The increasingly incorrect (longer) exposures at lower and
lower light levels sounds exactly like circuit board leakage from
contamination or amplifier input leakage. The Olympus circuit uses a special
low leakage amplifier input device mounted on a special low leakage circuit
"board" (ceramic). Given the values chosen by Olympus for the
capacitor/amplifier for integrating the light it is amazing the circuit works
as well as it does most of the time. It needs extreme cleanliness to work
correctly. If the board was handled with bare fingers instead of finger cots
while being repaired the skin oils could easily cause the symptoms described.
Heavy condensation could also move contaminants onto the board . To clean the
board toxic ozone destroying solvents are often used. Cleaning it youself may
cause more harm than good. A competent technician who really understands
board cleaning could do it. The sort of board cleaning done by technicians
for normal circuits is generally inadequate for the cleanliness required.
Another possible source of leakage is electrostatic discharge from say your
finger if poking around the photocell in the mirror box base. This is less
likely and would require board or part replacement. After any soldering for
part replacement the board would need proper cleaning again to remove solder
residue. I seem to remember that in some of the revisions Olympus reduced the
potential for leakge by placing a moisture barrier on critical board areas.
As a general rule for very low light levels the OM2 will have a slightly
shorter exposure times than expected as the long exposure shut off mechanism
works by biasing the circuit in that direction. In the OM2n that is not true
except very close to the exposure limit time.
Regards ,
Tim Hughes
Hi100@xxxxxxx
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