Re: [Leica] MR4 Meter Circuit

[Dennis Painter <dwp@deltanet.com>]

Frank, and others interested in alternatives to mercury batteries,

Thanks for your post on this.  One thing I would add is that the
alkaline battery does not have the stable voltage over its life that the
mercury battery has. The stable voltage of the mercury battery is what
allowed such simple meter circuits. The alkaline declines with use.

If the alkaline battery is used instead of the mercury battery I think
the user would be well advised to change them frequently. This might be
detectable in a meter recalibrated for the higher voltage of the
alkaline battery by the battery test indicator. 

Regards, Dennis



Frank Filippone wrote:
> 
> The MR4 meter is a simple circuit without voltage regulation.  It consists
> (simplistically) of a battery, amp-meter, CDS Cell, and variable resistor
> in its operational mode.  Basically, the CDS cell varies its resistance
> with light intensity.  The amp-meter measures the variable current from the
> circuit.  The variable resistor  is used to set full scale adjustment of
> the meter.
> 
> Why is this important?  Because if you have a meter that is calibrated for
> the old mercury cells (1.35 V) , and want to use it with the new 1.50 V
> batteries, you either must adjust the variable resistor or set your ASA
> lower.  How much lower?  I would say 10%... 1/3 of an F-stop is fine.
> Does this remain linear with increased light?  Yes it does... one ASA
> setting change will make all your readings accurate, no matter what the
> light reading.
> 
> So.... either get your tech to adjust the variable resistor or decrease
> your ASA by 1/3 speed... ASA 100 to 80, for example.
> 
> Your third choice is to buy the CRIS battery adaptor... this will work
> without adjusting anything.
> 
> I would review the CL and M6 meter as well if someone would make that
> available, although from my previous life I know that the M6 is a different
> animal.
> 
> Thank You
> Remove "ZZ" form auto reply address or
> please reply to address below:
> 
> Frank Filippone
> red735i@worldnet.att.net

- -- 
Experience is a tough teacher. It gives the test before the lesson.
- -unknown