Pentax 645 mirror moves only sometimes up

[ant!]

Hi,
I just bought a Pentax 645 (original model). To test it, I removed the film holder and used two sets of fresh AAA batteries. The first few shots everything went fine, but after 5 or 10 something strange happened: The mirror will flip up not every time, and therefore the shutter does not fire and the cycle doesn't go on. I seem some very small movements of the mirror. To complete the cycle, sometimes a second times pressing the shutter release helped, or pointing the camera lens down (this one helped always). All display settings look as expected without film, as far as I can judge.

Is there an easy answer/solution to this? Is it a spring which got weak? My fresh batteries not fresh enough (they measure good though)? I guess I should replace the internal lithium battery one day, but doubt this is the problem. The mirror damper feels dry, not sticky, and the mirror doesn't stay in the up position but down, so I guess it's also not this.

 

[Auer]

Battery holder issue?

I'm no expert of 645's (I have an 645N) but it's my understanding that the battery holders are a weak link and that could cause poor contact that could cause erratic behaviour.

 

[ant!]

I'll try to check this again and give all battery and holder contacts a cleaning. But since it gets at least some power, could this really be the issue?

 

[ant!]

I cleaned now the battery contacts and the holder contacts with an eraser and with some alcohol, no change.

Here some photos, the first one before pressing the shutter release button.

And here after the shutter was pressed. The thing marked green flips down and the mirror moves slightly up, the shutter does not open. When in this state, I have to move the camera to point downwards, the mirror flips up, shutter opens & closed, mirror back and motor is audible. So something makes the mirror stuck there, but I can't see anything obvious.

 

[mklw1954]

If you removed the film holder and got it to fire, you must have installed the rear body cap; if not it shouldn't fire at all. With the rear body cap in place, the LCD reads M1000 until you start pressing the shutter. You have to make sure the rear body cap is on properly or the camera won't fire and the C/S switch must be set to S. It uses 6 AA batteries, not AAA. The lithium battery is only for maintaining the LCD info (number of shots taken, ISO, etc.) if the main batteries run down, so this shouldn't be a factor. If the battery contacts are not right it will not fire. I once had a contact in the battery holder that was bent down and this resulted in intermittent power; when I bent it back up, everything was fine. The contacts can be bent down due to the nature of how some of the batteries are snapped into the holder. After that, I don't know what could be wrong.

 

[ant!]

Thanks for your answer. According to the manual, without a film holder and without the body cap 1/60, 1/1000 and bulb are firing, that's what I try. In don't have a rear cap, so could only try to use a film (firing with lens covered and trying to rewind in a changing bag later?).
The contacts look all good, but I'll try to bend then a bit more inwards. So you say a missing contact could lead to the display having power, but not firing properly?

 

[mklw1954]

I didn't know it could fire without the film insert or body cap at 1/60, 1/1000 or B (because I have the rear cap), so I tried it and the camera fired just fine. In the intermittent power situation I described it worked for a few shots and when it didn't the meter worked and the LED data was still shown, the shutter just didn't fire. Unless it's a battery contact issue, it sounds like something else is wrong.