Mamiya M645 Pro AE Prism indicates overexposure at all settings
When I went to use my Mamiya M645 Pro TL today, the AE prism indicated overexposure (flashing OVER in the viewfinder) at all aperture settings.
It was bright and sunny and I had 400 ISO film in one back and 100 ISO film in the other back.
I had the shutter speed dial set to 'A'. I tried all three metering modes.
I checked that the ISO dials on the backs were properly set, and that the lenses were set to 'A' and not 'M'.
I tried both backs and re-attaching the prism, to no avail.
I would appreciate any suggestions as to what might be the cause.