Mamiya 7II: Unstable Meter Display from a Loose ISO Dial
Originally posted on Instagram. Notes from the repair bench, kept here as a searchable archive. View the original on Instagram
This Mamiya 7II arrived with the complaint that the meter display was not stable and would change from the correct setting to the overexposure arrow. Sure enough, the display would change, especially when the shutter dial was slightly moved.
After removing the shutter speed dial, I noticed two things: 1.) there was quite a bit of residue on the ISO pads and 2.) the ISO dial could be easily moved. The movement of the dial would cause the ISO fingers to land on a between the contacts spot and cause the meter malfunction.
Photo two shows the ISO contact pads and the residue as I first removed the top part of the shutter dial. The video shows movement of the ISO dial which should be solidly locked in place and was not.
Fortunately for my client, this repair was fairly trivial and after cleaning and tightening, all is well.
The fourth photo shows a close up of the ISO dial of the Mamiya 6/7: the round contact in the center is the common rail and the ISO pads on the bottom are each connected to a 380 ohm resistor. The 19 discrete resistors provide a resistance of between 2.3K and 9.8K depending on where the ISO dial is set on the camera. This resistance setting is input into the processor and will be one of the factors in determining the correct exposure.
