Beseler 67 dichro blows bulbs help

Discussion in 'Darkroom Developing and Printing' started by ryan, Nov 21, 2003.

  1. ryan

    ryan Guest

    My old trusty Beseler 67 dichro keeps blowing bulbs. They last only a
    few seconds. I've tested the socket with a voltage meter and the voltage
    looks corect.
    Could this be a problem with the "stabilized power supply" unit? Has it
    gone bad?

    Any help is appreciated,

    Ryan

    PS Check out my "Airshow" photographs in this months Black & White
    Magazine. 12/03


    www.rfoto.com
     
    ryan, Nov 21, 2003
    #1
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  2. ryan

    RWatson767 Guest

    Ryan
    Could this be a problem with the "stabilized power supply" unit? Has it
    gone bad?

    This can do it. When trouble shooting put in a larger voltage bulb so if the
    regulator is bad the bukb will still hold up. Then troubleshoot as necessary.
    If repairing these things is not your bag email me privately.
    Bob AZ
     
    RWatson767, Nov 21, 2003
    #2
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  3. ryan

    br Guest

    Hopefully your not touching the reflective portion of the bulb. The oil from
    your fingers will cause the bulb to blow instantly. Use cotten gloves
    handling the bulb. Placing the bulb into the dichroic head it a pain in the
    ....

    Bill
     
    br, Nov 22, 2003
    #3
  4. ryan

    Norman Worth Guest

    Is the voltage at the bulb socket correct? If so, the power supply is OK.
    It would take massive overvoltage to burn out the lamp in just a few
    seconds. Are you sure you are using the correct replacement lamp? Does the
    voltage at the socket match the voltage rating for the lamp?
     
    Norman Worth, Nov 23, 2003
    #4
  5. Many enlarger bulbs these days are for 82 volts which is simply obtained
    through a half-wave rectifier with no filter condenser. If that
    rectifier shorts, you will get the full 120 volts on your bulb which
    would blow it right now. The place to measure the voltage is at the
    bulb, not at the line.
     
    Patrick Gainer, Nov 26, 2003
    #5
  6. ryan

    David Starr Guest

    The stabilized power supply contains a "analog optical isolator" chip.
    For the life of me, I can't remember the exact name, but that's a
    decent description of what it is. . This is what keeps the output at
    29 volts. When this chip fails, the output voltage will "run away",
    blowing the bulb. When this happens, you'll see the bulb rapidly get
    brighter & brighter until it blows.

    When this happened to my power supply, I emailed Beseler & they were
    kind enough to send me a schematic. They also told me these chips
    were the most common failure, and they were no longer available
    anywhere. I searched ebay regularly until I found a couple of these
    supplys. Now, I have one I use & one for a spare. One of the supplys
    I bought was a 220V unit. I took the circuit board out of it & put it
    in my original; the only difference is the transformer, & it's working
    fine.


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    Professional Shop Rat: 14,175 days in a GM plant.
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    David Starr, Nov 27, 2003
    #6
  7. Opto-Isolator chips are in current commerce. They are a lot alike, so if
    you are handy with electronics, you might guess another one that would
    work and just try it. You might have to recalibrate. All they are is an
    LED and a phototransistor (though some have amplifiers too) moulded into
    an opaque block of plastic with, typically, 4 pins. Typically cost under
    US$1.00 Here is the specification for one of these...

    http://www.fairchildsemiconductor.com/ds/MO/MOC3011-M.pdf
     
    Jean-David Beyer, Nov 28, 2003
    #7
  8. The majority of current Optos are made for digital applications.
    Some can be operated in a somewhat linear (analog) mode but not
    with very satisfactory results.

    The old linear units were tubular and made from an incandescent light
    at one end of the tube and a CDS cell at the other. If Beseler used
    one of this type it may be possible to make a new one if the bulb voltage
    and CDS characteristics could be known.

    Are there any identifying marks on the Beseler optos or any notes in
    the schematic or parts list? If so, then an old Clairex (the most common
    mfg.) catalog may have the characteristics listed.

    In any case, the repair will be a long haul. As David suggested, it may
    be easier to find another working power supply.
     
    Nicholas O. Lindan, Nov 28, 2003
    #8
  9. ryan

    Jorge Omar Guest

    They (standard opto isolators) can be made to work linearly over a fairly
    wide range through the use of two, inside a feedback loop.
    But this could mean a major modification of the existing circuit.

    If you do not find any other solution, and have a circuit diagram, let me
    know (post a note in the group - my 'email' is just a spam bait).

    Jorge
     
    Jorge Omar, Nov 28, 2003
    #9
  10. ryan

    David Starr Guest


    I just dug out the schematic Beseler sent me. It's dated 4-29-77.
    The isolator is a Vactrol 21L255.

    There are no notes or parts lists on the schematic. Components are
    identified on the schematic only.






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    Professional Shop Rat: 14,205 days in a GM plant.
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     
    David Starr, Nov 28, 2003
    #10
  11. That's an EG&G Vactec unit. This division of EG&G (of atomic
    bomb fame) has been sold to Perkin Elmer (of Hubble telescope
    mirror fame).

    The number is a bit funny for a Vactrol, though. The P/N usually
    is of the form VTxxxx. I hope the 21L255 is not a special
    design for Beseler, that would make getting the
    specs a bit iffy. It may be possible to tell by analyzing
    the schematic, though. If someone can send me a scan I will see
    what I can do. (Is there a number stamped on the part? - it
    is a black cylindrical thingy with two leads sticking out of
    each end.)

    Clairex still makes the CDS cells. It should still be possible
    to find a lightbulb of the right voltage.

    BTW: there is a lot of pent-up demand for the old Vactrol light
    bulb/CDS units for expansion/compression guitar amplifiers and
    automatic volume controls from the old days.
     
    Nicholas O. Lindan, Nov 29, 2003
    #11
  12. ryan

    David Starr Guest

    I emailed a scan of the schematic to Nicholas. If anyone else wants a
    copy, let me know.


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    Professional Shop Rat: 14,205 days in a GM plant.
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     
    David Starr, Nov 29, 2003
    #12
  13. With a true RMS voltmeter. Other types of voltmeters will read the
    rectified waveform inaccurately.


    --
    Clear skies,

    Michael Covington -- www.covingtoninnovations.com
    Author, Astrophotography for the Amateur
    and (new) How to Use a Computerized Telescope
     
    Michael A. Covington, Nov 29, 2003
    #13
  14. ryan

    Jorge Omar Guest

    Out of sheer curiosity, could you pls email one to:

    jorge_omar2 at yahoo dot com

    Thanks,

    Jorge
     
    Jorge Omar, Nov 29, 2003
    #14
  15. ryan

    RWatson767 Guest

    David
    I emailed a scan of the schematic to Nicholas. If anyone else wants a copy,
    let me know.


    I would like a copy.
    Thanks
    Bob AZ
     
    RWatson767, Nov 29, 2003
    #15

  16. Looking at the schematic Dave sent me it would appear the 21L255 could
    be replaced with a VTL3A27:

    Calculated VTL3A27
    value if VTL specification
    3A27 is used

    Max I 42mA 40mA

    Max V 13V 12V

    At 26 volts on the enlarger lamp there will be 7 volts on the
    3A27 vactrol input giving an output resistance of 300 ohms.

    300 ohms is the nominal control point for the lamp control
    circuit. There is a trim pot in the circuit that should allow
    tweaking the power supply so that 29V is available at the lamp.

    VTL3A27's may be available from Oak Ridge Radio:

    http://users.rcn.com/oakridge.ma.ultranet/Products/Vactrol.html

    Caveat emptor: I am making no guarantee this substitution will work.
    I would recommend testing with a dummy load of 10ohms/100Watts
    first. Place a variac at the input to vary the input voltage and
    check regulation.

    Or, put the 3A27 in, turn it on and see if it works/does nothing/
    blows up.

    Keep a fire extinguisher handy in either test.
     
    Nicholas O. Lindan, Nov 30, 2003
    #16
  17. ryan

    David Starr Guest

    Nicholas, thanks for all the work you put into this.


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    Professional Shop Rat: 14,205 days in a GM plant.
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     
    David Starr, Nov 30, 2003
    #17
  18. A simple incandescent test lamp designed for 120 v. will do. If the
    diode is doing its job, the lamp will be much dimmer than if you plug it
    into a wall socket.
     
    Patrick Gainer, Dec 17, 2003
    #18
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