Minolta MOD III Lamp/Power Modification

Discussion in 'Darkroom Equipment' started by ic-racer, Dec 6, 2009.

  1. ic-racer

    ic-racer Member

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    Back in the late 70s the voltage regulated 19V power supply for my Minolta Mod III enlarger went bad. I converted it over to a 1/2 wave rectified non-regulated supply with a big diode. It has a 24V transformer, so half of that is around 13 measured volts.

    I used the enlarger like that all these years. Now I letting my kids use that enlarger. The "Kodak Projection Print Scale" works great with kids, they can just pick out the pie that looks good. However, our printing times were not always under 60 seconds with Y/M multigrade filtration dialed in.

    So, I decided to look into optimizing the light output.

    The enlarger has used the EKG bulb all these years. That is the original type bulb and it is rated at 80W at 19 volts. Well, since I only have 13 volts, I thought a 12 or 13V lamp would get me back to optimum intensity.


    A search of various bulbs led me to the DED bulb. It is 85W at 13.8V with the same GX5.3 2-PIN base. Cost about $5.00. It is on order, but I'll post my results when it arrives.

    [​IMG]
     
    Last edited by a moderator: Dec 6, 2009
  2. kinghl

    kinghl Member

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    Mod III power supply options

    Any suggestions of how to fix a power supply?
    Blown the circuit board after my unit was in storage for many years.
    Cannot find replacement boards from OEM.
    Does not have schematic to fix circuit board.
    Any advises??
     
  3. ic-racer

    ic-racer Member

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    You can easily half-wave rectify the voltage from the 24V transformer with a large diode. That is how my unit was repaired in the 1970s by Minota service. Subsequently they re-engineered the power supply that shipped with new enlargers.

    I wound up buying a power supply with the newer circuit from an APUG user. How do you know if you are buying a newer circuit power supply? Easy, if it is still working it is probably the later version :smile:

    Another thread here: http://www.apug.org/forums/forum43/107717-minolta-model-3-enlarger-color-head.html
     
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  4. ic-racer

    ic-racer Member

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    There is a minolta power supply on ebay now for $30 but the text reads " I am not a professional on how it really works." which usually means it is broken and does not work.
     
  5. Peter Schrager

    Peter Schrager Subscriber

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    best small frame enlarger I ever used!! someone should start production of these again...
    best, peter
     
  6. kinghl

    kinghl Member

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    Do you have the specifications of the 24V transformer and the diode?
    Your description really does not tell me how to get component and how to put it together.
    Like to get my unit working so I can sell it to someone who really can use it.
     
  7. ic-racer

    ic-racer Member

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    If the term "half wave rectification of 24V to 12V with a diode" does not make sense to you then this procedure will be difficult and perhaps dangerous. The primaries of the transformer are 120V which is lethal voltage.

    Having posted the above, lets start from the basics. Do you have a soldering gun? If so then maybe you have an extra diode laying around. Diodes are rated per reverse voltage. If you have a 1N4001 (35v) that should do but if you need to buy one you could get a 1N4002 (70V) to have more of margin against failure.

    The principle of half-wave rectification is described here: http://web.physics.ucsb.edu/~lecturedemonstrations/Composer/Pages/64.57.html

    You need to find the secondaries of the transformer and connect the diode between one wire from the transformer to the lamp.

    After the procedure the device is no longer UL or CE approved. You may want to take that into consideration if you are going to sell it.
     
  8. kinghl

    kinghl Member

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    Are you referring to the original transformer in the power supply?
    Use the diode to by-pass the circuit board straight 12 v to the lamp?
     
  9. ic-racer

    ic-racer Member

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    Yes, the transformer in the box should be a 120V to 24V transformer if I recall correctly. The circuit board electronics are not used after the procedure.

    You could also look up the Beseler 23C III color head with "built-in solid state power supply" which I believe is just a rectifying diode (part #90).
     
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  10. kinghl

    kinghl Member

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    I remember (over 30 years ago) when I bought this, the ads said the power supply has "stabilized" feature to minimize lamp variation.
    Does the original circuit board suppose to do that? If the circuit board is by-passed, what effect will that have to the variation of the lamp?
     
  11. ic-racer

    ic-racer Member

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    Yes, converting it to run on half-wave rectified current makes the lamp just like any other lamp plugged straight into an outlet. If you don't notice a lot of dimming of your lights and brown-outs from your utility company, then it should not be a problem.
     
  12. Cruzingoose

    Cruzingoose Member

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    Volts, Amps and WHATS?

    The OP stated the original lamp is 80w at 19 volts. This lamp will draw 4.2 Amps. The NEW lamp proposed is rated at 85w at 13.8 which will draw 6.2 Amps which is overloading the transformer. Also, the OP did not state what diode was being used. A 1N400X diode is rated for only 1 Amp, not enough for either lamp. It is generally bad form to half wave rectify a transformer and use a lamp rated at one half of the un-rectified output, (13 volt lamp on 24 volt transformer). An better option (other than repairing the PS), is to use a 24 volt lamp like a common like as found here. http://www.lightbulbsdirect.com/page/001/PROD/MR16-50W24V/50W24VMU Even though the wattage is less than original, you will still have very close to the same lumens because the lamp is running on full AC rather than pulsed DC (half wave rectified). Another advantage is the transformer will not be overloaded or have a pulsed DC component on the secondary.

    I also repair power supplies........
     
  13. kinghl

    kinghl Member

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    Any ideas of why Canon configure the original power output of the PS to be 19V with the circuit board instead of running full 24V from transformer? Is this to stabilize the output and make the lamp more consistent? Was the 80w/19V lamp only available lamp back then?
    I was thinking either 12V or 24V would be more commonly used and available for current use also.
    Lumen differences between the original 80w/19v lamp and the proposed 50w/24v lamp will probably affect exposure timing and/or aperture setting and will need trials to determine. However, this change appear to be more robust than the 1/2 wave rectified method.
     
  14. ic-racer

    ic-racer Member

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    Thanks for the comments Cruzingoose. My mistake on the rectifying diode, it is 1N5401 (3A), not 1N4001. Again, that is what Minolta service put in back in the 1970s, probably want to go with a little more forward amperage rating for a de novo conversion. However, going straight to 24V with a different bulb seems like an excellent idea for anyone out there with a non-working power supply like this.

    Also, readers, realize this is the re-activation of very old thread about an experiment that has been subsequently aborted due to some of the issues pointed out by Cruzingoose above. (I got a new working 18V Minolta power supply for that enlarger a few years ago).
     
    Last edited by a moderator: Mar 14, 2014
  15. Cruzingoose

    Cruzingoose Member

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    There wew many goofy lamps in the old days. Some were made just to lock one into buying this special lamp. Some were chosen for brilliance or longetivity, who knows. A common approach would have benn a common 24 volt lamp marketed as a 19 volt lamp. In anycase, the output of 19 volts was a convenient and cheap way to build a stabilised power supply using off the shelf 24 volt components.

    As far as total lumen output between the two lamps, I figgure a old design 24 volt 80 watt lamp running on 19 volts will not be any brighter than a modern 24 volt 50 watt running on 24 volts.