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Re: [OM] [Re: OM-1/1N Meter accuracy] A new question...

Subject: Re: [OM] [Re: OM-1/1N Meter accuracy] A new question...
From: HI100@xxxxxxx
Date: Fri, 7 May 1999 02:54:59 EDT
In a message dated 5/6/99 10:19:43 PM Pacific Daylight Time, 
francescoarg@xxxxxxxxxxxx writes:

<< Dear Zuikers: 
 Somebody can give an aid on the subject of below? 
 Thanks
 
 Francisco Mostaccio
 La Plata
 Argentina
 > > 
 > >   This is the question:
 > >       In the local market I cannot obtain the 1N34A. They offer 1n60A
 > > to me. He is equal? The solution to the measurer of the OM1 is also been
 > > worth for others camaras that use mercury 1,35 V (Olympus 35RC,
 >>exposimeter of Pen F, PEN FT, etc.)? >>

Francisco,
                the 1N60A is a very similar device to the 1N34A. However, in 
the application of adapting
a silver-oxide battery to mercury replacement in an OM1,what counts is the 
forward voltage drop
at about 0.4mA. (This corresponds to the maximum current drawn using an F1.4 
lens under very bright conditions 
EV 16-17.). You can check whether the diode is a good replacement yourself 
using a digital multi-meter. 
Connect the diode in series with a resistor of 2700 Ohms ("2.7k") and connect 
it up to a silver oxide battery. 
The diode package has a line on one end this must be connected to negative 
battery terminal. 
Measure the voltage accross the diode with the digital multimeter. It should 
have a value of around 200mV. 
If it is very different say less than 175mV or greater than 235 mv then it is 
not suitable or is damaged.
        If you don't have a resistor, then connect the diode into the camera 
as you would normally and then with the 
baseplate still off the camera measure the voltage accross the diode with the 
camera pointed at a 100W 
frosted light bulb from a few inches away using a lens of at least F2. The 
voltage should be about 200mV.

        A better diode alternative is the collector base junction of a 
germanium transistor (like the the 2N1305 
that I tested.) They have lower leakage and thus work better at lower light 
(current) as well as having a 
smaller spread in characteristics.The downside of the  2N1305 is that it is a 
bit large to fit in the
 base of an OM1 and the metal case is connected to the collector making it 
easy to short to ground (case)

        In general, with a diode and without recalibration you will still 
have modest errors at 
high light levels in an OM1. Even with recalibration the diode voltage varies 
significantly with temperature 
and you will still have errors at temperature extremes. 

The diode fix may work better in other cameras in which the current does not 
vary over as wide a range as the OM1.

        I am using a somewhat more complicated low current regulator circuit 
for my own OM1's after realizing
the diode adapter performance is inadequate.


Tim Hughes
Hi100@xxxxxxx

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