Need help with Pentax 645 issue

[sooner]

I've been on a GAS binge lately and picked up a Pentax 645. The test roll showed half the shots almost completely blank. After a second test roll, I've isolated the problem to shooting in aperture priority mode at f/2.8. In all other modes, the camera works great. In aperture priority mode at f/4.5, for example, the shot was fine. Can anyone diagnose this problem?

 

[charjohncarter]

I have the 6x7 so I know nothing about the 645. This problems is real: you might check further.

https://datsphotographyca.wordpress.com/pentax-6-x-7-intermittent-shutter-problem-solved/

 

[Doug]

I have two Pentax 645NII but have not encountered that problem. Is yours the original 645, the 645N or the 645NII, as the systems are somewhat different.

 

[sooner]

It's a Pentax 645, not the N or NII. I know it's not a mechanical problem because the shots turn out great in manual mode. So is it an electronic issue?

 

[Rob-F]

After a second test roll, I've isolated the problem to shooting in aperture priority mode at f/2.8. In all other modes, the camera works great. In aperture priority mode at f/4.5, for example, the shot was fine.

So the frames in question were grossly overexposed? Maybe the camera ran out of shutter speeds, didn't have a high enough speed to expose correctly at f/2.8?

 

[sooner]

Hi Rob, no, actually, the frame is almost blank, not dark, so that suggests extreme underexposure.

 

[retinax]

If this camera uses a resistor strip with a sort of wire brush gliding along it to tell the meter about the aperture, it could be in need of cleaning or bending this wire brush into shape. This would be found somewhere not too far behind the lens mounting flange.
Edit: Or it might be that this is in the lenses in the P 645 system, they have electrical contacts after all.