[help] Mamiya RB67 coupling arm issue

Meerkieker

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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!
 

chassis

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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.
 
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Meerkieker

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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!
 

paul ron

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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.

 
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paul ron

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hahahaha thanks. thats my apug contribution.
 
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Meerkieker

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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?


 
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paul ron

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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.
 
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Meerkieker

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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.
 
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MattKing

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paul ron said:
Get rid of the yarn!
Now see if the problem went away?
Click to expand...
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.
 
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Meerkieker

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Having ordered some foam from Jon Goodman, i've pulled out the old wool yarn seals, but left behind there's a thin layer of dry glue which i can't remove with alcohol. How should i proceed?
 

paul ron

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scrape it out.

have you checked tge rev back pins to see if they operate smoothly?
 
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Meerkieker

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paul ron said:
scrape it out.

have you checked tge rev back pins to see if they operate smoothly?
Click to expand...
Tried to remove it with acetone and it worked fine.

The rev back pins run smoothly and 'oily', they seem well lubed. Still the problem persists only when the rev back is mounted. A mistery indeed and i cannot think of any cause atm, pity. Still, it's not such a big issue with the manual back, and even so with the motor drive, i only need to cock the body right after the shutter is being released. Whether or not it would be more handy and preferable to get it solved.
 

chassis

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Hi Meerkieker, I am following this thread with much interest. I am re-sealing my RB67 and am looking at the coupling arms for hours!

I think the problem is inside the camera, not with the revolving adapter. I agree that the problem is more frequent, with the revolving adapter. It does happen when the revolving adapter is removed, on my camera.
 
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MattKing

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When you refer to the "revolving back", what is it you are referring to?
Whether or not a back rotates depends on what adapter you are using. Some of the options rotate, while others do not.
To complicate things further, the rotating adapters come in different versions, as do the film backs.
I can put a Pro rotating adapter on my Pro-S body, with either Pro backs, or Pro-S backs attached.
When I had a Pro body as well, I could mix up things in a similar fashion.
The same probably applies to Pro-SD equipment - I don't have any to test.
The point of all this is that it just takes one component in each of these complex combinations to be out of spec to make everything work either poorly or in unexpected ways.
When I purchased the last package of RB equipment I had some problems. Replacing one of the Pro-S rotating adapters solved them.
 

chassis

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Matt, apologies for the poor terminology. By "rotating back" I should have said "revolving adapter". I edited and corrected the post.

My Pro S camera has the exact same symptoms as the original poster. The symptoms are more frequent when the revolving adapter is installed. On my camera the symptom does occur when the revolving adapter is removed. No mixing of Pro-Pro S-Pro SD components.
 
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Meerkieker

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Could it be that the springs need to get lubed, so they could reboud back into their position more easily once the arm gets into contact with an 'obstacle', such as the pins?


Sorry, i referred to the revolving adapter, which is a Pro-S aswell as the body. I use Pro-SD film backs, but showed itself not being the problem since the issue occurs already with the rev adapter alone.
 
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paul ron

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springs run dry and if they are a problem they get replaced.

The pivot inside may need to be oiled but the tabs move fine you say...
the delrin bearing that rides the cam may be deteriorated but the tabs move fine you say...

The only time the problem happens is when you use the rev adaptor... it has to be the rev adaptor then!

Do you have another revolving back to try on it?
 

paul ron

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How handy are you?

Maybe you can split the revolving back yourself n service it?
The only tough part of that is being organized.

where did the video go? I wanted to see it again. can you send me a link to it?
 
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Meerkieker

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paul ron said:
How handy are you?

Maybe you can split the revolving back yourself n service it?
The only tough part of that is being organized.

where did the video go? I wanted to see it again. can you send me a link to it?
Click to expand...
Video is here: Strangely i can still see it on the initial post.

I'll answer you better later as now i have to go. Still, thank you so much for your effort and help here!
 

paul ron

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i see it now.

seems it intermittently sticks without the rev back too.

id have it serviced. something inside as well as the rev back needs to be checked.

too bad you are so far away.
 
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Meerkieker

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Unfortunately not. Hope Chassis has one so he can try himself.

paul ron said:
How handy are you?

Maybe you can split the revolving back yourself n service it?
The only tough part of that is being organized.

where did the video go? I wanted to see it again. can you send me a link to it?
Click to expand...

I could do this, but still its not clear to me the reason to disassemble the rev adapter. I guess the arms only gets through it and excluding the locks to the body, no other mechanism is involved. Please correct me if i'm wrong.

paul ron said:
i see it now.

seems it intermittently sticks without the rev back too.

id have it serviced. something inside as well as the rev back needs to be checked.

too bad you are so far away.
Click to expand...

Pity indeed. But fortunately it's not going to change drastically my workflow, its a minor issue. Where i am currently, Italy, looks like there's someone who repairs RBs with pleasure, maybe in future i'll invest in a CLA.

How could you tell that the arms still stick out w/o rev adapter? Funnily i can't notice it at all!

Thanks.
 

chassis

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The symptom is present with a both Pro S and Pro SD revolving adapters, on a Pro S body. The symptom is far less present, although still occurs, without a revolving adapter.

It seems like the left coupling arm (viewed from the rear of the camera) has too much vertical play. It is getting stuck on something related to the revolving adapter, or the adapter alignment frame (black plate with round holes), but in my opinion the revolving adapter is not the problem. It seems to me that there is something loose, bent or out of adjustment in the left side mechanism that drives the coupling arm. This is the side of the camera opposite the cocking lever.

My view is that because the coupling arm has too much play (latest root cause in my view), it is getting stuck on adjacent parts (revolving adapter mounting frame, and revolving adapter).

So, why is there too much play in the coupling arm? Need to disassemble the left side of the camera to find out.

I have the service manual and parts diagram, and have decided it is a project that needs some time reserved for it. I don't have that time at the moment.

Meerkieker, are you able to remove the left side cover and take some photos of your camera?
 

chassis

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Did some more investigation. I am strongly of the view that the problem is inside the camera on the left (non cocking arm) side, in the mechanism that drives coupling plate RS2531.

I did six tests:

Pro-S body and revolving adapter:
1. Theory: revolving adapter to camera body surface is worn, causing revolving adapter to be too close to the camera body, causing the coupling plate to stick.
Test: apply metal tape, thickness about 0.5mm, to the machined metal contact pads between the revolving adapter and camera body. Assemble camera. Actuate body. Problem is there. Remove revolving adapter. Problem is gone (same as before the test). Test result: inconclusive.

2. Theory: pressure or force from the revolving back pins is too high, causing the coupling plate to stick.
2a. Test: remove film back, with revolving adapter installed. Actuate body. Problem is there.
2b. Test: remove rotating adapter, check pins for free movement. Pins move freely. Actuate body. Problem is gone. Test result: inconclusive.

3. Theory: pressure or force from the revolving back pins causes coupling plate to stick, because of a problem inside the camera with coupling plate RS2531 drive mechanism.
3a. Test: Remove film back and revolving adapter. Gently apply finger pressure to coupling plate RS2531. Actuate shutter. Problem is there (no film back, no revolving adapter).
3b. Test: Remove finger pressure. Actuate body. Problem is gone.
3c. Repeat 3a.: Problem is there. Test result: conclusive.

Pro-SD body and revolving adapter:
4. Repeat test sequence 3. Problem is not there. Result: slight finger pressure does not cause a problem with a body that did not already have the problem. Problem is not related to slight finger pressure on coupling plate.

Pro-SD revolving adapter with Pro-S body:
5. Repeat test 2a. Problem is there. Result: problem is not related to revolving adapter.

Pro-S revolving adapter with Pro-SD body:
6. Repeat test 2a. Problem is not there. Result: problem is not related to revolving adapter.

The left side Pro-S camera plate needs to be opened. I don't have time for this so it will wait. Hopefully Meerkieker will be able to do this with his camera.
 
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