Olympus OM-1n exposure meter conversion

Discussion in 'Camera Building, Repairs & Modification' started by Chris Douglas, Dec 18, 2011.

  1. Chris Douglas

    Chris Douglas Member

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    Yup, another discussion about mercury batteries, but different than I could find in the archives. I don't want to re-start the same old discussion on Wein cells and hearing aid batteries. The best solution to the mercury battery problem is to adjust the meter circuit to use silver oxide bateries. I say this after looking at the curves of voltage verses temperature and current of the series diode solution. The series diode is a "sloppy" solution. I am looking for someone who has adjusted the meter to use silver oxide cells who is willing to share the procedure. I have done camera repairs, CLA and electronics bench work, so I am not afraid to tackle the job, I just don't want to "re-invent the wheel". Thanks.

    Best regards,
    Chris
     
  2. paul ron

    paul ron Member

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  3. Jerevan

    Jerevan Subscriber

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    Can I assume you mean the zener diode solution (like the CRIS battery converter)?

    Maybe John Hermanson has any ideas? (www.zuiko.com)
     
  4. Chan Tran

    Chan Tran Member

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    I generally would simply forget about the meter and not using it. If I have to tackle this problem I would want first a schematic diagram of the meter circuit. From there one could study how the meter work before trying to figure out how to correct this problem.
     
  5. ic-racer

    ic-racer Member

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    Another reason I like the FX-3...built in voltage regulator 1.25V +/- 0.15V in the meter circuit.

    I'm sure there are a lot of busted FX-3s out there with intact voltage regulators. Perhaps it could be used in the OM1 circuit ("Vreg" on the diagram).

    Too much bother for me; I use zinc cells in all my equipment that requires mercury cells.

    [​IMG]
    [​IMG]
     
  6. BobD

    BobD Member

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  7. paul ron

    paul ron Member

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    I've been using the over the counter alkiline replacement batteries in my old meters n have no problems with my negatives or readings. I think if you just standardise your meter to your film, you will find the meters are very accurate n reproduceable. If you keep comparing meters to other meters, even the best of em will not agree msot of the time. Meters are for exposures, not racing against eachother. And BYW the battery last so long it will take several years to experiance the voltage drop off point. Rplace your battery regularly and you won't have any problems.

    I like that voltage regulator circuit in the FX3 idea but still takes up too much room.
     
  8. wiltw

    wiltw Subscriber

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    Oh, horrors! Many years ago I conducted a test with my OM-1n (I am the original purchaser), with mercury oxide cell and with alkaline cell. There was definitely an error in the meter reading when I used alkaline 1.5v battery rather than the 1.35v mercury oxide cell. The amount of metering error is NOT constant and cannot be simply compensated with adjustment of ISO value. The amount of metering error depends not only upon the precise voltage being out out by the alkaline chemistry (which varies as the cell depletes), but also based upon the amount of light in the scene being metered!!!
     
  9. ic-racer

    ic-racer Member

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    ???? the voltage regulator is a tiny, only a few millimeters long. It is "Vreg" in the second diagram and is so small it does not show up in the picture of the camera with the top off. I don't have a parts camera on hand at the moment but now that I think about it I suspect that it is a Zener diode and a resistor! Again it specs out at 1.25V +/- 0.15V. Why don't you like Zener diodes?
     
    Last edited by a moderator: Dec 19, 2011
  10. paul ron

    paul ron Member

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    WOW I'll definately have to see that regulator, sounds interesting. That will level the playing field for sure.

    Metering is not accurate compared to what? Can you reproduce the same readings of the same 18% gray card over n over in the same situations using alky bats? It only has to give you a reproduceable reading that you standardize to your film vs develping vs printing to make it do exactly the same thing every time. Meters are only as stupid as the user.

    Whetstone bridges are a balancing act of current. They don't know light... just curent. It reads the photo cell as a variable resistor limiting curent on one side vs the other so the same amt of current will make the meter deflect the same amt every time. Linear? No, log.

    Depleation of batteries is a few years down the line. I had batteries in my analog PentaxSpot meter for 5 years that kept saying the same florecent overheads in my shop are exactly the same exposure as the day I put em in. Change the battery regularly and you will never see the end of the curve where it breaks down. How long do your batteries last? Do they depleat every time you take a reading?

    BTW if you have any old spotmeters (maybe any meters or cameras) that use mercury batteries you want to throw in the trash because they are not accurate enough with alkys, I'll take em all off your hands, I'll pay the postage. PM me?

    Anyway... to each his own, how do your pictures look in the end is all that counts, not how accurate a meter you have. Many old timers never use meters, I hardly ever, except maybe as a referance starting point. I see it as a crutch like having a GPS take the place of my map n compass n the brain between my ears.

    .
     
  11. ic-racer

    ic-racer Member

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    Speaking of Wheatstone Bridge, here is a neat little meter circuit from the 16mm Minolta 16QT. Instead of a galvometer in the middle of the bridge there is a differential amplifier. The two bulbs alternate blinking when it is balanced. I'd give this one the award for "Most Elegant Meter Circuit" of the film age.
    [​IMG]
     
  12. John Hermanson

    John Hermanson Member

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    OMG, making it way too complicated. Schotkey diode 1N-6263 in series with the battery. Meter accurate and linear within 1/8th of a stop (after calibration) and since most are shooting print film these days, that "inaccuracy" easily falls within the plus or minus 1.5 stop (or more) latitude of the film. Then there's shutter speed latitude, film asa latitude, f stop latitude, the joker making minimum wage at the drug store pushing the "start" button on the "automatic" machine developing your film. John
     
  13. paul ron

    paul ron Member

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    Well I guess the diode is the way to do it. Nice to come up with a bettter solution though, maybe something a bit more accurate.

    BTW what is the voltage drop on that diode?... most are around 1 volt, how do you conpensate for it?

    .
     
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  15. ic-racer

    ic-racer Member

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  16. paul ron

    paul ron Member

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    The size of the transistors alone are way too big for a compartment, even the IC I listed still much smaller but also too big. Although the FX reg is by far the best of em all.

    Do you have any spares to experiment with? Maybe KEH "as is" bin has a busted camera for a few bucks I can glom one out of?

    .
     
  17. John Hermanson

    John Hermanson Member

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    As stated, any "accuracy" is lost in all the other latitudes. My OM-1/1N meter conversion is on the money at BV15, 12 and 9. at BV6 it's about 1/16th of a stop off. Plenty accurate, and to get more complicated is wasted labor (especially with negative/print film). This is perfect even with slide film. Voltage drop is about .2 volts, so working voltage from a silver oxide 1.55V battery (which is really around 1.6V) is approx. 1.35V. John
     
  18. Chan Tran

    Chan Tran Member

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    I never really care about the accuracy of these old cameras' meters but for the sake of it I think may be one can build a battery grip using AA with room (and current) for a rock solid 1.35V supply.
     
  19. paul ron

    paul ron Member

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    Hey Chan that is a great idea, a grip with the regulator buiilt right in.

    I have never used the meter in my OM1MD or any of my 35mm cameras. For some reason they just nbever seemed accurate and were always finiky.
     
  20. ic-racer

    ic-racer Member

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    I had a look at an original circuit board from the FX-3 and it looks like I made a mistake. The Vreg in the diagram is just a test point. The actual regulator is incorporated into the integrated circuit and is not a separate component.

    I think one could make a tiny regulator based on a Zener diode like this: http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/zenereg.html
     
  21. paul ron

    paul ron Member

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    So the Shotky is the best solution for now.. or the regulator in the grip idea.
     
  22. ic-racer

    ic-racer Member

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  23. paul ron

    paul ron Member

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    SMDs are the way to go n very little perifery needed.

    Nice find.
     
  24. wblynch

    wblynch Member

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    I think John's solution is the simplest, proven and most cost effective. I mean 1/16 of a stop is more accurate that the camera itself. Why complicate things?
     
  25. Chan Tran

    Chan Tran Member

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    1/16 stop? I have never seen a meter with that much accuracy not even hand held meter. Handheld meter has resolution of 1/10 stop and some of them are accurate to that much but not all those that can display reading in 1/10 stop.
     
  26. John Hermanson

    John Hermanson Member

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    Why complicate it, indeed. There is no 1/16th stop obviously, but for the sake of "anality" how accurate does it have to be? ;-) In this example, completed repair at BV 6 is one needle thickness off center (and exactly where IS the EXACT center mark in the OM-1 meter display?) There isn't one, so I estimated "how close is this to zero on this $5000 Kyoritsu EF5000 shutter tester? Crap, it's a needle thickness, let's say 1/16th stop" The "error" is so minute it just doesn't matter. I've been servicing OM for 34 years, and in the 15 years using this 1N-6263 conversion with silver oxide battery, not one complaint regarding "accuracy". John