Only remaining alternative I can think of is drilling out the two frozen screws. Boy, I'd surely like to avoid that! My best guess is that, during factory assembly years ago, the screws were overtorqued. Now, all these years later, they will not budge.

Such a nice lens. Such an intractable problem. Sad

Last edited by guardian on Thu Aug 08, 2013 10:47 am; edited 1 time in total


PostPosted: Sat Jun 22, 2013 10:50 am    Post subject: Reply with quote
Before you drill, hold the hot tip of a soldering iron on the screw head until it gets hot, then let the screw cool a bit and try to unscrew it again. If it is really stuck maybe try heating with the soldering iron then rapid cooling with a tiny drip of ice cold water - just a drop from a syringe. The expansion / contraction of the brass or steel screw will be different to the aluminium or plastic body part and should shock itself free.


PostPosted: Sat Jun 22, 2013 3:18 pm    Post subject: Reply with quote
Lloydy wrote:
Before you drill, hold the hot tip of a soldering iron on the screw head until it gets hot, then let the screw cool a bit and try to unscrew it again. If it is really stuck maybe try heating with the soldering iron then rapid cooling with a tiny drip of ice cold water - just a drop from a syringe. The expansion / contraction of the brass or steel screw will be different to the aluminium or plastic body part and should shock itself free.


Great idea! I'll try it. Thanks!

I've reconsidered my theory regarding factory over-torquing. With one screw out I can confirm 100% it is steel. A magnet attracts it. The mount itself is aluminium. So we have steel in contact with aluminium for forty years = galvanic corrosion! That is what I'm now guessing is holding these screws so darn tightly.


PostPosted: Thu Aug 08, 2013 3:01 am    Post subject: Reply with quote
Lloydy wrote:
Before you drill, hold the hot tip of a soldering iron on the screw head until it gets hot, then let the screw cool a bit and try to unscrew it again. If it is really stuck maybe try heating with the soldering iron then rapid cooling with a tiny drip of ice cold water - just a drop from a syringe. The expansion / contraction of the brass or steel screw will be different to the aluminium or plastic body part and should shock itself free.


I am offering herewith an update, but first:

A hearty and grateful "Thank You" to Lloydy! Much appreciated!!

Now, the story:

This thing really had me buffaloed. A beautiful, virtually new, lens . . but needing aperture cleaning. How to get it apart given the two frozen screws.

I followed the spirit, but not exactly the detail, of Lloydy's counsel. Actually I did try a soldering iron: not enough heat. Whereupon:

I went to my garage, retrieved and unholstered my propane torch. Removed rubber grip from lens. Remainder near to frozen screws was metal and glass.

I torched the first screw for at least ten seconds. It broke free. Ditto second screw. Nothing. Still tight.

Cooled lens. The heat had spread widely. I wanted a cool start for the next try.

The next application of (extreme) heat did the trick. The final screw came free. Remarkably I did not damage the lens or even the heads of the frozen screws.

For other repairers who might read this:

This is a 300mm Sigma-Z. It's not a bad lens at all. However:

I now believe Sigma had used locking compound on those screws. Once things came apart there was "stuff" I had to deal with, i.e., clean up. It's all good now, but I hate when they (in effect) glue lenses together. I think the extreme heat finally destroyed the compound. In addition, I think the screws were somewhat corroded. I had to get those suckers really hot; applied tip of the propane flame . . . hundreds of degrees. It was lucky the lens took the heat, but it is old enough to be made right and not to be crap (no plastic).

Anyway, I have removed the rear lens group. Cleaning the aperture with naphtha is next. I am a happy camper. Very Happy


PostPosted: Thu Aug 08, 2013 7:32 am    Post subject: Reply with quote
The propane torch sounds a bit extreme, but it worked! Great news. I suppose the paint they used on these old lenses was good stuff that would take a lot more heat to peel off?
Hope you get the lens fixed.


PostPosted: Thu Aug 08, 2013 10:43 am    Post subject: Reply with quote
Lloydy wrote:
The propane torch sounds a bit extreme, but it worked! Great news. I suppose the paint they used on these old lenses was good stuff that would take a lot more heat to peel off?
Hope you get the lens fixed.


Thanks again, Lloydy. The basic idea of employing heat was yours. And it worked.

As for paint, there is no paint. The metal portions of this lens are anodized aluminium, black in colour. I did anticipate damage but I was at my wit's end and went ahead anyway. But there was no damage visible. Only possible trouble: I might have overheated the helicoid grease a bit. But I first rotated the helicoid in a manner which moved all glass elements as far as possible from the mount I was torching. I also used moving air to cool the lens's innards between heatings. While the lens barrel became warm, it was not overheated I hope.

The mount became really, really, hot, though. Those screws, and especially the last one, were immensely recalcitrant.


PostPosted: Sun Aug 11, 2013 6:15 pm    Post subject: Reply with quote
Epilogue:

Here, once again, is a link to the original auction wherein the aperture problem was fully and honestly disclosed:

Click here to see on Ebay

The lens is, for all practical purposes, brand new. With the aperture now cleaned and functioning the lens has been reassembled and restored to service. It's a nice, pristine, 300mm lens with an m42 mount.

All is well that ends well. Smile


PostPosted: Sun Aug 11, 2013 10:41 pm    Post subject: Reply with quote
Wow, worth it for 99c. Well done. Now does it perform as well as you'd hoped?


PostPosted: Mon Aug 12, 2013 2:11 am    Post subject: Reply with quote
philslizzy wrote:
Wow, worth it for 99c. Well done. Now does it perform as well as you'd hoped?


Thanks. Yes, well that's another matter entirely, isn't it! While I like to search for bargains, buy, and repair lenses . . . actually taking photographs with them is, for me, not what you would call an "action item". Part of this is because, as a photographer, I have zero talent. I do enjoy the old MF lenses, but primarily just for their own sake. Perhaps this situation will change as I become older. But if I become too much older I'm gonna die for sure!! Laughing
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