Weird problem only appears on film

The Pentax Spotmatic curtain speed is 14.3 milliseconds to travel the 36mm frame distance. This falls into the 1/60 - 1/75 shutter speed range. Other makes of cameras with a horizontal travel FPS should have a similar curtain travel time.
Have you looked at the mirror curtain travel from the front of the body with the back open and a bright light behind the body? Slower speeds may reveal more than faster ones.

shutterfinger said:

The Pentax Spotmatic curtain speed is 14.3 milliseconds to travel the 36mm frame distance. This falls into the 1/60 - 1/75 shutter speed range. Other makes of cameras with a horizontal travel FPS should have a similar curtain travel time.
Have you looked at the mirror curtain travel from the front of the body with the back open and a bright light behind the body? Slower speeds may reveal more than faster ones.

Click to expand...

I have reason to believe that the SRT shutter is marginally faster in terms of curtain speed than the Spotmatic/K1000 models, but not enough to raise the minimum sync speed by a full stop. That being said... I have looked through the front a couple of times but not as much as I've looked through the back. The next thing will be to look through with a strobe synced up...

Slower speeds have not been particularly revelatory... and 1/60th is where you start to see the non-returning mirror on this camera, unless it's very cold, so that may have other problems associated with it.

It was the non-returning mirror that led me to think the mirror may be rising slow -- if it's sticking or dragging one way, it almost certainly is the other.

I don't know what is causing that, but the picture in your first post is really nice. I wish more people would post pictures of problem negatives like that.

Donald Qualls said:

It was the non-returning mirror that led me to think the mirror may be rising slow -- if it's sticking or dragging one way, it almost certainly is the other.

Click to expand...

Well, see... I don't know how it works on this camera, but it seems like there's just not enough force in the shutter mechanism when it's set to slow speeds to consistently trigger the mirror cam to release the mirror. I feel like it's the cam mechanism, not the mirror hinge itself that's running slow. Now, that doesn't mean that it couldn't be running slow both ways... also, it's not locking up most of the time at 1/60th and never above 1/125th, whereas I know some of the pictures on that roll were taken all the way at 1/1000th.

At any rate, I'm about to look through the shutter some more at 1/60th with a powerful strobe this time. Failing that, I'm going to put another roll through it and see if it's still doing it. I think there's every chance that it was a little sticky from sitting and that exercise has loosened some stuff up.

I think that mechanical cameras have a lever that should release the shutter when the mirror reaches the end of travel to the up position, electronic controlled cameras use a switch.
Mechanical wear or dried grease is likely the cause.
https://learncamerarepair.com/downloads/pdf/SRT-101ServiceManual.pdf

ic-racer said:

I don't know what is causing that, but the picture in your first post is really nice. I wish more people would post pictures of problem negatives like that.

Click to expand...

I knew people would complain if I didn't, and honestly it would be hard to explain that curved shape without illustration