[Jessieben2021]

My ttl prism switch is not turning on. When I slide it on position it doesn't stay on the "on" position. Please help. Does it needs to be repaired? Battery check on the body and the shutter work.

Anyone knows where I can find disassembly photos or videos of the ttl prism I can follow please? I found one in you tube but not showing every step.

---------- Post added 04-16-21 at 08:35 PM ----------

I tested body battery contacts to prism and the contact on the right together with the middle is not budging the needle of the tester. Is that normal? Only the one on the left and middle together works.

[ProfessorBuzz]

When you say it doesn't stay "on" - it's either on or off.

Is the centre sliver bolt head with the 2 holes moving when you move the black notched "handle" of the switch?
Any chance the silver bolt has just come loose?

If you go here, and take a look at the photos:
Daren Sefcik: Pentax 6x7 TTL Prism Repair and Exposure Meter Adjustment
you can see the switch mechanism is just a springy long piece of metal that pushes against a fixed contact. So likely the switch bolt or cam is not in the right position or is loose.
Good luck with the repair!

[Jessieben2021]

When you say it doesn't stay "on" - it's either on or off.

*It just springs back in between on and off.

Is the centre sliver bolt head with the 2 holes moving when you move the black notched "handle" of the switch?
Any chance the silver bolt has just come loose?

It just springs back in between on and off. I've tightened the silver disc with 2 holes in middle but it does make any difference.

[Jessieben2021]

Thank you Professor Buzz for the the reply and link.

[tuco]

*It just springs back in between on and off.

That's correct and normal behavior. The switch returns to a neutral position after you toggle it on or off. Note too the TTL will turn itself off automatically after a short time period. Rub the contacts common to the prism and the body with an eraser to clean them.

[Jessieben2021]

Thank you Tuco. I will do that. Fingers cross, hopefully it'll work. If not, it has to be opened up I guess. I appreciate all the replies everyone. Thanks again.

[johnha]

I have two 6x7 era metering prisms. One places the needle at the top when it's off, the other places the needle at the bottom when off (no idea why). One only meters in stop down mode - I forget which.

The above means that it's easy to mistake extreme under/over exposure with the meter not working until I remember which prism I'm using. Make sure your base settings are in the right ballpark.