Jobo motor controller - reduce speed more?

Discussion in 'Darkroom Equipment' started by Dave Swinnard, Mar 29, 2006.

  1. Dave Swinnard

    Dave Swinnard Subscriber

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    Does anybody know of a means to modify the speed controller system for the JOBO CPA-2 processor?

    It's slowest rate is ~28 rpm (timed).

    It has been suggested a slower rotation speed might clear up an issue I had with 35mm Acros and Pyrocat-HD in a series 2500 tank. (extra density near the edges, just like that which used to happen with the old, non-invertible, twirly-stick dev. tanks. e.g. GAF from my younger days)
     
  2. OldBikerPete

    OldBikerPete Member

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    I have a CPE-2. I don't know offhand what sort of motor the Jobos have and I'm at work, away from my developing gear at the moment. I'll perform an experiment tonight and let you know the results.

    If the Jobo's motor is a synchronous motor (and I don't think it can be if the same motor is used world-wide on different flavours of electricity) then the only way to vary the speed is to vary the frequency of the AC that the motor uses. This can be done but it is a bit expensive for what amounts to a variable frequency sin-wave generator and an amplifier capable of outputting 110 or 240 volts RMS of AC power.
    If the motor is a so-called 'universal' motor and doesn't have any electronic speed control already then its speed may be controlled using a simple electronic circuit called an 'electronic drill speed controller'. I built my own from a circuit published in an electronic hobbyist magazine. There are probably many circuits published on the web and you may be able to buy a kit or a finished article from Radio Shack (Tandy Electronics) or the like, or even a hardware store.
    Worth a try.
     
  3. ras351

    ras351 Member

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    I don't have a Jobo processor so I'm not sure what will work but if the speed control uses some sort of variable resistance (ie either a potentiometer or a set of selectable resistors) which is used as input to a pulse generator you might be able to modify the resistance to slow down the speed. Assuming a low voltage DC motor (likely seeing as it's reversible) being driven by something like a H-Bridge you could also use a frequency divider or 555 to decrease the duty cycle going into the driver circuitry. If there's feedback from the motor to a controller it gets a little more complicated. It's doable but without knowing the circuit it's a little difficult to suggest the easiest option.

    Roger.
     
  4. pentaxuser

    pentaxuser Subscriber

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    Dave I have the Rotary Processor Manual written by John Tinsley in the early 90s. He covers all the Jobo equipment including CPA-2 which he used himslef and recommends the fast speed for all film processing which is about 70rpm. To quote" Thus slow speed can give an uneven develop especially near the edges of roll film. The P position is much better, giving a rotation speed in the order of 70rpm with even development across the film.

    So it looks like he advocates the very opposite of what has been suggested as the cure to your problems.

    Whether he ever used Acros and Pyrocat is an unknown. Did Acros exist in 1992 when the book was written? I don't know but based on his book, I think I'd re-examine why a slower speed should solve a problem with Acros when he felt it was likely to cause the same problem with other films mentioned such as PanF, FP4 and the Delta films.

    pentaxuser
     
  5. Neal

    Neal Subscriber

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    Dear Dave,

    I would like to second Pentaxuer's advice. Speed up to the 70-80rpm range as a test before trying to modify the processor.

    Neal Wydra
     
  6. DeanC

    DeanC Member

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    I think 80rpm and a staining developer would be a recipe for out of control B+F densities.