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06-28-2020, 01:14 AM   #1
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Pentax MX asa knob not locking on certain speeds

Hi all,
I recently purchased a Pentax MX and it seems to have a small problem with the shutter speed/asa knob:

When I turn the knob with the small button pressed down to set the asa it functions fine but when I depress the button and start turning the knob to adjust the shutter speed, the knob “slips” and keeps setting the asa instead. This only occurs on certain asa numbers and in certain directions, i.e. 400 is fine clockwise but slips counter-clockwise, the knob next to it (500 I think) works fine, then the next (700?) slips in both directions and 800 is fine again.

I wouldn’t care if it wasn’t for the fact that I shoot at 400 and not being able to have an accurate light meter kind of renders it useless.

Any ideas why this is happening?

06-28-2020, 01:59 AM   #2
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QuoteOriginally posted by agregate Quote
When I turn the knob with the small button pressed down to set the asa it functions fine but when I depress the button and start turning the knob to adjust the shutter speed, the knob “slips” and keeps setting the asa instead
You should not be pressing the little button when changing the shutter speed.
06-28-2020, 02:07 AM   #3
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QuoteOriginally posted by pschlute Quote
You should not be pressing the little button when changing the shutter speed.
I think you have misunderstood my wording.
I pressed it down to change the asa, I let go and started changing the shutter speed and this problem is occuring.
06-28-2020, 03:36 AM   #4
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Ah I did misunderstand you. It sounds like that due to the age of the camera the button is not returning fully and locking the ASA ring. Short of having it serviced, I would suggest blowing the area with canned air and press/release the button many times to try and free it up.

06-28-2020, 08:42 AM   #5
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QuoteOriginally posted by agregate Quote
Any ideas why this is happening?
The button is sticking in the down position. Cleaning the mechanism might prove to be a ready fix and is something that would not be difficult for someone with the proper tools and expertise to remove and disassemble the knob. Having said that, my advice is to not attempt amateur camera service unless one is interested in learning the craft and willing to sacrifice the camera in the attempt.

Instead, conventional advice on this site is to invest in a CLA (Clean, Lube, and Adjust) service when a recently acquired vintage camera presents with unexpected quirks or failure. CLA is usually flat rate and fixing things like your malfunctioning film speed dial would be included in the basic charge unless replacement parts are required. Replacement of light seals and mirror bumper should be included as well. A proper CLA involves full disassembly and removal of old lubricants and accumulated gunge, cleaning of mirror and viewfinder optical path, adjustment and calibration of shutter and meter, and a thorough cleaning of the exterior too. FWIW, the light seals and mirror bumper are probably toast and needing replacement anyway.

Yes, a CLA will often cost more than one paid for the camera, but once done on a mechanical camera such as the MX, one can expect trouble-free operation for many years.


Steve
06-28-2020, 10:14 AM   #6
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Steve is correct...........Cleaning the mechanism might prove to be a ready fix and is something that would not be difficult for someone with the proper tools and expertise to remove and disassemble the knob. Having said that, my advice is to not attempt amateur camera service unless one is interested in learning the craft and willing to sacrifice the camera in the attempt.

Common Sense is a great guide....Think first. I've repaired cameras since graduating from National Camera back in 1973


This is a MINOR repair...... Use a small pin spanner or round nose pliers to carefully spin off the center pin face spanner. all fits into tabs on this, the assembly is spring loaded, unscrew slowly and nothing will fly out. sounds like one tab is slightly bent, usually will straighten. Clean and reassemble in reverse order.



good luck
06-29-2020, 12:28 AM - 1 Like   #7
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Thank you everyone for the suggestions!
I’m fairly certain the previous owner has had a service done to it in the past (particularly the foam because it looks brand new) but I may have to go in to get this done myself.

I tried to unscrew it with a screwdriver, paper clip and other tiny pins I have around but to no avail. (Is it reverse threaded?)

All the best to you!
06-29-2020, 01:56 PM   #8
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QuoteOriginally posted by agregate Quote
Thank you everyone for the suggestions!
I’m fairly certain the previous owner has had a service done to it in the past (particularly the foam because it looks brand new) but I may have to go in to get this done myself.

I tried to unscrew it with a screwdriver, paper clip and other tiny pins I have around but to no avail. (Is it reverse threaded?)

All the best to you!
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