Gossen profisix light meter repair.

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awty

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AgX said:
So this way of calibating then is very different from that of the Lunsasix 3.


low, linearity, high

versus

offset , slope
Click to expand...

Which is off set and which is slope and what do they do exactly?
The middle trim pot marked in red is for the battery test.

 
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awty

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Also have a Lunalite from the same bundle.
The middle LED stayed on most of the time, underexposed LED occasionally would flicker and the overexposed LED didn't come on at all.
I unscrewed the back plate, checked the soldered joints, resoldered everything that looked suspicious. Still no change. Unscrewed the four screws and unsoldered the three legs on the main pot.




And it came apart easy.



Unsoldered the overexposed LED to test if it were dead. Checked fine.





Reinstalled.
Gave all the trim pots a blast of circuit cleaner, marked their position and turned them to and fro till I got the overexposed LED to light up. Did the same with all till they were all working.....Yippee!
Started reassembly
Cleaned the sensor glass and sensor and incident cover.


Put it back together, with screws and resoldered the three lugs.



Adjusted the trim pots as best I can till both meters were the same.

Both meters are very responsive and work, now have 2 very good working meters.


Profisix manual
 
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AgX

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awty said:
Which is off set and which is slope and what do they do exactly?
Click to expand...

I hinted at the repair manuals for the older Lunasix 3 models
There this terms are used.

As a graphic is more telling than words, here the gaphic taken from the manual:


So you take a deliberate luminance (best at one side of the range) and adust the offset until the displayed value is correct. Then you adjust the slope until all other values are right too.



At the Lunasix F (and likely the Profisix) you instead adjust for offset one value at either end of the range. Then you adjust the curvature (linearity) of the line inbetween, by adjusting for some middle values.
So here you need 3 pots instead of 2 at the older model.
 
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awty

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So using a Nikon f100 on spot meter and a grey card. I was able to determine full sun on a cloudless day was ev15, a darken room was ev2 and a shaded patio was was ev8.
The pot marked in red has something to do with voltage, but I do not know where and how to measure, which has an impact. The other two need to be adjusted to. After a lot of going back and forth I have both within a 1/3 of a stop of each other. Good enough until AgX does a full demonstrational you tube video to the music of Kraftwerk!
 

ags2mikon

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awty, you did very good here. You have been a big help to the rest of us. I have 6 of these things and all but 1 is within 1/3 of a stop of the others. 1 is 1 stop off. So it needs some tweaking. Thank you for the write up and pictures. What a great forum!
 

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The art of adjusting all three pots so that the response across the light range agrees exactly without and drift or non linearities is an art to someone without all the laboratory testing environment and a career in that field.
 
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awty

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I would suggest it might be just a poor contact somewhere, you could mark the orientation of each pot and just turn them in and out a few times, usually helps. If not circuit cleaner will help clean.
These are great meters that going by the price tag on one used to go for $275 40 years ago, that was a week's wages for me back then. Working ones still sell for a good price, worth keeping them going.
 

AgX

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awty said:
The pot marked in red has something to do with voltage, but I do not know where and how to measure, which has an impact.
Click to expand...

awty said:
The [two marked in yellow] need to be adjusted too. After a lot of going back and forth I have both within a 1/3 of a stop of each other.
Click to expand...

Which of the two yellow marked ones affects what end of the range?
 

AgX

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To avoid a misunerstanding,
meanwhile we are no longer discussing the Profisix, or the Lunasix F (both with that big round hole in the circuit board to access the mechanical zero adjust of the dial), but are now talking about the Lunalite in same casing and with same dial, but instead of a pointer and a graded nulling-scale, just with a three-lights nulling-indicator, and thus no mechanical adjusting from the rear, thus a different circuit board.
 
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awty

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Yes the hole on the back to access screw is zeroing the needle or putting it on green marker like photo.




Appart from that the two circuit boards are similar. The one in the middle adjusts the voltage and the other two adjust the frequency. You need to first set the voltage then adjust the frequency and checking the voltage at same time. I adjusted the voltage so the LED and needle were in the middle when pushing the battery tester. I have no idea if this is correct. Someone could tell me where the needle rests when testing battery.

I'll take the back off the pro later and take a photo, but the trim pots on both are in similar position, one is just harder to find in the only photo I have
 
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ags2mikon

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Most of mine rest over the "a" in "batt" with a fresh battery. As the battery starts to go it goes to the "b" and then off the box.
 
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awty

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ags2mikon said:
Most of mine rest over the "a" in "batt" with a fresh battery. As the battery starts to go it goes to the "b" and then off the box.
Click to expand...

Thanks. I would think that there would be a measurable voltage, just dont know where or what. If you turn up too high you lose adjustment on the other two trim pots......but I haven't delved in too deep.
Its easy to set to a grey card, but with real world reflections you lose a bit more accuracy with out knowing values to set to.
 
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awty

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Gossen profisix
Same set up as the lite.
Set trim pot in red for battery so needle is in green box.

Adjust the blue trim pots for light, one in more sensitive to bright light. I adjust one in outside light and other for inside light. Keep adjusting till the reading is accurate for both.
Used a Nikon f100 with centre weighted metering to compare. Mostly it is now with in a third of a stop, may need further fine tuning. It's not rocket science, I'm bored with it now, time to fix something else or make a picture.

Profisix
 

AgX

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So we now got two different calibration approaches using different potentiometers...


awty: for the Profisix, Lunasix F, and Lunalite

the three pots accessible, after openening the casing, from the rear of the circuit board


our french collgue (@post #24): for Lunasix F (and seemingly Profisix)

three pots (out of five) accessible from the battery compartment
 
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