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  1. #1
    polyglot's Avatar
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    Print consistency: mains voltage

    I did some prints last night and then again tonight; identical exposures (according to my electronic timer) and brand new developer in each case.

    The densities are different. Our mains voltage varies a bit and I have a traditional lightbulb enlarger, so my suspicion is that that has caused the exposure error of about 0.15 stops - quite visible in the final print.

    So I can see that having some sort of brightness-feedback system as is required for (highly variable) cold heads might also be of some value when printing with tungsten.

  2. #2
    Steve Smith's Avatar
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    Is it possible that your timer varies a little bit? It depends on how many cycles of a clock count it uses to count seconds and would have more effect on short times than long.

    I made a timer a few years ago and made the mistake of using a free running one second oscillator to advance the counter. At the time of pressing the start button, the next pulse could be anything from immediate to one second away so it had an accuracy of +/- one second. Linking the start switch to a clock reset would have cured this but I didn't get round to doing that!

    If it is the mains voltage, would it be possible to replace the bulb with a low voltage type and use a regulated dc power supply?


    Steve.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

  3. #3
    markbarendt's Avatar
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    When I first set up my darkroom this issue popped up. I was on the same circuit as my home office in a converted closet. I unloaded the circuit of other things and that helped, still and yet when the laundry is in process or the dishwasher, I still get it.

    The other issue I get is related to how well the enlarger is warmed up. I've got a Beseler PM2L meter and adjust my exposure for each print with it. I can actually watch the exposure needle swing as the bulb warms up.
    Mark Barendt, Beaverton, OR

    "We do not see things the way they are. We see things the way we are." Anaïs Nin

  4. #4

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    I know that Gordon at point light uses a voltage stabiliser for all his enlargers.

  5. #5
    polyglot's Avatar
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    Steve: the timer is good to 10ms (on a ~25s exposure in this case), i.e. half a mains cycle since it uses a triac. See timer link in my signature.

    I note that later DeVere enlargers have stabilized supplies; mine is the 1978ish model (Mk.I) 504 that precedes such niceties. Certainly a regulated DC supply would solve the problem, the only drawback being $$$ for a 12A 24V supply.

    I think my solution will be "make a quick test strip before blithely exposing that 16x20 I setup 18 hours ago". I only got into this situation by fogging (during development) the final sheet I made last night and giving up in disgust; came back to redo it tonight and had my little issue. I suspect the airconditioner.

  6. #6
    Monophoto's Avatar
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    I presume that the bulb in your enlarger is a conventional incandescent bulb.

    The light output from incandescent bulbs varies SIGNIFICANTLY with voltage and is likely to be the cause of your exposure problem. A 5% reduction in voltage (which is within the range of variation that a utility may apply during normal operations) results in a nearly 20% reduction in light output. A 10% reduction in voltage (which is unusual, but possible) can reduce light output by up to 70%! And as the light output varies, the color temperature of the lamp also changes - the output becomes noticeably more yellow as voltage drops.

    The impact on timer accuracy not nearly as profound. As you say, if your time looks at zero crossings, then the timing rate is controlled by frequency rather than voltage magnitude, and frequency is unlikely to change dramatically (unless you are in India where frequency is notoriously variable). Voltage variations can affect the timing accuracy of motor-based timers, but I would not expect that magnitude of error to be nearly as significant as the magnitude of lamp output for a given variation in voltage.
    Louie

  7. #7
    Mike Wilde's Avatar
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    I lucked into an old ferroresonant transformer for the right price. It is bolted to the wall, high, and out of the way in the darkroom, and has just enough capacity to power the timer and enalrger light.

    They might be rare as hens teeth, but find them , and they work.
    my real name, imagine that.

  8. #8

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    Print consistency: mains voltage

    I have this problem in my own darkroom. I have a heater with a thermostat built in. When the heater kicks in, the lights dim. I don't seem to have a problem with the Leitz enlarger which has a built-in transformer, but I'm sure the simple Durst condenser model is affected. Trouble is I need the heater!

  9. #9

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    Quote Originally Posted by polyglot View Post
    Steve: the timer is good to 10ms (on a ~25s exposure in this case), i.e. half a mains cycle since it uses a triac. See timer link in my signature.

    I note that later DeVere enlargers have stabilized supplies; mine is the 1978ish model (Mk.I) 504 that precedes such niceties. Certainly a regulated DC supply would solve the problem, the only drawback being $$$ for a 12A 24V supply.

    I think my solution will be "make a quick test strip before blithely exposing that 16x20 I setup 18 hours ago". I only got into this situation by fogging (during development) the final sheet I made last night and giving up in disgust; came back to redo it tonight and had my little issue. I suspect the airconditioner.
    You can get this one which is not exactly cheap but reasonable in price.
    http://www.automationdirect.com/adc/.../PSM24-REM360S

  10. #10
    adelorenzo's Avatar
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    I have a couple of heavy boxes that came with my enlarger heads. They make a humming sound when the enlarger is turned on and I belive they do something to control the power levels. Voltage regulators maybe? At any rate, I have them so I figured I should use them, although I am nowhere near that level of precision in my darkroom work.

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