Dirk writes:
<<
>This design concept is reflected in the meter circuitry of the OM-1/1n
>and 2/2n, which eschews "variable resistors" to adjust the meter.
>Instead, only fixed resistors and precise quality control are used to
>assure accuracy in the meter circuit.
Yes, pots can get dirty and effect the circuit, so fixed resistors are best.
I assume they used 1% tolerance or better?
>>
Actually the "fixed" resistors are selected in a calibration procedure for
the particular photocells in each camera which have significant production
tolerances. There is a relatively complex iterative procedure to get the
right values over the whole EV range.The service manual thus lists a large
number of resistor values in the parts list for each resistor.
The meter uses two photocells with each cell also split in two
interdigitated sections. One section is for higher light levels the other for
low. Resistors are selected to provide the correct linear EV response over
the whole range when the output of the two cell sections are combined with
the calibration resistor network. Having two cell sections gives the
calibration procedure more degrees of freedom for accurate adjustment over a
wide range and probably improves the cell response times at low light levels.
Regards,
Tim Hughes
>>Hi100@xxxxxxx<<
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