[Friendscallmedad]

The metered prism on my Pentax 67 is reading as dead, but only when the lens is attached to the body.

When I remove the lens and active the meter it appears to work with no issues. The aperture link chain seems to be intact and couples/works well when lens attached with no prism. The meter reads dead in both manual and auto mode. In the past the meter has worked fine but today I changed my lens from 55mm to 105mm and the meter no longer worked.

Desperate for any advice or resources that might point me in the right direction. I feel like I am missing something obvious here...

So far I have tried:

- Removing the lens and then prism and placing both back on. Several times.

- Replaced the battery

- Cleaned contacts on the body and prism

-Check for broken, damaged chain

Is this an issue anyone has come across before?

Thanks!

[pentaxus]

When you remove the lens the meter goes into stop down metering mode because of the position of the feeler in the prism. That feeler moves a wiper across a variable resistor to provide aperture information to the meter. Maybe some dirt is keeping the wiper from contacting the resistor strip. Yes this requires disassembly.

[Friendscallmedad]

Thank you very much for your reply. Just to confirm, this potentially dirty resistor is found in the prism itself or camera body? Any resources for disassembling either would be greatly appreciated. I do have a quality repair person in the city who has done similar work for me before so I could bring it to them if need be.

Only confusing bit to me is that the meter worked fine up until a few days ago and the camera was not exposed to any dirty conditions in between then.

Thanks again!

[pentaxus]

The aperture resistor is inside the prism. The prism top cover has to be removed to access it. I have seen two versions of the meter and both have it. The silver thing you see inside the slot when when you take the prism off the camera is part of it. It could also be the circuit board but the resistor is more likely. Everything works until they stop working. Sometimes it doesn't make sense. Some versions have the VR coated onto a ceramic board which can have fractures that show themselves some time after the fracture is generated. Your repair guy will most likely be able to deal with it. Hopefully you don't need a part.

[Silent Street]

There is something akimbo. My thinking is there is a problem with the linkage between the camera and the lens, reflected in your statement that the meter works fine when a lens is not mounted. Is there any evidence of the lens mount (on the camera) having been interfered with? Likewise, is there evidence of tampering with the mount on either/both of the lenses you have?

If those checks are clear, move on. We must appreciate that it is a good 30+ years since the last of the newer generation TTL meters first came out. Derangement of the meter must be suspected, especially after heavy professional use or mistreatment (indicated by substantial dings, brassing, dents, cracks etc.); indeed, it is very common with those meters from the last years of the 1960s, and many owners simply discarded these blighty meters and bought another -- a choice you still have today with plenty of meters available on the used market, albeit requiring astute observation and questioning to ensure you get one that works now and works well into the future.

The inside of the TTL meter should not present a big problem to a skilled service person, but it should be disassembled in an aseptic environment, free of static influences. Post-assembly, it must be calibrated for correct exposure over its 5-stop range (+2.5, -0-[centre], -2.5). I'm hoping that a simple adjustment or clean is all that is required, but an open mind must be kept considering the age of these cameras and their meters. Touch wood!