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Johnboothe
Tinkerer
Username: Johnboothe
Post Number: 4
Registered: 04-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, April 07, 2011 - 12:35 am: | 
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I have stripped down a Pentax MX to work on some problems with the shutter. The first curtain was coming to rest within the picture frame area. I have solved this now. However, the other problem was that capping was occurring at 1/500s (ie the width of the shutter aperture was changing as it moved across the frame), and I was getting no exposure at all at 1/1000s (ie curtains overlapping throughout their travel). I'm currently trying to solve this problem and need some help with it.
I have tried increasing the speed of the first curtain, but at the point at which adequate overall exposure is attained at the 1/1000s setting, the first curtain is travelling significantly faster than the second curtain, resulting in capping (with more exposure towards the end of travel).
Basically, the problem is that when the speed of the two curtains is equal, they overlap slightly and don't expose the frame. My analysis is that the second curtain is being released too early - a timing problem. As I understand it, high speed timing adjustment on the MX is done with a tiny screw on the high speed arm assembly, which adjusts the distance between the two arms and therefore adjusts the time before second curtain release. However, I'm not certain about this because the MX repair manual is very vague here. On attempting to make the adjustment, I stripped the screw slot and so am unable to adjust this any more. There is a slotted collar around this central grub screw. Should I have loosened this first?
Can anyone confirm that this is indeed the correct method for adjusting high speed timings? Can anyone suggest any other ways to adjust the timings or might I have something else misaligned somewhere, causing the mis-timing? The front edges of the shutter blades when cocked overlap by around 2mm. Is this correct? My thinking is that if I reduced this overlap then I might get a correct-sized gap?
Many thanks for any contributions. |
Steve_roberts
Tinkerer
Username: Steve_roberts
Post Number: 64
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, April 08, 2011 - 02:30 am: |
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John,
I'm not familiar with the adjustment on the MX but the slotted collar around the grub screw you describe sounds like a lock nut. Pentax are very fond of these, often accompanied by a dab of red paint. All may not be lost, as if you release the slotted lock nut the grub screw should come out easily, even given its lack of slot. Whether that adjustment is the source of your problems, however, I'd have to leave to those with more experience than I have!
Good luck,
Steve |
Johnboothe
Tinkerer
Username: Johnboothe
Post Number: 5
Registered: 04-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, April 08, 2011 - 12:55 pm: |
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Thanks for that Steve. Yes I think you're right it probably is a lock nut. I have had a go at releasing that but had to stop for fear of stripping that one as well. I'll have to make a special tool to stand a chance of removing that one. I have resorted to filing down the high speed timing cam a little and it seems to be making a difference. I know this is bad practice but I got the camera as a learning project so I won't be too bothered if I mess things up. In fact, it'll be a miracle if I manage to get the thing back together with everything working! |
Gez
Tinkerer
Username: Gez
Post Number: 273
Registered: 09-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, April 09, 2011 - 06:00 am: |
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The problem you describe seems to affect the MX model to a greater ot lesser degree. Are you releasing the shutter with the mirror box out? The exact timing of the ist curtain latch seems to make a difference so that capping is worse with the mirror box removed than when it is reinstalled and the shutter released normally. |
Johnboothe
Tinkerer
Username: Johnboothe
Post Number: 6
Registered: 04-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, April 10, 2011 - 01:50 pm: |
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Thanks Gez. Yes I was originally doing my adjustment with the mirror box out. I kind of understand what is happening with the mirror box. As you probably know, there are two catches involved in the release of the shutter - both of which must be released to fire the shutter - the pin underneath the shutter release button and the lever protruding towards the mirror box, which is released when the mirror reaches its 'up' position. If the mirror lever is released without the pin pressed down (I was doing this manually for testing), the first curtain gears move a bit - as though taking up slack. They are then released fully by pushing the shutter release pin. Under normal curcumstances however (ie. with the mirror box in), when the mirror triggers its lever, the pin is already down (due to the photographer's finger on the shutter release) and the shutter fires instantly, so the shutter mechanism does not reach this intermediate state of release. On my camera, shutter speeds were markedly faster (smaller gap) when first releasing the mirror lever and then the shutter pin sequentially, than when I held down the shutter pin while releasing the mirror lever. This second method is more like the 'real' situation, so after I understood this effect, I did all my testing like this until I got the mirror box back in. I must admit that speeds did seem to increase slightly with the mirror box in, compared to my tests with the second method of release, from around 3mm to 2mm at 1/1000s. I think this is a secondary effect, which I don't yet understand.
Now with the camera largely back together, when I use the mirror lock 'feature', speeds revert back to what I was getting with my initial tests using the first method of release (or slighly worse because of the mirror box effect). This is what you would expect because the mirror lever is being triggered before the shutter release pin. The differences I am seeing are around a 2mm change in the size of the gap between curtains during exposure. At 1/1000s this will cause a big difference in exposure while at slower speeds the difference (in exposure) becomes less. At 1/1000s for example, when using mirror lock, I get either no gap or a hairline gap, whereas using standard release, I get around 2mm - still a bit faster than it should be, I'm guessing.
My observations and analysis do seem to be consistent with what you say, Gez. With the mirror box out, if you test by releasing the mirror lever first, followed by the pin, you're going to see a significant decrease in speed when the box is in place because the release sequence is changed.
So now I've got the camera largely back together, I think I have a reasonably accurate set of shutter speeds, and pretty even exposure across the frame, though I think the faster ones will be a bit too fast still. I will test with film and then sand down the high speed cam further if I think it's necessary. I think the problem of higher speeds while using mirror lock is probably inherent in the design of the shutter and not something I will be able to change. After all, the camera was not designed to have mirror lock. This will not be a major problem since mirror lock is mainly useful at slower speeds where differences in exposure will be minimal.
Gez, when you say that this problem affects a lot of MX models, do you mean the problem of uneven exposure across the frame (a function of curtain speed), or of too small a gap at the highest speeds (a function of release timing)? In my copy, it seemed that the curtain speed had been altered to to compensate for a gap which was inherently too small - increasing overall exposure at the expense of even exposure (ie. so that the gap got bigger towards the end of travel). Is this something you have observed before?
I apologise for the very long post but these things are hard to explain in just a few words. Any further contributions on this topic would still be welcome. My main problem now is with the electronics - they've stopped working! I'll be posting another question on this if I can't solve it.
By the way, the initial problem of the first curtain coming to rest around 2mm before the edge of the frame was a problem with the bounce stopper. For anyone with the same problem, this is very easy to adjust once you get inside the shutter mechanism on top of the camera, beneath the shutter speed dial. There's a horizontal screw which adjusts the resting position of the stopper, and screwing this further into the camera body means that the first curtain will travel further before coming to rest. The difficult part is removing and replacing the plate covering the mechanism, as it affects the coupling between the shutter dial and the slow speed escapement shaft, as well as the shutter speed indication wheel (what you see in the viewfinder). |
Gez
Tinkerer
Username: Gez
Post Number: 274
Registered: 09-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, April 11, 2011 - 01:31 pm: |
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John, thanks for the full description of the problem you encountered, it will be a valuable addition to the archive.
The problem I encountered with servicing the MX shutter was both a narrow gap, only 1-2mm wide, at 500 and 1000 and the gap narrowing and closing completely at the frame edge, typical indications of dirty or dry bearings. Gentle cleaning and the minimum application of oil usually solves the problem, but in one case I made the mistake of over enthusiastic cleaning with Ronsonol which actually made the problem much worse and caused the first curtain to bounce out of engagement with the brake. You may have noticed, after removal of the speed setting plate, that a single brake mechanism serves for both cutains. The surface of the brake lever is lubed by an oiling wick contained in a very small plastic tube. I found that applying a couple of drops of oil in the tube and letting the camera stand for a few days allowed a thin film of oil to coat the brake lever and this made all the difference to the proper functioning of the curtain brake. |
Johnboothe
Tinkerer
Username: Johnboothe
Post Number: 7
Registered: 04-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, April 12, 2011 - 11:32 am: |
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So that's what that little tube was for! I couldn't for the life of me think what that piece of plastic and string was there for. I thought someone had maybe put it in there to hold something in place and forgotten to take it out when replacing the plate. So perhaps all I should have done was oiled this. Drat!
I think most of the changes I made were probably work-arounds rather than addressing the root cause of the problem - down to inexperience no doubt. Perhaps the problems I was having with curtain speed and timing could have been sorted out by cleaning and lubricating the right parts as you suggest - certainly a better solution than sanding down the timing cam! It seemed in this case, though, that getting to the right parts was too daunting for me with my current level of knowledge. Are the bearings you talk about at the ends of the shutter pulley shafts or in the cog spindles of the timing mechanism?
Thanks for sharing your experience - it's very hard to find this kind of information anywhere.
I'm sure any seasoned repairmen (like yourself?) reading my account probably cursed and tutted as they read - understandably, but for now at least, I have a fully working camera and have learned an enormous amount in the process, so I'm pleased (the meter sprang back into life shortly after my last post). |
Gez
Tinkerer
Username: Gez
Post Number: 276
Registered: 09-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, April 14, 2011 - 02:30 pm: |
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The bearings are the ones holding the geared spindles in place, there are also bearings on the bottom of the spindles and these are accessible with the mirror box out, but can also be cleaned from under the baseplate after pulling the slow speeds escapement which often needs cleaning if the slow speeds are hesitant.
I don't know about the availability of Pentax spares in the States but here in Britain the friendly people at www.harrowtechnical.co.uk have a good supply and do repairs too. |
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