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Pentacon 300mm f/4 disassembly/repair
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PostPosted: Wed Jun 19, 2013 6:17 pm    Post subject: Pentacon 300mm f/4 disassembly/repair Reply with quote

Hi all, I recently obtained a Pentacon 4/300 in M42 mount. It had a little bit of oil on the aperture blades so I had to disassemble it to get at the blades to clean them. However, I couldn't find anywhere detailing how this could be done so I decided I would make a guide for those who seek to disassemble this lens after me. I had previous experience disassembling the Pentacon 135mm f2.8 (15 blade, preset version) before so it wasn't all that daunting, and was remarkably similar.

This is the lens in question.

Now, in my case the oil could be seen through the front and rear elements so I couldn't simply access the blades through the front, however this is entirely possible if there is just something on the front of the blades.
The image below shows the front element. Simply use some form of calipers or even a compass to unscrew the metal casing holding the front element in. There will be a similar screw-in casing for the next element that can be unscrewed in the same way (Note: the second element is very large and heavy and will require caution when removing it).
These elements can be cleaned if there is any dust. Once the front two elements are removed you can close the aperture blades down and they can be cleaned with relative ease.

However, if more repair is needed or the aperture needs to be completely disassembled and cleaned then follow these instructions.
(I do not recommend disassembling the aperture as I did this and took a VERY long time reassembling all of the 19 aperture blades after cleaning them)

Before doing anything, focus the lens at infinity and open the aperture fully. This makes for easier disassembly and protects the aperture blades during the process.

The first step is to unscrew the retaining ring for the M42 mount. The mount may be different depending on what camera you use the lens on. It unscrews in the normal anti-clockwise direction.

Once this is done the conical mount is free and can be lifted off. There is a little rut in the mount that sits over a screw with the purpose of stopping the mount from rotating.

Once the conical mount is lifted off there will be 5 screws visible. The one raised screw and then 4 recessed screws. Mark the positioning of the raised screw as it should be placed back in the same position after reassembly. This is not completely necessary but will ensure that the lens is rotated the right way round when mounted on a camera.

Unscrew the 4 recessed screws; you can leave the raised screw in because it doesn't actually hold anything down.

Then simply lift the ring off, it should be pretty easy and should look as below.

The next step is to lift the next section of the lens off as shown below. This will feel harder to lift off and will feel scratchy because there are two springs at opposite sides of the lens pushing against this section. These springs push two pinions into the anti-rotation groove on the main focusing helicoid. You must lift this section slowly to stop these springs from flying out, as they are very small and will be hard to find again. Notice in the picture below how the orange arrow is in line with the infinity symbol and there is a small vertical groove on the part beneath. Remember this for reassembly.

You will now be able to see both the springs as below.

Turn the section anti-clockwise SLIGHTLY as shown below to force the pinions and spring out. Remove them and keep them safe. Look through the hole that the pinions came from and rotate the focus ring slightly back and forth, you should be able to see the groove the pinions are meant to be in. Remember this for reassembly.

Now comes the trickiest and most important part of the disassembly. It's not too hard if you follow exactly but if you don't you risk losing infinity focussing.

Rotate the focus ring clockwise SLOWLY and it should begin screwing off from the main lens body but DO NOT unscrew it completely before reading on.

Now, before you unscrew the focus ring from the helicoid, view the lens from above as shown below and SLOWLY unscrew the focus ring from the helicoid and stop the very second you feel it come free of the helicoid. MARK the position it becomes free VERY CARFEULLY (make a mark both on the focus ring section and the rear element casing as shown below at the bottom of the picture. I actually used one of the grooves on the rear element casing as my marker there, but mark it as there are two grooves).
The marking on the focus ring section is actually out of focus but you can see the white scratch I made, as well as the scratch in the groove of the rear element casing.

The sections will come apart as shown below.

At this stage you may want to unscrew the casing retaining the rear element (being carefully not to lose track of your marking scratch) but this is only necessary if you wish to clean in between the two rear elements.

On the main body of the lens, below the helicoid, you will see a TINY screw. Unscrew it with an equally tiny screwdriver, but not completely as it will be very hard to get back in. If you decide to take it out, keep it safe as it is very small. The second image below shows how far I unscrewed it.

Once the screw is taken out a little, the cap it is in should be easily unscrewed from the main body of the lens and removed and a very greasy section and another screw should be revealed. This screw couples the aperture ring to the actual iris of the lens allowing the turning of the ring to open and close the blades. This screw should be holding the section below it down, as this section is being pushed up by a spring. Simply push down on the spring loaded section and unscrew the screw.

Lift the aperture ring off and another section is revealed with yet another screw holding down the spring loaded section below it. Remove the screw while pushing the section below downwards.

Now remove this section and there will be a lot of grease and a large wire which acts as the spring which pushed the above sections upwards.
You can remove the grease from all of the greasy parts and reapply new grease if you wish (if you have the right grease) or like me you can just leave it there and reassemble with the original grease.

Amidst all of the grease there is yet another TINY screw as shown below. Simply unscrew this (again, without taking it out completely) and the two sections should unscrew until they are apart as shown in the second photo below.

You now have the front two elements in one sections and the aperture mechanism in the other. Below are a few photos of the aperture mechanism and how to control it for cleaning.

Use small screw driver to put in the small hole and rotate it round, closing the aperture blades for cleaning.

While not advised unless experienced, if you wish to disassemble the aperture blades for cleaning then you simply remove the unclosed metal circle that sits above the aperture blades, hold the aperture mechanism in the lens body and turn upside down and remove slowly. The whole section is removed and the aperture blades can be taken off and cleaning thoroughly. I also advise cleaning the moving part of the aperture mechanism as a lot of oil can gather here. You can clean with isopropyl alcohol or, as I chose to do, simply use mild soap and warm water, being very careful and drying thoroughly.

Once everything is apart you can degrease, clean and dry everything thoroughly if necessary before reassembly. Just remember to regrease everything you degreased for smooth operation and protection!

Reassembly is simply the above steps in reverse, however some points are worth mentioning.

Keep the aperture open.

Ensure the orange circle is above 22 on the aperture ring as shown below. This is the preset aperture marker and since you opened it with it set at 22, you should keep it at 22.

If you have any questions just post below!

Last edited by lholmes549 on Wed Jun 19, 2013 8:34 pm; edited 1 time in total

PostPosted: Wed Jun 19, 2013 6:19 pm    Post subject: Reply with quote

Many thanks for this great guide!

PostPosted: Wed Jun 19, 2013 7:52 pm    Post subject: Reply with quote

Excellent guide.

I regret selling mine.

PostPosted: Wed Jun 19, 2013 7:54 pm    Post subject: Reply with quote


Thank you very much for documenting and posting the hole process, very useful indeed!



PostPosted: Wed Jun 19, 2013 8:38 pm    Post subject: Reply with quote

Thanks for the positive feedback guys!
I just changed on of the photos as it was repeated instead of the photo described.
It was quite annoying that there was no guide on the web anywhere for such a great lens (although it made it a bit more fun), so I thought I'd give it a go!

PostPosted: Wed Jun 19, 2013 9:47 pm    Post subject: Reply with quote

Excellent job, that's a great tutorial.

PostPosted: Thu Jun 20, 2013 7:18 am    Post subject: Reply with quote

beautiful lens and excellent job

PostPosted: Tue Aug 02, 2016 7:21 pm    Post subject: Reply with quote

The blades in this lens are supposed to be a bit oily. I don’t know why you had to clean them.

PostPosted: Wed Aug 03, 2016 10:25 am    Post subject: Reply with quote

Eugen Mezei wrote:
The blades in this lens are supposed to be a bit oily. I don’t know why you had to clean them.

I confirm (by personal experience) what Eugen says.
I have the same lens, purchased some years ago at a flea market, and it had dry blades when I got it. It soon started to develop an unusual iris problems, as the blades were bulging inside with a "pop" while closing the iris, and bulging outside while reopening. After a few weeks of use this led to some iris blades displacement that forced me to disassemble the lens.
A thorough cleaning of the blades gave only a few weeks of good working, before the problem develops again.
Finally I tried the opposite way, oiling the blades with some gun oil and then cleaning them with a clean and lint-free towel in order to leave only a very thin oil film on them. This happened more than five years ago and the problem never reappeared since.
Actually I didn't know this is how it was supposed to be. I thought my lens' blades had some strange problem to need to be oily to work fine... Rolling Eyes

PostPosted: Wed Aug 03, 2016 12:45 pm    Post subject: Reply with quote

No, diaphragm and shutter blades should work with absolutely no lubrication! In a shutter or an automatic diaphragm, the blades need to move very quickly in times as short as one millisecond. It may seem strange, but due to viscosity, any oil film introduces a tremendous dynamic friction, preventing the blades to slide quickly one over the other.

In a manual iris, the viscous friction of an oil film is not so critical, but inevitably the oil dries out and becomes a glue with time, preventing the movement of the blades. So, even in a manual diaphragm, lubricating the blades is not a good idea. If the blades do not slide well, look for the problem elsewhere: rust, dirt, burrs, deformation of blades, etc.

Below a photo of the diaphragm blades of a Meyer Telemegor 400mm F5.5, which has a diaphragm similar to the Pentacon 300mm lens. The Telemegor's diaphragm blades have worked for over 50 years without any lubrication. The marks you see are due to blades rubbing one against the other.

Last edited by Gerald on Wed Aug 03, 2016 2:37 pm; edited 1 time in total

PostPosted: Wed Aug 03, 2016 1:35 pm    Post subject: Reply with quote

Nice, thx

PostPosted: Wed Aug 03, 2016 4:41 pm    Post subject: Reply with quote

So I was probably correct suspecting some other problem at the blades of my Pentacon 300, and took the wrong way by oiling them... maybe, but since it's still working perfectly after 5 years, I'm not going to open it again to look for the actual cause of my "popping blades" issue. At least, not yet Wink
It would be interesting to know where does Eugen information comes from. Question

PostPosted: Wed Aug 03, 2016 10:24 pm    Post subject: Reply with quote

Both of my Pentacon's that share this design, 135 and 200, ( actually I have 3 135's ) have all at some time or another had the blades jump out, and it's been oil every time. My everyday user 135 which is half Pentacon and half Meyer Optic has been dismantled and cleaned more than once. I shall do it properly one of these days. Rolling Eyes

PostPosted: Thu Aug 04, 2016 8:23 am    Post subject: Reply with quote

I disassembled and cleaned the aperture as there was some sticking. Whether you relubricate is up to you but mine worked fine with no lubricant.

Also yes this is a very similar process to the pentagon 135/2.8!

PostPosted: Wed Oct 05, 2016 9:04 pm    Post subject: Re: Pentacon 300mm f/4 disassembly/repair Reply with quote


Thank you for posting this, that's very helpful. I just got this lens last weekend, and have to fix the aperture mechanism. Actually, I have a couple of old CZJ lenses that have the same problem - the aperture ring just spins without opening or closing the aperture. I took the 180mm Sonnar apart, but couldn't really figure out how the mechanism works. Seemed like it is closed by a spring, and opened when you turn the aperture ring, but I am not sure... I would be grateful if you could provide some advice.

thank you