It appears to be working normally inside now. Keeping it inside my coat is certainly an option, though not exactly what I had in mind. I picked this thing up a year or so ago it is pretty much mint condition appearance-wise. I only mention that as it was never "used and abused." Mirror bumper was changed with the CLA. I took the lens off and the mirror would not come down with minimal force applied. Maybe I'll send a note to Eric Hendrickson, the guy who did the CLA.
It's not his fault. He could not have known. This is an adjustment situation inside the bottom cover. You can adjust the mechanism to give it more of a "hair-trigger" with the mirror release catch. Even then will not be a sure guarantee it won't happen again. Nature of design.
In addition to keeping your camera warm by using your body heat, I recommend placing the camera (and any other gear) in a ziplock bag before going inside. A camera bag works to. The idea is to avoid the pending condensation.
My experience is very limited since I live in Northern California. My shutter got stuck with my Super Ikonta IV in the cold. Just processed my film from my winter trip from Yosemite and my Rollei Automat IV. All the shots are slightly over exposed from a slow shutter. My Canon F1N is by far the most robust in cold weather. It's the only camera of the 3 that has an electronic shutter. Are mechanical shutters prone to cold weather issues and electronic shutters are better in the freezing cold? I'm just curious.
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Something I didn't see mentioned, but I kind of scanned so maybe somebody did, but did you check the battery? In cold temperatures battery power lowers, sometimes this could be to the point where your camera started off fine, but then would stop functioning simply because the battery wasn't activating the mirror properly, just figured I'd mention that. It's happened to me before, I was doing a 20 minute exposure in about 10° weather, and about 12 to 15 minutes into the exposure, the aperture end up closing because the electronic mechanism that was supposed to hold it open died of power.
Repeat: The Pentax mirror catch can be seen in action by removing the bottom cover and observation. Evidence will point out the problem. Inherent in the design. Though I stop short of calling it a "flaw" I have too much respect for the Pentax brand to do that. No amount of "CLA'ing" will spot the adjustment. Nobody does CLA's in 40 degree rooms. The problem usually rears its head at temperatures at 40 and below.
I think I've read somewhere that there are certain points below the bottom cover that may not be lubricated at all. May be this could be the reason. In mflenses.com under "Jammed Spotmatic" you can find a good picture of the inner mechanism of a spottie under the bottom cover.
I don't think this will be of very much help, but I had a similar problem once with an old Yashica FX-2. The evening started very damp and then the temperature plummeted to the teens. The problem never recurred under normal cold conditions. You will have a chance to see if the problem reoccurs on Monday.<g>
I had my Spotmatic CLA'd by the Pentax guru last year. I thought it would hold up in the cold weather. WRONG. Mirror got stuck after about 20 minutes in 15 degrees.
Is this normal? What temperatures were 1970s cameras supposed to work in? What did pros do? What can I do? I'm looking for a 35mm or medium format I can use in 0-20 degree weather.
What pros do is have their cameras they have use in extreme cold is have them winterised by a camera repaired having all the lubricants removed and replaced with one's more suitable for the extreme cold conditions.