light meter battery for a Pentax Spotmatic II

Discussion in '35mm Cameras and Accessories' started by spoolman, Jun 16, 2009.

  1. spoolman

    spoolman Subscriber

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    Does anyone know what type of battery powers the light meter in a Pentax Spotmatic II and where I can get one?

    Thanks,

    Doug:smile:
     
  2. BradS

    BradS Subscriber

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    they originally used a mercury 1.35volt PX400 cell.
     
  3. Rol_Lei Nut

    Rol_Lei Nut Member

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    Since their meters are relatively insensitive to voltage differences, you can basically use any battery in the 1.3-1.5 range which fits, including alkaline cells.

    Mine currently contains a LR936 watch battery, with a rubber ring on it so that it fits better.
     
  4. David Brown

    David Brown Subscriber

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    Google "Spotmatic battery" (or even search apug) and you'll get bunch of info.
     
  5. Anscojohn

    Anscojohn Subscriber

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    I found this on another website. I did not know this. And I have a Spot II and did not know this.

    Quote on:
    Availability of batteries for the Spotmatic is better than other cameras of the era because the Asahi engineers built in a bridge circuit in the metering which makes it battery voltage independent. This means you can use the original 1.35 mercury PX400 battery, or a 1.5V silver-oxide replacement without problems. An exact fit PX400S is available, or you can use a #392 cell with a small rubber-O ring (purchasable at any fine home repair center or DIY store) as a spacer. I believe a #397 also fits as well.
    Quote off
     
  6. altair

    altair Member

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    I have an SP500,and like u,i was stumped about the battery too.tried alkaline LR54 cells,but it made the needle of my sp500 work backwards,how's that for weird.then i tried silver oxide cells,Maxell SR936SW,and it worked fine.try it.
     
  7. Steve Smith

    Steve Smith Member

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    As posted above, the Spotmatic is a bridge circuit which is fairly independant of actual voltage.

    This is because a bridge circuit works with a centre zero meter and correct exposure is obtained at that centre point. This is the point where no current is flowing through the meter, therefore the voltage is irrelevant.

    However, the Spotmatic circuit is offset slightly so the correct exposure reading is slightly off of centre with a small amount of current flow. This was done (I think) to enable the battery check function to work).

    What this means is that the Spotmatic circuit is slightly voltage dependant but not by enough to worry about in normal use.

    In practice, any 1.5v cell which fits and connects o.k. should work. I too have an SP500 and I have had some perfectly exposed transparencies from it using a 1.5v alkaline cell.


    Steve.
     
  8. Vincenzo Maielli

    Vincenzo Maielli Member

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    In my Pentax Spotmatic II i use a Varta 392 silver oxide battery (1,55 volt), with a proper blue ring of the original PX400, to block the battery in the compartment. If you don't have the blue ring, use a proper circular black O-ring. The internal circuit of the all Spotmatics work very well with mercury oxide PX400 1,35 volt and with Varta 392 silver oxide battery, without any problem.
    Ciao.
    Vincenzo
     
  9. magkelly

    magkelly Member

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    I've used a 387S watch battery in my SP1 and it fits well and seems to work okay.
     
  10. Pumal

    Pumal Member

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    Try LR43
     
  11. Steve Smith

    Steve Smith Member

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    When experimenting with various cells in a Spotmatic, be aware that what appears to be the cortrect way round may in fact be upside down. It will not do any damage to the meter but a reversed cell will translate to reversed movement of the meter needle.


    Steve.
     
  12. holmburgers

    holmburgers Member

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    I bought a modern alkaline battery for a Spotmatic and at first it didn't seem to work, but the problem was that the original battery had a plastic insulator around it that prevented certain parts of it from touching the contacts. I wrapped a thin piece of masking tape around it and it worked fine then. So, if a battery fits, but still doesn't work, this is probably your problem.
     
  13. mgb74

    mgb74 Subscriber

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    My closest fine home repair center - Home Depot - used to have a display with about 3-4 dozen different size O-rings. Unfortunately, no more O-ring selection. All the more reason to visit your fine local hardware store.
     
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  15. jmschwartz

    jmschwartz Member

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    I've used this source for hard-to-find mercury cell replacement batteries for the Spotmatic and others: http://www.myoldcamera.com/
     
  16. CGW

    CGW Member

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    The not-so-hard-to-find but pricey Wein cells are zinc-air batteries. There are close fits among much cheaper hearing aid batteries for Pentax and other merc cell bodies.Alkalines will work with Spotmatics but be aware that they don't have the old merc cells' near-flat discharge curve. Tons of info on photo.net about merc cell replacement solutions. Just enter "PX625" or PX400" in the search box.
     
  17. Steve Smith

    Steve Smith Member

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    But as it has a bridge circuit it doesn't really matter.


    Steve.
     
  18. CGW

    CGW Member

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    Uh, it will when the cell's dying, dropping voltage, and giving "off" meter readings. Merc cells ran strong and just died, rather than fade. Zinc-air cells probably come closest to that if you're enough of a survivalist to futz with O rings and other hacks to keep an oldie working.
     
    Last edited by a moderator: Jun 21, 2010
  19. Steve Smith

    Steve Smith Member

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    As long as it has enough power to move the needle one way or the other when over or under exposing then the correct exposure will be indicated correctly.

    The Spotmatic's bridge circuit shows correct exposure when the same current flows in each side of the circuit. The meter shows the difference in voltage between the two middle points of the bridge and at correct exposure no current flows through the meter so the needle rests at its centre point. X - Y = 0 when X = Y. The actual value of X and Y is irrelevant.

    So this circuit will work well with mercury, alkaline and silver cells. You could even wire up a couple of potatoes with iron and copper nails as conductors if you wanted to!


    Steve.
     
  20. CGW

    CGW Member

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    Guess it's why I retired my old Spots and Minoltas since the difference between a working meter and an accurate became evident. Some proved linear but off +/-. while others were erratic and undependable--neither being surprising for 40+ year-old cameras.
     
  21. Steve Smith

    Steve Smith Member

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    Minoltas like the SRT series definitely need a good 1.35v source. Using a silver oxide cell with a schottky diode wired in series with the battery is a common modification.

    If you are using a 1.5v cell on a bridge circuit camera like the Spotmatic, although it will be accurate at normal exposure, the needle will go to a different point if it's under or over exposed so if you had a point to which you always set it e.g. for two stops extra exposure for a backlit scene, you will need to re-learn its over expose position.

    Steve.
     
  22. granathg

    granathg Member

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    This is a belated post. I too have an old Honeywell Pentax Spotmatic (stamped ASAHI Opt Co. Japan) that originally had the mercury PX-400 battery. I bought what my batteriesplus rep said should be a compatible replacement (silver 394X with a white ring, not a blue one - supposedly 1.5v). Meter still doesn't work. Does this suggest a camera meter circuit problem or is there something else I can try? It has been several years since I used the camera and the last time I replaced the battery I used a European-made mercury type. Worked fine.
     
  23. jochen

    jochen Member

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    Hello,
    i have a Spotmatic (ca. 1967) with the 394 battery and a self made plastic-ring which I have cut from an old ballpoint pen. It works and the calibration is o.k. I compared it with a Gossen Lunasix F. Did you insert the battery with the correct polarity? Maybe you should measure the electrical circuit with an Ohm-meter.
     
  24. granathg

    granathg Member

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    My Spotmatic is approximately the same age. I think that any 1.5V silver oxide battery of the correct size should work (I have several plastic rings) but neither of the V400PX silver oxide batteries made by Dantona work. I have an ohmmeter. Would I connect one lead to the contact in the camera and the other lead to the body?
     
  25. granathg

    granathg Member

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    Jochen,
    I tested the battery receptacle with ohmmeter and there is no continuity - an open circuit. I tried two other 1.5v silver oxide batteries and the exposure meter still does not work.

    When you do this continuity test on your own Spotmatic, what kind of reading does your ohmmeter give?

    I'm puzzled that after sitting on the shelf for 10 years that the bridge circuit would, for no apparent reason, go bad. I will wait for your reply to my question (please let me know). If you can confirm that the ohmmeter test proves that the bridge circuit is bad, I will see if a camera shop can repair it for a reasonable price.
    Gary
     
  26. Rol_Lei Nut

    Rol_Lei Nut Member

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    A hint: the likely culprit is the contact on the meter switch (assuming the battery compartment contacts are clean).
    Try "excercising" it a bit.

    If you like the camera, then it's time for a CLA anyway...