I just got a Spotmatic F that seems to be behaving a bit weirdly. Stick a super-takumar on the front (not smc) and the meter is reasonably accurate. However if I hit the stop down switch the meter immediately dives by about two stops, even if I haven't stopped down the lens. My understanding was that with the F there was no connection between the stop down switch and the meter electronics so I'm at a loss to understand how this is happening.

I have a couple of SPs as well so I understand the difference between the operation of the switch in the different models. Anyone any ideas ? I've got to disassemble it anyway to clean it up so I may poke around in the meter electronics at the same time.