Vintage Spot Meter Refurbish

Discussion in 'Camera Building, Repairs & Modification' started by wombat2go, Sep 16, 2013.

  1. wombat2go

    wombat2go Subscriber

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    I found a vintage "HoneyWell Pentax 3/21 Exposure Meter" and purchased for $25, non working with a nice case and the original instructions.
    The problem with these as with other old exposure meters is that the batteries are either unavailable or equivalents are costly and of different sizes to the originals.
    This one needed a 1.3 V Mercury cell as well as a 22.5 V Dry cell.

    Mainly a hobby project just to get it working again, the meter will be useful with my Graflex, the home brew cameras and the old Nettar folder.

    The unit was dismantled and the lenses and body were cleaned with i-p-alcohol.
    The optical circuit consists of a 100 mm f/2.5 lens , a semi silvered and greenish mirror at 45 degrees, reticle with a scale, an eyepiece and a Light Dependent Resistor -LDR (Cadmium Sulphide Cell).
    The eyepiece (I think called parfocal, not sure in this context) allows all distances to appear in focus, also the scale and meter needle.
    Some light goes up at 90 degrees into the eyepiece giving an upright but mirror image, to allow the small 3 degree circle to be positioned for a measurement.
    The remaining light goes straight through to the LDR.
    Generic LDR have a log10 gamma of about 0.5 and a dark resistance about 1 MegOhm and a bright resistance of 10kOhm of so.
    Testing the LDR indicated that it was probably still OK. Testing the meter indicated good, its impedance was 4160 Ohm and full scale deflection was 40 microAmp.

    Tracing out the circuit showed that it is 2 separate one loop circuits, the primary one using the 1.3 Volt cell and a set of load resistors, with a microswitch closing if the button is fully depressed to select the 22.5 V battery and another set of load resistors for low light levels. No compensation is provided for ambient temperature or reducing battery voltage.
    There were 2 metal battery containers.
    The larger one was a good fit for the standard 522 9V battery...... Ahh..So...

    To convert the meter for operation on a single 9V battery, a simple one transistor voltage follower circuit was used, using the microswitch to switch an appropriate load resistor in series with the LDR, to replicate the meter currents of the original 2 circuits. The LDR is now sourced from a zener stabilized 5.1 V supply to maintain accuracy when battery is cold or partially discharged. The gain of the transistor allows reduction from 22.5 volts to 5.1 volts in low light mode, as well as acting as an attenuator, at 5.1 volt, for the 1.3 V normal mode.

    Circuit simulator "Qucs" was used to optimize the component values.

    It probably would not have been feasible to use a single transistor in the early 60's when this thing was designed, due to the lower betas and thermal drift in those days.

    The battery case for the 1.3 Volt cell was removed to allow for a small prototype circuit board to be fitted into the space.
    The biggest difficulty and time waster on this project was re-wiring the microswitch and getting it assembled correctly in the confined space.

    The completed unit was then calibrated by its trim pots using the Pentax k-01 camera in spot metering mode.
    After calibration, a comparison was done to compare the modified HoneyWell Pentax 3/21 Exposure Meter with the Pentax K-01 and Pentax MX camera over various light conditions as follows

    Object---------------------------------Gray House-----------White Garage Door----------------Concrete Floor indoors---------Cloudy Sky-----------------
    Pentax K-01 @ iso 400------------------f/8 & 1/400----------f/8 & 1/640----------------------f/8 & 1/15---------------------f/8 &1/2500----------------
    Pentax MX @ iso 400--------------------f/8 & 1/60-----------f/8 & 1/125----------------------Low range----------------------f/8 & 1/1000---------------
    Pentax 3/21 Exposure Meter , iso 400---f/8 & 1/250----------f/8 & 1/640----------------------f/8 & 1/10---------------------f/8 & 1/3200---------------

    The MX exposes well, it looks like I have calibrated the meter about 1 stop under.
    The K-01 was in spot mode, histograms show it is metering for about 127/255, 50% of its saturation.

    I put up the circuits in case anybody wants to modify a similar meter
    The original Circuit
    https://app.box.com/s/taapfr676p2qcue30818

    The new circuit
    https://app.box.com/s/w5vw6gr73n5fke4018k2

    The case open showing the new circuit board
    https://app.box.com/s/1wrt2omxo2uvo8lktox9

    The finished meter with its original case and document
    https://app.box.com/s/esmm6jilqnw92i5aj8z3
     
  2. E. von Hoegh

    E. von Hoegh Member

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  3. wombat2go

    wombat2go Subscriber

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    Yes, Eveready 412 is listed as a replacement. But I see it is not currently available on that site
    When I drilled out the rivets for the contacts of that battery, the holder became a neat fit for the 9V smoke alarm battery plus its insulated press on terminals.


    The Eveready mercury Cell number was E-1
    The holder for it was corroded and I removed it.
     
    Last edited by a moderator: Sep 16, 2013
  4. shutterfinger

    shutterfinger Subscriber

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  5. Tom1956

    Tom1956 Inactive

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    If you cut open a 22.5V battery, I wonder if you'd find 15 batteries somewhere in the AAA to AA size range. I've made Hasselblad EL packs and RC flight-controller packs and such.
     
  6. dmschnute

    dmschnute Member

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    Nice modification to the old Pentax meter! You clearly know more than a little about electronics; apparently it was entirely worthwhile. Thanks much for sharing the details and schematics. Very nice of you!
     
  7. ic-racer

    ic-racer Member

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    You must be thinking of a bigger battery. Those 22.5V batteries are about the size of a single AA cell. I'm not sure what is in there, perhaps a stack of thin button cells. BTW those 22.5v batteries are also used in the Horseman electronic solenoid shutter release.

    To the OP, thanks for sharing this project.
     
  8. Tom1956

    Tom1956 Inactive

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    I believe I was thinking of the 510V battery. Sorry. Now the bigger question is what's inside a 22 1/2 volt battery, but not opening the topic for an answer. Just not a need-to-know priority for me. Thanks. Undoubtedly a cell stack of some kind.
     
  9. wombat2go

    wombat2go Subscriber

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    Tom, I remember pulling those old carbon zinc batteries apart as a kid, not sure if exactly that part number though.
    They consisted of rectangular cells stacked in series to make the "pile" and clamped by a wrap.
     
  10. Chan Tran

    Chan Tran Member

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    Wombat2go! I would need your help here! I am trying to figure out your circuit which I believe that it works. How do we have the current thru the meter movement proportional to Log of the light value hitting the LDR?
     
  11. wombat2go

    wombat2go Subscriber

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    Chan,
    That is already done courtesy of the properties of the Cadmium Sulphide.
    The surface resistivity reduces in an exponential way with an increase in the photon density. The photons raise electrons to the conduction bands and increase conductivity, and resistivity is the inverse of conductivity.
    The conventional exposure steps of shutter and aperture are also exponential.
    So we don't have to worry about that in the circuit which is why they originally were able to just use a passive linear circuit to drive the meter which is also linear.
    There is a circular slide rule on the lens barrel that allows conversion of the meter reading to shutter and aperture setting
     
  12. Bill Burk

    Bill Burk Subscriber

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    I had a flash that required a similar, but shorter, 15 volt cell. You guessed it. Inside are 10 thin button cells. When I couldn't find 15 volt, I would buy a 22.5 volt and take out 5 of the cells and fold the case back around it...
     
  13. E. von Hoegh

    E. von Hoegh Member

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    I have some early radios from before the days of plug-it-in-the-wall. For a "B" battery of 135v, I went to Shadio Rack and purchased on sale 90 CZ "C" cells, which I soldered in series and tapped at 22.5v and 90v, to provide for all the high voltage needs of the various radios.
    The look I got from the clerk was absolutely priceless.
     
  14. Roger Cole

    Roger Cole Member

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    It's funny to me to read a 9v battery referred to as a "smoke alarm battery" as I knew them as 9v "transistor batteries" for the then common pocket "transistor radio" long before I'd even heard of a home smoke alarm.
     
  15. E. von Hoegh

    E. von Hoegh Member

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    I remember them as transistor radio batteries from when smoke alarms simply did not exist. They were developed and made specifically for transistor radios.
     
  16. wombat2go

    wombat2go Subscriber

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    I have to amend, a little, my reply #11 to Chan
    The electrical circuit and meter are basically linear however I see that the meter scale is somewhat non-linear, probably to correct the transfer function of the CdS LDR to match EV.
    The EV scale of this meter ranges from 2 (iso 100 f/5.6 and 8 second) to 18 (iso 100, f/5.6 and 1/8000)
    As to whether, back in the day, they designed the scaling by trial and error , and whether the LDR CdS cells were specially made for exposure meters, I don't know.
     
  17. Tom1956

    Tom1956 Inactive

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    I think you did a bang-up job on the project. Restoring any kind of exposure meter and achieving anywhere near original linearity is enough to make you pull your hair out. I've pulled it off a few times, but not without spending days at it. On many jobs you just have to pick on end of the dial or the other, or somewhere in between where your linearity situation "bunches up" on you and you just have to live with it. But when you think about it, I suppose a lot of them came out of the factory not accurate from the depths of a coal mine to the surface of the sun.
     
  18. wombat2go

    wombat2go Subscriber

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    Tom, This is first time I have laid hands on an exposure meter. I will be trying it on weekend on some 120 Fuji 160 and 400.
    Like a new toy!
    I will report back, hopefully with good results!
     
  19. stanblute

    stanblute Member

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    Thanks a million for the great article. I got hold of a meter in good shape except for a sick photocell. I substituted a cell from a dead Gossen meter and followed your diagrams to build a circuit board. My cell has a different response slope so I had to change two of the fixed resistor values to get the calibration into range. After an hour of back and forth tuning the high and then low ranges it turned out to be amazingly accurate. It's within 1/2 stop of the readings from my Lunapro SBC reference meter over the top 4/5 of the scale on both ranges. I'm waiting for the weather to warm up so I can go out and play with it.

    Charles
     
  20. wombat2go

    wombat2go Subscriber

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    Good job, Stan,
    Let us know how the photos exposed with it turn out.
     
  21. wombat2go

    wombat2go Subscriber

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    https://app.box.com/s/ztpau5uww1dbafyq4hqn
    https://app.box.com/s/1wrt2omxo2uvo8lktox9

    Update:
    I post the above links for apug member Ofrayray who is restoring a vintage 3/21 and who pm me while I was away.
    Ofrayray reports different /resistor values/ maybe connections/ in the original circuit of his unit compared to my unit.

    I checked my unit with the modified electronics,(calibrated Sept 2013)
    today in Michigan at 10:30 am in the blue sky .>> EV 13 ~ EV 14

    I have been on a trip to Australia with a Pentax MX and some lenses, and with a Century Graphic 2x3
    I decided not to take the Honeywell 3/21 because it may resemble undesirable images in my carry on scans.

    I used Ektar 100 and Portra 160 125th f/16 in bright and estimated down from there in evenings.

    I would be interested in info about accuracy of these old Pentax meters -- thanks
     
  22. RalphLambrecht

    RalphLambrecht Subscriber

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    the ones ,I've tried were al within 1/3 stop:cool: