Pentax 6x7 MLU damages meter?

[steve_singleton2]

Based on my advice, a friend & client purchased two Pentax 6x7 bodies

with mirror lockup some years ago and recently a new 300mm lens.

Neither metered prism works now with fresh batteries and I'm wondering

if mounting the new lens and inadvertently pushing the lockup button

could have caused the damage--which he and I both managed to do

independently. That seems to be the common denominator. There looked

to be a short length of brass chain in the lens mount area on one

camera with the lens removed, as if the broken chain might have

severed the connection between the lens coupling and meter prism.

Anyone have any ideas about how this happened and how to avoid the

problem in future?

[steve_rasmussen]

The TTL meters generally fail slowly by giving incorrect readings. They will usually not be inaccurate in all EV ranges when they do start to act up. For your two meters to suddenly fail after the mounting of the 300mm and hitting the MLU is strange. The only thing I can think of, is that the 300mm has some machining mistake (dimensional) where the lens mounts to the body. That dimensional problem could be causing the chain break or.....

There is a pin in the lens mount face of the body that needs to be pushed in by the mounted lens. If your 300mm is not activating that pin, your TTL will not work.

[skygzr]

The chain couples the aperature setting on the lens to the meter. There's a little plastic tab on the chain that grabs a lever in the prism as you change the fstop. The prism needs to be on the camera BEFORE you mount a lens, or the tab won't engage. If the chain breaks, the lens no longer communicates with the meter (the meter still responds to light but it won't change as you stop down).

 

I hear complaints about the meter but mine is prretty accurate.

[skygzr]

Forgot to mention.....as far as I can tell, MLU has no effect whatsoever on the meter working or not working.