Gossen Lunasix F / Luna Pro F Calibration

[Don Promillo]

I am attempting to calibrate my Gossen Lunasix F light meter.

In the battery tray, there are 5 pots. On earlier models these would be the sensitivity trims across the range, but so far the 5 pots on the Lunasix F do not make any sense.
The outer pots do not change anything, even on extreme values (almost pitch black and almost supernova explosion)
The three inner pots, lets name them 2,3 and 4, all have an effect on all light values and interfere with one another. If you change 1 by a tiny bit, the two others are completely off. Number 4 seems to have an additional influence on the battery reading during battery test.

The Lunasix F uses a 9V Battery, so the mercury battery problem is not the problem.

Does anyone have any hint, manual or blueprint how to calibrate it?

 

[John Koehrer]

Multiple pots are to balance particular points in the meters range.
Usually they're set to high, low and medium light levels and they DO affect each other. If memory
serves LV 9, 12 & 14 were the settings on the tester we used. That's IF the memory worked.

 

[Don Promillo]

If memory
serves LV 9, 12 & 14 were the settings on the tester we used. That's IF the memory worked.

That is something to work with, thank you a lot. I will try to dial it on these light values.

Even though it is a big beast, I like this meter, especially because it uses 9v batteries and is easy to read. For everyday use I have smaller meters, which work fine, but it is a nice challenge to make the Lunasix F work, too

 

[wombat2go]

Here is a link to a circuit diagram that I traced from a Luna Six.
It is too rough,sorry, but may be legible.
https://app.box.com/s/df96ofgvhh8bfybkt1n3tw474mblfu8y

Looking at bottom of the Unit
Adjustment Potentiometers: Left to right P1, P2, P3, P4, P5

P1 (10k) is battery test current adjust.

P5 ( 20 k) is high illumination coarse adjust. (P5 is used when the nd filter covers the CdS LDR)

P2 ( 75 k) is low illumination coarse adjust. (P2 is used when the CdS LDR is directly exposed via the lens)

P3 ( 10k) is low illumination meter shunt adjust

P4 ( 10k) is high illumination meter shunt adjust

P3 and P4 need to be set near mid point.

The circuit is crude, just a passive resistive divider that approximates
the log function as a reciprocal resistance function.
The meter scale graduations are non linear to more closely approximate to the logarithm.

The meter scale graduations on the one here are divided into 1 ~ 22 in 2 ranges
and the circular slide rule converts those to EV from EV-5 to EV+17.5

In bright sun about EV 14 or so, with the dome on, (incident mode) and ND filter on, the LDR resistance is 520 Ohm.
Dark resistance is about 20 Megohm.

There was no compensation for battery voltage or tempco of the LDR.
The D'Arsonval galvanometer is of 25 uA fsd and internal resistance 2600 Ohm.
The meter needs to be upward and horizontal when taking a reading, as it has a counter weight which swings in a clamp.

The original battery was two by PX13, each with a stable voltage of 1.35 Volt over life and temperature.
The battery current is in range 10 ~ 60 uA, which is not enough to drive a small signal Scottky diode toward saturation,
so there would be accuracy problems by using the often suggested schottky diode dropper in series with modern cells.

If the plastic rear case of the unit is lifted away from the printed circuit board,
the rocker switch pivots come adrift and re-assembly is tricky due to the springs. ( I suppose there was a service jig)

 

[Don Promillo]

Post #4 is describing the Gossen Luna Pro, not the SBC.

Yepp, but still interesting to read.

The similar sounding names for different designs and two different names for the same design in the US/EU are annoying and really confusing if you are looking for an old meter or doing some initial research.

 

[wombat2go]

Yes I posted info about the much older LunaSix before realizing there was an "F" version. The 'F" version is probably using a more advanced electronics.
As it will cause confusion to o/p's query and as later archive, I should delete my post #4 -??

 

[AgX]

The similar sounding names for different designs and two different names for the same design in the US/EU are annoying and really confusing if you are looking for an old meter or doing some initial research.

Especially annoying is that Gossen themselves not only intruduced that double-designating (or did so on request from the importer) but also do not hint at that or offer a name table to look things up.

 

[Don Promillo]

Yes I posted info about the much older LunaSix before realizing there was an "F" version. The 'F" version is probably using a more advanced electronics.
As it will cause confusion to o/p's query and as later archive, I should delete my post #4 -??

I think it's quite clear, that you described a different model. I'd leave your description as it is.

I was able to tune the meter to plausible results. Now it's time for a field test and fine tuning.

 

[CobbPerrin]

As per my expereince multiple pots are there to balance particular points in the meters range.
Usually they're set to high, low and medium light levels and they DO affect each other. If memory serves LV 9, 12 & 14 were the settings on the tester we used. That's IF the memory worked.