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Message |
    
Martinsmith99
Tinkerer
Username: Martinsmith99
Post Number: 6
Registered: 11-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 04, 2009 - 12:15 am: | 
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I need to fix a slow aperture on a Pentacon 50mm. Anyone have any details of how to get this guy apart and repair?
Thanks |
Steve_s
Tinkerer
Username: Steve_s
Post Number: 144
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 04, 2009 - 03:04 am: |
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Do the diaphragm blades appear oily? If not take the lens-mount off and check that everything is moving freely at the back. If you need to clean the diaphragm I can give you some directions. |
Martinsmith99
Tinkerer
Username: Martinsmith99
Post Number: 7
Registered: 11-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, February 05, 2009 - 06:30 am: |
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There are very minor black traces on the diaphram blades.
I think cleaning would help if you would be kind enough to give me some directions. |
Steve_s
Tinkerer
Username: Steve_s
Post Number: 145
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, February 05, 2009 - 12:04 pm: |
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O.K. First unscrew the ident. trim at the front. On the part below it you will see a single grub-screw. Slacken this and you can unscrew this section. Then, using the outer pair of spanner slots unscrew and remove the front optical assembly.
You now have access to the diaphragm, but to avoid the risk of damaging the lens you really need to take out the rear elements before you take it apart, so take off the lens-mount and the aperture-ring. There are click-stop balls and springs for the aperture-ring and auto/manual switch, so don't lose them. The rear elements are held in a way that allows radial adjustment. So that you can put them back in the same position follow the directions for Meyer lenses at http://www.ydo.abelgratis.co.uk/rear_element.html#01
To dismantle the diaphragm you just have to take out the 3 screws visible from the front and lift off the plate, but before you do, mark the plate and the barrel with a scriber at 3 points around the circumference so that you can put it back in the same position. It is adjustable, and its position affects the diaphragm opening.
Before assembly I always rub the outer edges of the rotating diaphragm ring with dry graphite and then brush and vacuum any loose powder off. I once assembled one of these diaphragms that just would not work 'dry', though that may not apply to all of them.
The bad news is that if you have oil on the diaphragm, it probably means the helical grease is separating! When you get the lens apart, if the helical looks very oily and you feel like re-greasing it, I'll see if I can put some directions together for that. It's quite an easy job. |
Steve_s
Tinkerer
Username: Steve_s
Post Number: 146
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, February 05, 2009 - 12:07 pm: |
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P.S. I'm assuming this lens is an M42. It's just occurred to me it could be Prakticar bayonet! I'm not sure what differences there would be. |
Martinsmith99
Tinkerer
Username: Martinsmith99
Post Number: 8
Registered: 11-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, February 06, 2009 - 12:23 am: |
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Yes, it's M42. Thanks for the info. Much appreciated. |
