Durst M707 color - What electric circuitry should be inside ?

Discussion in 'Darkroom Developing and Printing' started by Philippe Lauwers, May 11, 2004.

  1. Hello,

    About 1,5 years ago, I purchased a second-hand Durst M707 color enlarger.
    After a (more or less) monthly printing session, I had a 'meltdown' last
    weekend ...

    When opening the head of the enlarge, I found a) the wires connected to the
    lamp were brazed to some kind of connecting plate and b) the wires from the
    cable entering the enlarger were bolted to that same plate.
    Apparently, the tin connecting the wires coming from the lamp had melted,
    and the wires had come loose.

    This made me start considering wether there should be some kind of
    overheating-protection in the enlarger-head, as the temperature can become
    very high (as my old chemistry books tell me tin melts @ a temperature of

    Since there's nothing to be found except for a few wires connected to each
    other I wonder :
    1. What was originally in there.
    2. If the state of my enlarger more or less ressembles the original state,
    can I make an improvement so that the enlarger only needs some cooling down
    instead of opening and fixing it when the temperature gets too high.
    3. Would it be a good idea to build in a small fan in the enlarger-head (as
    I've heard when using a laborator @ school). Since the enlarger came with a
    12 V transfo, I could maybe use a fan similar to those used in computers
    (which would be my first step into the digital era ;-) ) ...


    Philippe Lauwers, May 11, 2004
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  2. Durst designs there enlargers rather well, so it is hard to imagine the
    enlarger getting as hot as you describe. Be sure, though, that the right
    lamp is being used in the enlarger. It may be made for a 50W and have a
    100W installed, which would lead to big time overheating.

    Or, there may have been a short in the lamp wiring and the 10-20 amps the
    transformer can supply (for a short time) was enough to melt the solder

    But "brazed" -- or do you mean soldered? Brazing is half way between a weld
    and solder and I doubt you melted a brazed joint - the aluminum housing
    would have to melt first (IIRC).

    Solder is the stuff radios and circuit boards are put together with.
    Around the house it is a soft wire made of 60% lead and 40% tin.
    That _is_ pretty hot, in any case.
    Besides the solder melting, you mean...

    There are thermal cut-off switches:


    I am sure you have something just like them in the eu. The usual application
    is to have a self-resetting disc thermostat in series with one or two thermal
    cutoffs. An old coffee maker will yield both the thermostat and the cut-off(s).
    If it really runs that hot, yes. Most small computer fans run on 5VDC, the
    enlarger transformer provides 12VAC so you would need to build in a power supply
    circuit. OTOH, you may want to have the fan run all the time, this is the
    normal way air-cooled equipment works. A small 'wall wart' 5VDC power
    supply would run the fan and most folks have a box full of the things from
    old electronic gadgets and the like.
    Nicholas O. Lindan, May 12, 2004
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  3. Philippe Lauwers

    Budwich Guest

    Typically, in appliances where heat / current is an issue, crimped
    connectors (and brazing) are used to prevent "de-soldering" although as
    another poster indicate, its hard to imagine that an enlarger draw enough
    current to cause this but maybe in combination with the heat from the bulb.
    Budwich, May 12, 2004
  4. I meant soldered. Life isn't always easy without a dictionnary @ hand ;-)
    Philippe Lauwers, May 12, 2004
  5. Philippe Lauwers

    John Walton Guest

    Have Nick run down to Electronic Surplus on Broadway and 55th and get you
    some inexpensive Teflon wire.
    John Walton, May 13, 2004
  6. Philippe Lauwers

    Jorge Omar Guest

    Or a miscontact in some of the wires, and due to the high current (12V
    bulb), lots of heath.

    Jorge Omar, May 13, 2004
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