[Rollei] OT: Gossen Light Meter Calibration

[ad7i (Paul Benjamin Newland)]

Greetings...

I'm in the processes of trying to calibrate a Gossen CdS Super
Pilot meter and I need some advice.

I purchased this meter on ebay to use with my Rolleicord.  The
seller told me it was in great condition.  When I got it I
found that the battery was dead and corroded.  I cleaned it
up, put in a new mercury battery, and then found that the meter
read about 2 stops low on the high range and about one stop low on
the low range.  I contacted the seller, telling him of my problems,
and asked how did he determined that the meter was in "great
condition"?  In the end, we agreed that we'd call it even if he
rebated me all but $12 on the purchase price of the meter.  At
that point I had a working but inaccurate meter for $12.

I then called Gossen to see about getting it calibrated and they
told me that they no longer worked on this meter and it was not
repairable or recalibratable.

Being the tinkering sort, I took the cover off the meter to peek
around a bit.  The mechanics are very interesting but I found
that the electronics seem to be surprisingly simple.  I didn't
fully disassemble the meter but it looks like it's just a simple
series circuit with a single point calibration.  I could be wrong
about this since I didn't look on the bottom side of the PC board,
but in checking all the circuit nodes with a ohm meter (low voltage,
low current, suitable for checking sensative meter movements
without harm) , it looks like the circuit is as follows.


                batt
                test
               switch
        ---------|-----CdS-------
        |     | nc              |
        |     |                 |
        |     --><---Reostat----|
        +       no     R3       |
       BATT                     |
       1.35V                  Reostat
        -                    Coarse R1
        |                       |
        |                       |
        |                    Reostat
        |                     Fine  R2
        |                       |
        |                       |
        -------------------------

My guess is that the calibration routine is as follows....

1.  Set R2 to mid scale.

2.  Place meter in known flux density of light
    (maybe EV12 at 100 ASA would be a good value).

3.  Adjust R1 to get meter to indicate as close as possible to
    EV12.

4.  Adjust R2 to get meter to be dead-on EV12.

5.  Press batt test button and adjust R3 to indicate
    middle of "batt OK" range.

6.  Compare this newly adjusted meter to known good meter in light
    values from EV 5 to EV 20.  If it is within 1/3 stop then it's
    OK to use.

End

Does this seem reasonable.  Anything I'm overlooking here?
Any and all advice is welcome.

Paul

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