22 Sep 2014

Mamiya C33 TLR Repair - Part 3 (Sekor 80mm f2.8 Twin Lens)

In the last part of the 3-partite re-commissioning journey of my Mamiya C33 TLR, I work on the Sekor 80mm f2.8 lens in Seikosha shutter. Is this the dreaded silver-fronted one that every repairs person hates? I do not know. You tell me.

The problem was simple: aperture was completely jammed in place. The lever wouldn't even budge the slightest. I did not expect this to be oil on blades because it was completely frozen in place. I expected another mechanical problem, dirt in gears or something, so on to disassembly.

First of course you need to take the lens off the main plate. Using a lens spanner or a vernier caliper, turn the rear holding ring to unscrew it and pull the lens out. You cannot simply remove the bottom taking lens on its own. You have to start with the four screws around the viewing lens in order to free the sync-port which is connected to the shutter mechanism in the viewing lens.

Taking lens assembly with sync port

Then remove the yellow ring from the back to reveal the three little screws under it. These screws hold the external aperture arm. Get that out of the way.

The peripheral screws hold the whole assembly together. Take those out and brace yourself for disaster. Pull the front and back apart and LO! there are your shutter blades, all over the place!

These springs will also pop on the sides. There are four of them. If you opened the mechanism from the back like I did, do not worry about them as they will not go anywhere.

There are five blades in the shutter. I read somewhere that there is a fifth retaining blade (half-blade) but I only found five in mine. Maybe the sixth was already stuck to one of them? Maybe different models?

There was nothing to fix in the shutter mechanism or blades as it worked fine. So I left them aside on a white paper along with the front part of the mechanism, and headed towards the rear part, which is the aperture. This little arm below is what controls the aperture but it was completely jammed in place.

On the inside, this is the aperture. Remove the screws holding the black thing from the metal housing.

On the other side of this ring, the aperture blades were completely stuck as if they were glued. The dreaded oil was gummed up

To clean that up, I soaked the blades in isopropyl alcohol, and used cotton q-tips to clean them off, making sure they are layed flat on a hard surface to avoid irreversible accidents. These blades are extremely thin layers of metal (iron, they can be picked up with a magnet) and bending them the slightest is not an option.

This is the other wide of the aperture retaining ring. This is the part which turns when you slide the aperture arm. This stays in the metal housing, no need to remove it.

Putting the blades back in place is a very tricky business and you should get it quite right. Layer the leafs/blades as they should be, carefully install the upper retaining ring or tracks ring, make sure everything is in place, be very careful, double check, install the screws.

It should be very smooth with almost no resistance. Do not worry about it. Resistance will be back when you assemble the whole thing. But before you can put the front and rear part together, you need to put the shutter blades in place.

Putting the shutter blades in place is similar to assembling the aperture, but please note the side to which you put blades. I took the following pictures but I think I reversed the direction of the blades afterwards. Please note the pictures may be wrong, so I share them for illustration purposes only. Also for showing off.

The last part is to put the front and back parts of the entire lens together. You need to make sure the shutter blades stay in place and slide safely into the track ring as you close them in together. Keep the front part with the shutter blades flat on the table and mount the rear part upside down on top of it. Use a screwdriver or something to push the springs into the mechanism before you close it well, and fit the four screws.

This finishes the difficult part of the job and all that is left is to put the lens back together. The sync type (X and M) arm/ring is the trickier of the two. See if the following pictures can help you figure out how it should go. See that part part which bends inside the assembly and goes just behind the shutter release? You'll need tweezers or a screwdriver or anything similar to guide a small metal 'petal' into that bending part. This gives this arm its "click" when you switch the sync type.

See the sync arrow is pointing towards X. This is the best indicator as to how this ring should go.

Aperture arm, the last screws in the assembly and then the yellow ring back in place.

With the aperture arm and the yellow ring back in place, all screws fitted, the whole assembly is ready to do onto the main twin lens plate.

Fit the taking lens first. Very easy to orient it according to the little hole in the back. It is the same stud that holds the big yellow ring above. Simply fit correctly and screw on the rear ring.

Then fit the sync port. Do not forget the metal plate that connects the lower rear screw to the sync-electrode.

The viewing lens should be fine long as you keep the shims as they are.

Align the upper stud with the slit, and screw on the rear ring.

If everything goes well, you have a working twin lens.

In fact, the calibration process that I talked about in the last post was done after this aperture cleanup was finished. I wanted to make sure the two lenses were perfectly aligned. It is difficult to align them wrong, just don't strip your rear holding rings.

And because it all went well in my case, you get two more extra pictures.


  1. Would you mind reposting these images? This is a great post but I can't see the images! Thanks so much.
    kelvination at gmail.com

    1. Hi Kelvin, thank you for your reply. It seems all images have been lost from my blog due to a phone sync issue. Google does not give me a way to "restore" images, so I will have to re-upload them one by one. Thank you for pointing this out, I am now working on it but it may take a short while.

      Best regards

  2. Hi Noah,
    I would love to be able to reassemble my lens, hav eno clue on how to do this, but your post would definately be very helpful... with photos, of course :) Could you?
    Thank you very much in advance.

  3. Hi Noah,
    Wow what great info on the repair. I am planning to do the same and it would certainly help if you could repost the images to accompany the text.
    Thanks and here's hoping you can find time.

  4. Oi Noan as imagens não estão aparecendo

  5. Hello Noah,
    Your repair seems awesome. But I cannot see any images? Would it be possible to repair the blog and put in the images or send it to me? Please?