A while back I e-mailed the list mentioning that I had an MX that would
completely drain the batteries after a few of days, even though the LEDs in
the viewfinder were off.
I was able to fix it with the help of the MX repair manual that a PDML
member very kindly posted at http://www.robertstech.com/pentax.htm
The key piece of info that this manual revealed is that if the voltage drops
below 2.7, the LED lights go off. What this suggested to me is that the
meter could be drawing a considerable amount of power even though the LEDs
are off. (My electronics knowledge is a bit sketchy, but I think the light
meter uses a resistor, which presumably would use the energy to generate a
bit of heat. It doesn't have to create a lot of heat to drain two tiny
batteries.)
I saw from the diagram that the meter coupling to the shutter release lever
was right down at the bottom, and accessible by unscrewing the bottom plate.
Down there is a tiny switch that consists of two springy pieces of brass
colored metal that touch to make the LEDs light up. What happens when you
press down on the button is that a tiny arm that holds up one half of this
switch allows it to go down and contact the other one.
When I opened the bottom plate of my camera, I found that these two tiny
pieces of metal were in contact even when the shutter release was not pushed
down. All I had to do is bend one of them slightly to keep them from
contacting when they shouldn't. I should note that the distance between
the open and closed position is less than a millimeter. Pentax really
gambled on close tolerances here, in the hope that nothing would disturb
these tiny, fragile pieces of metal.
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