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Re: [OM] OM-2S and reciprocity failure

Subject: Re: [OM] OM-2S and reciprocity failure
From: "John Hermanson" <omtech@xxxxxxxxx>
Date: Fri, 3 Dec 1999 10:57:27 -0500
The small circuit in the bottom of the 2N (referred to as the "P" board)
controls TTL flash 1/60 cutoff and the 3.5 minute limit on time exposures
and battery drain.

John
________________________________
Camtech, Olympus Service since 1977.
21 South La.  Huntington NY 11743
631-424-2121 http://www.zuiko.com
Free Olympus Manuals: 1-800-221-3000
--------------------------------------------------------------
----- Original Message -----
From: <HI100@xxxxxxx>
To: <olympus@xxxxxxxxxxxxxxx>
Sent: Friday, December 03, 1999 7:27 AM
Subject: Re: [OM] OM-2S and reciprocity failure


> 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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>


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