Electronic Engineering commentry follows,
I previously commented on possible causes of erratic long exposures :
> 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).
Dirk replied:
the board in the bottom of my om2n is fiberglass (green), not ceramic.
It has an early type of surface mount chip on it.
My original comment was not explicit enough. I was referring to the OM2 in
regard to the ceramic circuit.
The OM2S has a polyimde type flex circuit I believe. In any case the leakage
requirements are pretty horrendous.
For example we can estimate the current output from the photocell by looking
at the integration capacitor used
and knowing the integrator output voltage swing can be no more than
approximately 2V or so. The capacitor is
470pF. At 120seconds exposure time this implies a photo detector current of
very approximately 10pA (!0^ -11 A) and hence
total board leakage must be less than this. (dV/dt = i/C) This implies
board leakage resistances of hundreds of Giga Ohms and Mosfet
gate leakage currents at pA levels. In practise for non ceramic boards this
is almost impossible so guard ring techniques
and/or conformal coating is probably necessary. The OM2N has a slightly
larger
capacitor (680 pF) which eases the requirement a little but not much.
The Olympus OM2 service manual for example says :
" The shutter amplifier (M circuit board) requires very high insulation
resistance on its every part, and must be kept free from dust, smudges etc."
There are dire warnings elsewhere about electrostatic damage etc if not
carefully handled.
Regards,
Tim Hughes
Hi100@xxxxxxx
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