Gossen Luna Pro sbc does not Null

[HpS]

Pulled out after many years my Luna Pro sbc, loaded a new 9V battery and found:
1.) The battery test button works.
2.) On reading the light the needle responds but I can not Null the needle to the Zero mark. The needle freaks out at the Zero and does not stay there.

Anyone here knows how to go about this please?

 

[PaulDalex]

I had the same problem withe the Lunasix F
The culprit is the potentiometer. The meter is clearly based on a Wheatstone bidge and the needle is zeroed by means of a potentiometer which, aging, becomes noisy
There are two possibilities
Either you clean it (I used alcohol and other methods). I have just done it and I fixed it. Well almost because the needle zeroes correctly if you turn the wheel slowly
Or you substitute the potentiometer. This is easy, provided you can find one. I am thinking of trying a meter for parts.
The disassemble is quite easy although tricky at certain stages.
I intend to post about the disassembly as soon as I can take my time to do it.

 

[HpS]

Thank you PD.
Look forward to your post on how to take it apart.

 

[PaulDalex]

BTW welcome to the Forum

 

[PaulDalex]

I will try to do my best.
I have taken photos. The problem is it takes time to reduce their size to post them.
I can anticipate that no fancy tools are needed. Just a plain screwdriver for watches.
Actually the design of this meter is amazing for me.
The Wheatstone architecture is a high precision approach, because the meter is used at a single point of the scale.
Unfortunately potentiometers do not age well. And Gossen has interrupted selling parts. Too bad!

 

[PaulDalex]

Please notice I have a Lunasix F
The Luna Pro SBC looks quite similar but I cannot be sure that they are identical inside, or anyway similar enough to be fixed the same way.
I got mine from a 100% Ebay seller many years ago, and the seller claimed full functionality.
The rating and claims made by such sellers clearly mean very little. The seller did not answer my mails protesting that the needle was jumpy. So I put the meter aside hoping to have one day the time to try to fix it.

 

[PaulDalex]

This afternoon (here in Rome) I will post about the disassembly and fixing

 

[HpS]

I had the same problem withe the Lunasix F
The culprit is the potentiometer. The meter is clearly based on a Wheatstone bidge and the needle is zeroed by means of a potentiometer which, aging, becomes noisy
There are two possibilities
Either you clean it (I used alcohol and other methods). I have just done it and I fixed it. Well almost because the needle zeroes correctly if you turn the wheel slowly
Or you substitute the potentiometer. This is easy, provided you can find one. I am thinking of trying a meter for parts.
The disassemble is quite easy although tricky at certain stages.
I intend to post about the disassembly as soon as I can take my time to do it.

Thank you for your welcome.

You are right, the two look identical, as are the two buttons on the left, for battery test and power. I guess the potentiometer should be a fundamental assembly and perhaps common to both the models.

Your Lunasix F indicates it is good for flash light readings. My Luna Pro sbc is an ambient light meter. (To take Flash readings one has to use an accessory that clips on the light cell/diffuser end).

When you said "cleaned it with alcohol and other methods" - did you mean you cleaned the Potentiometer only?

And, on the Lunasix, does the front 'Sun & Moon' circular plate, unscrew or is it glued to be pried open to get to screws underneath, to open the front dial?

Thanks for your replies!

 

[PaulDalex]

Yes the potentiometer only.
No disassemble of the front wheel!
I will now illustrate the procedure. Naturally I don't assume any responsibility i anyone follows my indication. Attempts to fixing is at your own risk, and the possibility of doing serious damage is concrete.
Here are the photos of my meter:

 

[PaulDalex]

And the photo of rear

 

[PaulDalex]

Now I recommend that you take many photos going on and that you have ready a container for small parts to avoid loosing any. I use a film box for that.
Look at the rear. My guess was that the back panel were glued (or also glued).
I was wrong! So no need of an hairdryer and most of all DO NOT STICK THE BLADE OF A KNIFE UNDERNEATH THE REAR METAL PANEL.
The panel is kept in place by a single tiny golden screw which you see at the center of the panel.

 

[PaulDalex]

Remove the screw. But before doing this note the other black screw. This is the mechanical zero adjusting screw. It is blocked by the panel and will be loose when you remove the panel. Take note of its position and store safely in place.
Now remove the panel. Under the panel you will see a circuit board and you will note 4 screws

 

[PaulDalex]

What You See After Removing The Panel

 

[PaulDalex]

Now be careful. There are four golden screws at the perimeter of the circuit board. In the photo three are removed one is left. This is above the circled A. The other two are one above the R39 sign and the other symmetrical on the opposite site.
Remove the screws. After that DO NOT TRY TO REMOVE THE BOARD!
In fact the screws are there to fix the two outer shells to each other!
Now gently separate the two shells. This will loosen both the incident light dome and the transparent screen that protects the cell. Take good note of how they are positioned at this point.

 

[PaulDalex]

Here is the image of the front shell.

 

[PaulDalex]

In the center sits the potentiometer. You cannot miss it: the 3 golden prongs are the contacts of the potentiometer. These prong engage three contacts on the other shell.
To be in the safe side I cleaned the prongs although I think they are innocent.
Now as you see there are two golden screws that fix the potentiometer.
Unscrew them and the potentiometer comes out.
NOTE THAT THE CURSOR OF THE POTENTIOMETER HAS A HOLE THAT IS ENGAGED BY A SHAFT UNDERNEATH THE WHEEL IN THE FRONT OF THE METER.
THIS COUPLING MUST BE RESTORED WHEN YOU REASSEMBLE

 

[PaulDalex]

In this photo you see the potentiometer removed

 

[PaulDalex]

Note the shaft on the rear of the wheel and the hole on the cursor of the potentiometer.
Now you can try to clean the potentiometer.
THERE IS NO GUARANTEE THAT THIS WILL RESTORE FUNCTIONALITY.
The only safe way is to SUBSTITUTE THE POTENTIOMETER WITH ONE IN PERFECT SHAPE.
Anyway this precision potentiometer is realized as a circular coil around which a metal cursor moves

 

[PaulDalex]

Very very gently clean the coil. LET THE CURSOR ALONE!
I used a cotton handkerchief with a little pure alcohol on it. Then I made a second pass with one of those pens to relieve the batteries contacts.
I made a third pass with Marine CRC.
When I was a boy I used Carbon Tetrachloride, which should be very efficient for cleaning electronic parts. However, this is an extremely toxic and cancerogenic substance and I am sick on my own and have no intention worsening my health.
Once you are done with cleaning. you can reassemble the meter.
In my case the potentiometer is still noisy, but if I turn the wheel (i.e. the cursor) slowly the meter zeroes properly.
Good luck and best regards.
Fell free to ask any question, I will be happy to help

 

[HpS]

Thank you so much PD. Truly appreciate your detailed advise.

Armed with all the knowledge from you, I contacted a trusted TV repair engineer known for over thirty years and showed him my meter, sharing information you have provided.

You will be surprised by the story now:
1.) He said he has never repaired such a meter and immediately understood and agreed the fault lies with the potentiometer.
2.) Then he advised that we should not disassemble it, as he felt that old plastic gadgets can get brittle and be risky, as something else can break apart while disassembling. (Especially with extreme temperatures in India).
3.) Keeping in mind the fact that my meter has not been used for 20+ years, he advised that definitely the carbon slider of the potentiometer is oxidised.
4.) And has asked me to keep using the meter for a few days and keep moving the dial left and right. He said the movement will clean off the oxidised carbon strip. And told me to take readings in bright sunlight as the condensers have also got discharged over all these years of not being used.
5.) Now, I have been doing that since yesterday and you will be surprised to know the needle has already begun to respond to the turning of the dial.
6.) In bright sunlight readings the dial Nulls the needle now. Its still is a little crazy in low light readings.
7.) I am going to work on this for a few more days and share with you how it behaves finally.

I want to ask you one more question: Have you opened the front dial? How does it disassemble? I want to clean out some dust that can be seen moving inside the dial.

Thank you so much for your help PD!!