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02-23-2017, 05:41 AM   #1
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Removing glued trim / retaining ring on Prakticar zoom lens

I've acquired a few lenses that were parts of bigger lots I bought, and it's about time I sell some of them. Before doing so, I'm dealing with any minor maintenance issues.

One of the lenses is this Prakticar Pentacon 70-210 f4.5-5.6. It's in very good shape except that the inside of the front element has the first signs of light fungus around the edge. I've been trying to get the trim / retaining ring off using a rubber ring remover, but it won't budge. Looking carefully, I think it has been glued into place.

My question is, what solvent can I use to try and loosen the glue without damaging the ring and the printing on it?

Financially, the lens isn't worth the effort, but I'd like to have a go just to build on my lens repair experience.

Thanks in advance

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02-23-2017, 06:54 AM   #2
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QuoteOriginally posted by dcshooter Quote
First step is is to look closely at the base of the front barrel to see if there are any set screws. On some pentacons, the hood and trim ring are one piece that unscrew from the bottom.

If you see no set screws, and presuming the ring is engraved metal, I would recommend using a drop or two of methyl ethyl ketone. It is available at hardware stores in the U.S. I am not sure about U.K. availability, so you might need to check around. Wear gloves and use it in a well ventilated space. If the lettering smears while using it, clean it with MEK or acetone, and if you need to refill, use a Markall Paintstik.
Thanks yet again, Brian - very helpful indeed

Yes, I'd checked for set screws... none to be found even with the barrel fully extended.

I'll see if I can find some MEK or equivalent...
02-23-2017, 08:21 AM   #3
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Just because there aren't any set screws doesn't mean the whole front barrel doesn't unthread.

I've disassembled lenses of this type, but I can't recall if the front barrel on yours is just threaded on tight. Some are.

With the focusing set to minimum (to get the barrel fully extended) take a firm grasp of the front barrel and see if it comes unscrewed. Don't use superhuman force if nothing comes undone. Be sensible.

But you may just find the front unthreads and gives you access to that front element.
02-23-2017, 08:22 AM   #4
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QuoteOriginally posted by dcshooter Quote
a drop or two of methyl ethyl ketone.
Nail varnish remover (=acetone diluted somewhat with water) is easest and cheapest to get, and works fine on most thread locker. 50p a bottle from that discount chemists. Use eg a small screwdriver to dip in, pick up and place drops around the thread, leave for a while to soak in.
I do use it for cleaning as well, good for that resinous residue left by deteriorated helicoid grease. Being diluted means its less likely to start taking off paint etc but pure acetone is better for lens elements.

02-23-2017, 02:37 PM   #5
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QuoteOriginally posted by dcshooter Quote
Once you use MEK, you will never go back. It works about 100x better than acetone-based solvents on most threadlockers. It dissolves better and evaporates more slowly, meaning you don't have to keep applying. I do like acetone for cleaning lens elements, though.

The best thing to clean helicoids is a heated ultrasonic cleaning bath in a heavy duty degreaser, which will remove gunk that you didn't even realize was still in the threads. My preferred one to use is Krud Kutter, although I'm not sure if that is sold in the UK. Avoid the temptation to use this method on exterior anodized pieces, though, since it can fade or discolor them. 2nd best is hot soapy water with a toothbrush. While slvent son their own can do a good job of removing the oil fraciton of the grease, they will leave behind the clay, along with any solid particles that have made their way into the threads.

One other thing to remember is to never leave parts that can rust soaking in acetone. Even "100%" acetone will absorb moisture from the air and cause corrosion if you leave parts in it too long.
Thanks for this, Brian. I'm a sponge for all the information I can get right now.

I've used the soapy water and toothbrush method on the helicoids I've done in the last week or so, and it's worked really well. I had to use cocktail sticks to get some of the hardened grease out of the threads, but otherwise, no problem.

Re acetone, how safe is it with the glass and coatings? I've also heard of white vinegar being used... does that work well?
02-23-2017, 04:15 PM   #6
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I can't really help on the disassembly issue, but I can say for sure that this Prakticar is from the late Cosina made series. This lens looks pretty equal to the Cosina 70-210/4.0-5.6 which appears under dozens of names (e.g. Miranda). So probably someone posted something somewhere in the internet....
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