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01-08-2016, 02:17 PM   #1
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MX shutter sticking/locking over 1/60 shutter speed

I've had a Pentax MX for a while, never really used it since there is nowhere to develop film around where I live besides walgreens

Decided I might try to sell it so I started testing it out. Turns out the mirror gets stuck very frequently if the shutter speed is set any faster than 1/60". It's like it does 90% of the shutter operation and then sticks.

If I switch back and fire at 1/60 or slower it works fine, getting stuck very rarely. So it doesn't get permanently stuck.

When I do this with the back flap open, I can see that the shutter opens and closes but the mirror stays opened. Also it feels like the shutter doesn't close all the way (can see the end of the curtain, but there is no light gap).

Anyone have an idea as to what might be going on? I wouldn't want to sell it in this condition and might end up keeping it for any future film endeavors so I would like to get it fixed if possible.

01-08-2016, 04:22 PM   #2
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QuoteOriginally posted by NitroDC Quote
Anyone have an idea as to what might be going on?
Sticky mirror bumper foam? You can test the bumper by touching it with a toothpick or similar wood splint. If it is sticky or crumbly, it is time to replace. Continued use if bad carries the risk of slinging goo onto the focus screen and/or mirror or casting crumbs into the works.


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01-08-2016, 10:53 PM   #3
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If the curtain doesn't seem to go all the way, it could be that the second curtain is going too slowly which doesn't kick the mirror mechanism. GENTLY push the curtain the rest of the way and see if the mirror goes down. When cameras are stored cocked, the springs lose tension. Curtain speed adjustment may be needed.

---------- Post added 01-08-2016 at 10:00 PM ----------

Also, high speeds use a different timing mechanism than slower speeds. The problem could be there also.
01-10-2016, 02:05 PM   #4
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QuoteOriginally posted by pentaxus Quote
If the curtain doesn't seem to go all the way, it could be that the second curtain is going too slowly which doesn't kick the mirror mechanism. GENTLY push the curtain the rest of the way and see if the mirror goes down. When cameras are stored cocked, the springs lose tension. Curtain speed adjustment may be needed.

---------- Post added 01-08-2016 at 10:00 PM ----------

Also, high speeds use a different timing mechanism than slower speeds. The problem could be there also.
Okay, so I tried pushing the curtain (it needs just a 2-3mm push) and it worked! The mirror came down.

01-10-2016, 03:53 PM   #5
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A few possibilities:
Could be dried, sticky lubricant in the closing curtain mechanism, which a CLA by Eric would fix.
I've seen a number of cameras of this vintage where storage or display with the lens off allowed long term environmental exposure of the closing curtain to stiffen the rubber coating and cause that curtain to drag. This calls for a curtain replacement.
Some MXs were made with plastic gears on the tension adjustment "worm" gears, and I've seen the plastic deform and allow the tension adjustment to slip, reducing the spring tension on a curtain, that can also cause this. The MXs with metal gears are more trouble free. I had one plastic-geared MX lose its closing tension not long after a CLA by Eric, and that's how I found the problem. I fixed it with a metal gear from a parts body.
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