Pentacon Six Random Light Leaks

I have a Pentacon Six, and I must say when everything "clicks" - pun not intended -, it produces fantastic results in tandem with its Carl Zeiss Jena Biometar 80mm f/2.8 MC standard lens, like these:

However, some frames are affected by what seems to be light leaks. Exhibit A (cropped to 645):

And here's the full frame, with the problem intentionally exaggerated so that it's easier to see:

Do you think this is a light leak? Or is it a development/lab issue? As I've said, this is something that affects some frames and not others - but there always seem to be 2-3-4 affected frames per roll.

The blueness of the streaking would point more to a processing fault IME.

If it's a light leak, then there would be two possibilities, since it affects the out-of-frame edges of the film.

1) The camera back bulging open, or loss of light-trap material from the back.

2) Sloppy winding of the film onto the takeup spool. Allowing light to leak into the loosely spooled film when taken out of the camera.

I suppose the blueness could be caused by fogging the film through its backing only. Seems unlikely though.

My money is on a processing fault, and my first reaction, after thoroughly examining the camera back, would be to find a different processor.

I would try two things: first, expose another film (not on a subject you care too much about) and send it to a different lab. If the new lab still gives you the problem, then it's probably not the processing. Second, try taping the edges of the door with black insulating tape; if that gets rid of the problem, replace the door seal material. On a camera this old, that's probably something like soft, thick black string, and you should be able to do it yourself with (say) embroidery wool and a tiny amount of glue.

My Exa leaked at the hinge end, which took me a while to track down.

Thank you both - I will tape the edges of the door and try a different lab.

I will recommend you only test one parameter at a time. If you do both you won't know the cause of the problem.

only test one parameter at a time

For SURE.

The main problem I have with my Pentacon Six TL is that if the film is even a little bit loose in the camera, spacing of the negatives is poor, though that's true of many 6x6 cameras in my experience (maybe, I'm just 'loaded' poorly).

LINKS

Pentacon six TL

Pentacon 6TL with 180mm Sonnar f/2.8