[help] Mamiya RB67 coupling arm issue

Discussion in 'Medium Format Cameras and Accessories' started by Meerkieker, May 17, 2017.

  1. Meerkieker

    Meerkieker Member

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    Hello folks!

    I'm new here even though i've been lurking and following you since i got my first MF camera, a beautiful RB67 Pro-S. After a month of inconsistent results due to a then unknown problem with the system, i've been recently trying to detect it. Basically the issue involves both the top left (as seen from behind/film back) coupling arm and the revolving adapter. Here three different scenarios in which the problem emerges:

    1) no revolving adapter attached (in video from 00:00 to 00:30) - The coupling arm behaves normally, no irregularities whatsoever.

    2) Revolving adapter attached (in video at 01:01 and at 01:06) - coupling pin stands out a bit longer than it should. I presume this is because of the underlying coupling arm being stuck for a time long enough to undermine the proper working of the motor drive back, which i own. I've lost two roll of film because of this, as the motor drive continued to wind up skipping most of the frames before the coupling pin went back to it's retreating position.

    3) Revolving adapter tilted (in video at 01:23) - although this is not an usual shooting situation, this is to show how the problem emerges, with the coupling arm not getting back to its resting position. Usually it does so after a couple of second, enough to undermine, as already said, the proper working of the motor drive.

    Meanwhile i've got a pro-SD manual back with, and it does not seem such an issue anymore since i usually wind the film after enough time to let the pins get back to their resting position.
    What puzzles me about this is how inconsistent the problem comes in light, it can happen four times in a row and the disappearing for another 10 frames.
    Second, i'm struggling to get why does this happen only when the revolving adapter is sitting on the body. As shown in the first part of the video, in its abscence, all is working regularly without any trouble for 20+ shots.

    Before peeling off the leatherette and disassembling the left panel, i'd like to ask you help. That would be extremely appreciated. Thanks!
     
  2. chassis

    chassis Member

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    Hi Meerkieker

    I have this precise problem on a Pro S body. It has to do with something out of adjustment on the left side (looking at the camera from rear, opposite the cocking lever) of the camera. Google around for the repair manual and you will see how the signal arm (the part that is the problem) is connected to the mirror mechanism.

    I have not opened my camera body, instead I bought a Pro SD body. I would like to fix it. Here is what I think could be root causes:

    - sector plate slipped (radially adjustable plate)
    - something bent
    - something needs lubrication
    - something not noted above needs adjustment

    To get the left side plate off doesn't seem difficult but rather somewhat involved. Some leatherette needs to come off and that is something I am procrastinating on.

    The intermittent nature and correlation with the revolving back tells me that the arm is ever so slightly out of alignment and is hanging up on something. If you look at the window in the revolving back through which the arm needs to protrude there is not much room for error and the arm can get stuck. I tried to bend the arm a bit and it didn't help. The arm feels a little floppy in the side-side direction so again I think something is worn or out of adjustment.

    Hopefully Paul Ron chimes in on this.
     
  3. OP
    Meerkieker

    Meerkieker Member

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    Well i hope its just about lubrification, but the relation to the revolving back isn't in favour of that guess. Both arms and pins seem working well and not cloggy at all. I bought my RB67 in Near Mint condition and it looks and feels like it has been barely used. Hope its neither something bigger, since i don't have anyone who could CLA it, nor do i have the money atm.

    This is what i'm thinking of actually. It could be that the arm is being pulled toward the tight window's wall on the revolving back. Tried to bend it outwards aswell but i didn't want to force anything.
    Thanks for your precious feedback!
     
  4. paul ron

    paul ron Member

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    That could be a few different things causing those tabs to hang. They run dry so lube, a very light oil (watch oil) may free it up a bit and cure your problem. The camera may have sat unused for several years n just needs a jump start... oxidation or whatever. I may have a picture of how those "arms" link inside so you can understand the mechanism.

    The revolving back.... those chrome tits sometimes get crud in them when people put the camera on the revolving back without a film mag. The pins either get bent slightly n cant run free, or crud gets in and hangs them up. Clean the hole n push the pins from teh back of the revolving adapter as you work in a very very tiny dab of WD40 in each.

    Note... to open this side of the camera, you will have to replace internal seals. This side has a double panel.

    the bushing is plastic delrin and rides the cam. This isnt a heavy spring load... its a delicate load as you can feel when pulling the tabs. If its not snapping back, something is bent, or the bushing is worn or a spring is popped.

    TABS.jpg
     
    Last edited: May 18, 2017
  5. xtolsniffer

    xtolsniffer Member

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    Where would us RB67 users be without your help Paul? You always come up with solutions.
     
  6. paul ron

    paul ron Member

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    hahahaha thanks. thats my apug contribution.
     
  7. chassis

    chassis Member

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    Thanks thanks very much Ron, your comments are highly appreciated!
     
  8. OP
    Meerkieker

    Meerkieker Member

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    First, i have to thank you for the all the precious effort you make on this site, especially when it comes to the RB67 noobies like myself.

    Now, back to the topic, today i've been trying to localize the problem avoid to take a further step, namely removing the side panel. Both mechanisms, the pin and the arm seem pretty slick; there's no apparent rubbing nor friction in their movements. I've however given the pins a tiny coat of WD40 as recommended.

    What is puzzling me is that that issue is happening only when the revolving adapter is attached to the body. Otherwise, namely w/o revolving adapter nor film back, both arms are running perfectly. Therefore i guess that the problem is caused by the revolving back. I came to this conclusion when i turned the revolving back at around 45° as shown in the video; the upper left arms gets stuck somewhere on the revolving back, blocking the springs (or something else) in their position. Then i tried to attach a cotton thread to the arm so i could push it or move it whenever it locked forward. From what i felt and seen it seems that the plate which is attached to the arm gets stuck around that whole which leads to the pins.

    Considering that when i bought the beast, i've surspisingly found that the previous owner has replaced the light seals on the revolving adaper with a thick wool yarn. Isn't it possible that the arm - or better -, that small metal plate which lies just in the middle of it, gets either stuck or stopped by that thick thread?

    [​IMG]
    [​IMG]
     
    Last edited: May 19, 2017 at 6:15 PM
  9. paul ron

    paul ron Member

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    Get rid of the yarn!
    Now see if the problem went away?
    Replace it with the proper foam seals.

    BTW that little square thing under the arm is a light baffel. That side of the camera has to be light tight because the mirror arm is open to the internals there... see the slot inside? that tab you are tugging is also open to the inside.
     
    Last edited: May 19, 2017 at 6:34 PM
  10. OP
    Meerkieker

    Meerkieker Member

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    no difference at all, that arm still gets stuck. I don't know what to think of at this point, im 90% sure it's not something mechanic but else..i don't know.

    And thank you a lot for clarifying me what that square tab is.
     
    Last edited: May 19, 2017 at 7:15 PM
  11. MattKing

    MattKing Subscriber

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    For clarity, there are some parts of some cameras where yarn does work well, but it needs to be the right type of yarn - dense and black and without frilling - and that yarn is certainly not the right type.