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William Martins
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, May 15, 2005 - 10:24 pm: | 
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Can anyone with some experience calibrating a Pentax Spotmatic SP jump in here with me and learn me how to tweak the damn thing!
I know there is a calibration port under the rewind knob. Is it necessary to tear the camera down completely? Or will just gaining access to the port suffice. Also, I understand the needle of the ammeter must rest at the center of the notch when 3 microamps is applied. I don't have a power supply that will produce that kind of
load. Should the meter be centered without a battery or does it sag a bit toward the bottom? Any help appreciated. |
Tony Duell
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, May 16, 2005 - 12:14 pm: |
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I've never attempted to calibrate a Pentax Spotmatic (IMHO any manufacturer who uses a bridge circuit for the meter, and sets it up so that the state you're aiming for is not the electrically balanced state of the bridge is insane, but I digress...)
Anyway, if it needs 3 uA to centre the needle, then fairly clearly with no battery (and thus no current through the meter coil), the needle should not be centred. I _thionk_ it should be below centre.
To get the 3uA calibration current you can use a DC power supply (even a battery) and a suitable resistor. Remember that 1V will cause 1uA to flow through 1MOhm. So a 3V battery and a 1MOhm (brown-black-green colour bands) resistor in series will act as a 3uA source. Strictly, it's the _total_ circuit resistance, including the internal resistance of the PSU and the meter coil that should be 1MOhm, but I suspect the error in ignoring that is insignificant. |
Berk Sirman
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, June 30, 2005 - 02:14 pm: |
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In very early spotmatics the needle rests in the middle when the meter is off. In later models (65 and onwards) the needle sits on the bottom when meter is off. They modified it so that people wouldn't just look at the needle when the meter is off and think they had the perfect exposure. |
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