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02-19-2013, 12:25 PM - 1 Like   #1
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MX now working

Last month I posted here to announce that I had added an MX to my Pentax collection. I was really pleased, especially as I got it at a good price from a local seller. Unfortunately, on its first outing, it was obvious that the metering was intermittent and unreliable. Luckily I had transferred a 2/3 used roll from another body for these first test shots so I didn't waste a complete roll.

After a bit of investigation, I discovered that the aperture adjustment ring on the body was sticking. This is the thing that is driven by the aperture tab on the lens and it tells the meter what aperture is set. By checking my other Pentax bodies, I could see that this ring should be free to move and is sprung loaded in one direction (see photo showing stuck ring and tab and its correct position).

I contacted the seller who offered a full refund without quibble but I had already paid for, and fitted a light seal kit. Besides I really wanted to keep this sexy black MX. The seller offered a partial refund to cover the cost of replacing the spring which was damaged. I disassembled the front of the body and found that the screws holding the aperture ring were too tight and not allowing it to return under spring pressure. Long story short... the MX is now working fine and I'm a happy bunny

02-19-2013, 01:12 PM   #2
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Nice repair work. Looks great.
02-19-2013, 03:05 PM   #3
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Oh, that was all that needed to be done? Nice. I wonder how screws get "too tight" after tens of years.
02-19-2013, 04:37 PM   #4
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Maybe that's what's wrong with my two!

02-23-2013, 05:11 AM   #5
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QuoteOriginally posted by kcobain1992 Quote
Oh, that was all that needed to be done? Nice. I wonder how screws get "too tight" after tens of years.
Sorry for the late reply. I didn't get the impression that the camera had been disassembled before although I could be wrong. My own theory:- the screws and their slightly oversize washers form a sort of outer bearing race. They are located on the outside diameter of the aperture ring. Too tight and the ring gets stuck, too loose and the moving ring could foul on the body. I think that over time, the aperture ring may have distorted slightly, the spring will have weakened and any lube applied during initial assembly will have dried out.

I suppose that backing off the screws slightly was a "quick fix" but I'm not ready to go down the full CLA route yet but may do later as I get more attached to this body
02-23-2013, 07:24 AM   #6
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I had a similar problem with that lever on a used MX. Same symptoms: unreliable metering. I was all set to send it back, when I decided to compare to another MX I already have. That lever was bent just a little bit (outwards, too close to the mount) compared to my good MX. Sometimes it would catch on the corresponding bit on the lens; sometimes not, which explained the erratic operation.

I gently bent it to match the positioning on my working MX, and voila, completely fixed.

Feels good to sort out a problem like this on your own, doesn't it? :-)

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