Gossen Lunasix 3 circuit diagram

I have a few cameras that need a light meter. I have an old Lunasix 3 which was excellent when it was new and mercury batteries were still available.

Of course I would prefer a Gossen Digiflash but it is beyond my budget.

You will find tons of writings about replacing mercury batteries with other types and people try to sell you funny adapters to make alkaline buttons work in the Lunasix and others.

Since my Loonie had a severe corrosion problem I decided to open it and see how it works.

Above is the circuit diagram.

AFAIK the Luna Pro is the same thing.

There are no electronics in the Lunasix 3.

It is just a battery, a CdS cell, a meter movement and a bunch of resistors.

The only nonlinear element is the CdS cell. The rest is linear.

Therefore it doesn't matter if the battery voltage is 2 x 1.5 V (alkaline) or 2 x 1.35 V (mercury) as long as you calibrate the meter with the used battery type.

Load a new set of PX625G or similar.

Adjust the battery check so the needle points to the upper end of the red field when you move the bat.-check slider.

Calibrate both measuring ranges with a known good light meter.

Check the batteries regularly.

Replace when the needle is below the red field.

Alkalines don't discharge at constant voltage like mercuries used to do, so you have to change the batteries more often but alkalines are dirt cheap - they still last about one year of regular use.

  • Sean Tubridy PRO 7y

    Wait, where is the "get directions" button? I think I like google maps much better.
  • 8thcross 7y

    Very kewl. Thanks for the info!
  • Scott PRO 6y

    I have a Vivitar 45 meter that is made to use a 675 1.35V mercury battery. I read that to get accurate readings with 1.5V SR44 silver-oxide battery, I would need to put a 1N5711 schottky diode into the circuit to bring the voltage down closer to 1.35V.

    Inside, there is a circuit board above the meter CdS cell (or cells). There are three semi-circular things in a row on there which I assume are either variable resistors or potentiometers (they have a visible semicircular carbon track, with a brass dial riveted in the center which looks like it can be pushed clockwise or counter-clockwise) . Someone told me that one is probably to set the battery-check reading, and the other two are for the low and high-range readings. Does that sound correct?

    Would adjusting the battery-check be as good a solution as adding the diode?

    I don't know much about electronics, so I need a few tips. thanks!
  • Georg Holderied 6y

    I don't know how the Viv45 works - maybe similar.
    While it is possible to recalibrate Mercury based light meters for other battery voltages (Batt-check often !) it is not a good idea.
    I bought a modern light meter a while ago that is small, accurate and measures Flash too.
    It cost more than the average digital camera but it has reduced the number of wasted (Polaroid) film significantly.

  • Colin Johnson PRO 6y

    You can get batteries for these meters w/o having to modify them. The Wein MRB625 (PX625/13) Cell 1.35v Zinc-Air Battery is what I use and you can get them at B&H Photo.

  • Georg Holderied 6y

    You have to adjust such an old meter anyway so why not adjust it to a cheap alkaline ?
    Zinc-Air batteries aren't constant voltage and don't last long.

    BTW I've bought a Digiflash since..
  • Andipo 5y

    Nice work ! I don't understand the notation & inclination though. Does it have something to do with the potentiometer adjustment ?
  • Georg Holderied 5y

    Offset shifts the curve up and down on the Y axis.(offset)
    Inclination tilts the curve.(vernier)
    L and H relates to the low and high ranges of the meter.
  • Andipo 5y

    Thank you Georg. I'm replacing the LDR on my Lunasix 3. Since I can't get the exact type here, I have to use different type.

    I'm going to post it on my blog once finished. Could I use your schematic ? With link to original and proper credit of course.

    My blog :
  • Nazar Artykula 5y

    actually zinc air batteries have a better discharge curve than alkalines do, but sr44 are best
  • francesco locuratolo 5y

    ciao Polapix, vorrei chiederti una cortesia: Ho rotto il mio esposimetro lunasix3.
    E caduto rivolto a terra con la ghiera di regolazione. Il corpo non e rotto, ma e incastrato,
    e non riesco più a fare girare la ghiera di regolazione. ho interpellato vari laboratori di riparazioni con risultati deludenti. Ho anche mandato una e-mail alla GOSSEN, non ho mai ricevuto una risposta.
    Su Google ho cercato vari indirizzi, ed ho trovato te. Vorrei riparare il mio lunasix3, tu per caso sai come si smonda? per sbloccarlo?
    Comunque vadano i risultato del tuo aiuto ti ringrazio
    flocuratolo@yahoo.it f.locuratolo@alice.it
  • nahanni•whisky™ 4y

    the Lunasix 3 is one of the most reliable meters ever built. thanks.
  • Andipo 4y

    I have one where the Cds cell pin was broken. It is impossible to resolder. Any ideas for what kind of replacement I need ?
  • Mercedes 4y

    Alkaline batteries don't have a constant voltage over their lifetimes. The voltage when they're new is different from the voltage when they're old. Meters like this rely heavily on a constant voltage, so it's impossible to adjust it for alkaline batteries unless you want to be opening it and adjusting it each time you use it, as the voltage drops.
  • Georg Holderied 4y

    Mercedes So you have to throw away all CdS meters and -cameras..
    Please read what I wrote.
    You can use alkalines if you (1) calibrate the meter for the different cell voltage, (2) check the voltage regularly with the battery check function.
    It will not last 5 years like a mercury cell but -depending on usage 1 year or more.
    It will outlast a Zinc-air cell.
    To avoid those problems buy a modern Silicon based light meter.
    I am happy with my Gossen Digiflash though I wish those Lithium batteries would last longer.
  • gblagnys 3y

    What is nominal resistance of R4?
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