Luna Pro F -- Needle Pegs On Battery Check

Old-N-Feeble · Sep 20, 2014

I have a Gossen Luna Pro F that's accurate in ambient mode according to side-by-side comparison with my Luna Pro SBC. However, the needle pegs to the far right on battery check. This happens in both ambient and flash mode. The installed battery is a new Kodak alkaline. Does anyone know what's causing this?

Arvee · Sep 20, 2014

I seem to recall that the manual says the indicator should be to the right of the mark to indicate the battery is good. So, to the extreme right is okay. I have an SBC and an F and, if I remember correctly, the F goes all the way to the right with a fresh battery.

andrew.roos · Sep 20, 2014

I don't have direct experience with the Luna-Pro F but I expect there will be a resistor wired in series with the battery check button to limit the current through the meter when the battery check button is pushed. If the resistance has decreased (or worse gone short-circuit) with age, then this could cause the effect you describe. If the resistor is wired in series with the button as I suspect then you should be able to check the value with a multimeter with the resistor in circuit, while the battery check button is not pressed, since one side of the resistor will be disconnected by the open-circuit button. You could then compare the measured value to the nominal value determined from the resistor colour code, and replace if necessary.

Update: I found a circuit diagram for the Lunasix 3 at https://farm9.staticflickr.com/8258/8686556384_f936f35ce3.jpg and it is as I thought - the 12k resitor in series with the battery check button limits current. Better still there's a battery test calibration variable resistor in series as well. If the Luna-Pro F follows the same general scheme, then you may just need to find and adjust the battery test cal resistor.

(Only saw Fred's reply after posting this. Adjustment may not be required after all. On my SBC it does not peg to the right and I assumed the F was the same)

Old-N-Feeble · Sep 20, 2014

Fred Aspen said:
I seem to recall that the manual says the indicator should be to the right of the mark to indicate the battery is good. So, to the extreme right is okay. I have an SBC and an F and, if I remember correctly, the F goes all the way to the right with a fresh battery.

Thanks, Fred. I wonder why Gossen used the same scale as the SBC then... with a reference high/low "window". Why is the "F" so different than the "SBC" in that regard? BTW, the manual I downloaded states the battery test to be precisely the same as the SBC model. Is this a double goof on Gossen's part?

Old-N-Feeble · Sep 20, 2014

andrew.roos said:
I don't have direct experience with the Luna-Pro F but I expect there will be a resistor wired in series with the battery check button to limit the current through the meter when the battery check button is pushed. If the resistance has decreased (or worse gone short-circuit) with age, then this could cause the effect you describe. If the resistor is wired in series with the button as I suspect then you should be able to check the value with a multimeter with the resistor in circuit, while the battery check button is not pressed, since one side of the resistor will be disconnected by the open-circuit button. You could then compare the measured value to the nominal value determined from the resistor colour code, and replace if necessary.

Update: I found a circuit diagram for the Lunasix 3 at https://farm9.staticflickr.com/8258/8686556384_f936f35ce3.jpg and it is as I thought - the 12k resitor in series with the battery check button limits current. Better still there's a battery test calibration variable resistor in series as well. If the Luna-Pro F follows the same general scheme, then you may just need to find and adjust the battery test cal resistor.

(Only saw Fred's reply after posting this. Adjustment may not be required after all. On my SBC it does not peg to the right and I assumed the F was the same)

Excellent... I'll have a look. It's fairly rare that an old-style carbon resistor goes bad though, isn't it? However, adjustable resistors do get grungy.

andrew.roos · Sep 20, 2014

Old-N-Feeble said:
Excellent... I'll have a look. It's fairly rare that an old-style carbon resistor goes bad though, isn't it? However, adjustable resistors do get grungy.

Not sure about carbon comp. resistors. However I have failures of thin film resistors due to attack by sulphur dioxide outgassing from rubber seals over a period of years. Strange things happen!

Old-N-Feeble · Sep 20, 2014

andrew.roos said:
Not sure about carbon comp. resistors. However I have failures of thin film resistors due to attack by sulphur dioxide outgassing from rubber seals over a period of years. Strange things happen!

I doubt that's the situation here plus the current draw is so very tiny that I just don't see how a typical old-style carbon resistor can fail. Of course, stranger things do happen and if there's a variable resistor in the circuit then that could very possibly be the culprit.

Byclops · Sep 20, 2014

In a similar thread in 2009 Lee L mentioned that the German instruction manual (translated to English) says " or stays to the right" referring to the needle movement on battery check. My Luna Pro F has the same behavior. No sweat.

Alex Muir · Sep 20, 2014

Mine also does this.
Alex

Old-N-Feeble · Sep 20, 2014

Thanks everyone... but you're stating that the needle in your meters peg to the far right of the entire range... not just to the right of the "battery okay window" (see pic)?

George Nova Scotia · Sep 20, 2014

Yes that's the way my F is also. The SBC status in the green box.

AgX · Sep 20, 2014

The needle has to be within that marked field OR right of it. (Which can be read as that field indicating the minimum voltage range for good operation.)

For its twin model, the Profisix, the manual says that needle should be within the marked field.

Strange wording at least...

Kyle M. · Sep 21, 2014

Old-N-Feeble said:
Thanks everyone... but you're stating that the needle in your meters peg to the far right of the entire range... not just to the right of the "battery okay window" (see pic)?

View attachment 94922

My Luna Pro F does the same exact thing and it's dead on.

Old-N-Feeble · Sep 21, 2014

It seems this is a common issue with Luna Pro F meters but I don't understand what's so different from the SBC model regarding battery check. Maybe Gossen received a large lot of faulty resistors that only went onto the F models?? Anyway, I guess all this means is the battery check is essentially useless because it tests with no drain on the battery.

sevo · Sep 23, 2014

The voltage is stabilised, so a bit too much will not affect the results. Fresh 9V block batteries now often have 9.6V or more, I don't know whether that was the same back then or is the result of formula changes (today's affordable batteries have twice the capacity of the best alkaline batteries available in 1980, so they are not quite the same...

Old-N-Feeble · Sep 23, 2014

sevo said:
The voltage is stabilised, so a bit too much will not affect the results. Fresh 9V block batteries now often have 9.6V or more, I don't know whether that was the same back then or is the result of formula changes (today's affordable batteries have twice the capacity of the best alkaline batteries available in 1980, so they are not quite the same...

I'm sure that's true but this doesn't affect the battery test function of the SBC models. Something is still different about the F models.

Luna Pro F -- Needle Pegs On Battery Check

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