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  1. #71
    Leigh B's Avatar
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    Quote Originally Posted by ic-racer View Post
    10-2 and 10-3 circuits are each powered by the same type of capacitor. 250uf/475v
    How old are the capacitors?

    If they're more than ten years old, or if the equipment sat unused for a year or more, the caps should be replaced.

    Suitable replacements can be found at Mouser Electronics http://www.mouser.com

    - Leigh
    “Wise men talk because they have something to say; fools, because they have to say something.” - Plato

  2. #72
    ic-racer's Avatar
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    If they're more than ten years old, or if the equipment sat unused for a year or more, the caps should be replaced.
    Why?

    250uf/475V, 10uf/500V at Mouser?

  3. #73
    Leigh B's Avatar
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    Quote Originally Posted by ic-racer View Post
    Why?
    The electrolyte degrades with age.

    If the caps sit unused (no voltage applied) for an extended period, the bubble layer dissipates, causing increased internal leakage.

    This is a well-known phenomenon that's been with us for decades. Modern technology has not mitigated the problem.

    This is a problem with aluminum electrolytics (which yours are). It is found much less often in other capacitor types.

    - Leigh
    “Wise men talk because they have something to say; fools, because they have to say something.” - Plato

  4. #74
    ic-racer's Avatar
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    Doesn't seem typical of an establishment like EG&G to use problematic components. Have you had the cover off yours? Certainly NOT Made in Taiwan



    In view of the quality of the components used in the EG&G Sensitometers, the quality control procedures exercised at EG&G and the conditions to which precision photographic equipment instruments are exposed, little maintenance will be required. Fuse or Flashtube replacement may be required during the life of the unit and these are easily accomplished.
    Routine capacitor replacement is not mentioned. Its not like the device is holding open the relay on an ICBM launch start sequence where unexpected failure would be catastrophic. If a capacitor fails in a sensitometer, one can replace it at the time of failure.

    Even if one were to find the appropriate capacitor, how would you know it has not been sitting on the shelf for ten years?

    My unit did, however, require a replacement flashtube, so I can verify their prediction with respect to that.

    Service life is only known for failed components.

    My opinion is that it would be very foolish to remove and dispose of functioning capacitors in a working EG&G sensitometer. The difficulty of finding currently produced reliable replacement electrolytic capacitors of the proper specifications in the small numbers required would be great.
    Last edited by ic-racer; 07-15-2012 at 10:20 PM. Click to view previous post history.

  5. #75
    Leigh B's Avatar
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    Ten years is the term commonly used in the industry for equipment that's used intermittently.

    The bubble layer starts to dissipate as soon as voltage is removed from the cap, i.e. when you turn it off.

    In normal use (once a month or more often), the percentage loss is small enough as to be inconsequential,
    and the layer will reform normally upon the next application of power.

    However, if the equipment is off for longer periods, the layer can dissipate locally to the point of increasing
    leakage, which translates to heat generation the next time power is applied.

    Heat raises the internal temperature of the cap, which increases internal pressure, which in turn can cause
    the safety vent to rupture. Once that occurs, the electrolyte dries out and is no longer capable of forming
    the bubble layer. This causes a failure of the cap, possibly including explosion.

    The scenario I described is not a step change. A cap doesn't die after sitting for seven months and 22 days.
    Degradation occurs continuously, and will correct itself almost completely when the equipment is used.

    The ten year replacement cycle is a standard from Sorenson power supplies and other similar companies.
    It's a rule of thumb for equipment in intermittent use, not a hard and fast law.

    The rated life of the best available capacitors is only 2,000 hours for most products. That's only one year of
    9-5 office hours. There are a few hi-rel types with longer ratings, but the highest I've seen is 8,000 hours.

    Some years ago I worked for a company that did warranty service for Sorenson power supplies, and my own
    company was a warranty service station for Novatron studio strobes, so I'm pretty familiar with capacitors.

    - Leigh
    “Wise men talk because they have something to say; fools, because they have to say something.” - Plato

  6. #76
    Leigh B's Avatar
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    Quote Originally Posted by ic-racer View Post
    Doesn't seem typical of an establishment like EG&G to use problematic components.
    Routine capacitor replacement is not mentioned.
    My opinion is that it would be very foolish to remove and dispose of functioning capacitors in a working EG&G sensitometer. The difficulty of finding currently produced reliable replacement electrolytic capacitors of the proper specifications in the small numbers required would be great.
    Wow, are you off in lala land.

    Who said anything about "problematic" parts? I'm talking about the best available commercial components,
    equivalent to current MIL-spec parts but without the supporting paperwork.

    As to replacing "functioning capacitors"... You don't know whether they're functioning as designed or not.

    High-quality modern capacitors are available in single quantities from a number of commercial distributors.

    You posted while I was writing. You need to read post #75.

    - Leigh
    “Wise men talk because they have something to say; fools, because they have to say something.” - Plato

  7. #77
    Leigh B's Avatar
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    Quote Originally Posted by ic-racer View Post
    Routine capacitor replacement is not mentioned.
    In looking over your photo, a couple of things come to mind.

    The six selenium rectifiers (blue finned things in the middle) should be replaced. These have a
    high failure rate. They've not been used in new production since the mid-1960's, indicating the
    product is at least that old. If one fails the stench will make the area uninhabitable for months.

    There's an oil-filled paper capacitor mounted to the chassis at lower right. I expect that's the 10uf
    cap that you mentioned. These caps have excellent reliability; many are still as new from WWII.

    The 250/475 is probably the pair of aluminum can electrolytics at lower left labeled C1 and C2, and
    rated 125uf each. This style of cap is no longer manufactured, so you could not get modern
    replacements. The style available would be stud-mount cans.

    Cornell-Dubilier makes a suitable replacement, rated 240uf @ 500 volts, here
    http://www.mouser.com/ProductDetail/...yh4r6vjddNY%3d
    if it would fit in the available space.

    You've indicated an unexplained deviation of output from the specified values. Both the selenium rectifiers
    and the electrolytic caps would be prime suspects as the cause of that problem.

    - Leigh
    Last edited by Leigh B; 07-15-2012 at 07:34 PM. Click to view previous post history.
    “Wise men talk because they have something to say; fools, because they have to say something.” - Plato

  8. #78
    Photo Engineer's Avatar
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    IC;

    That photo of your sensitometer looks totally unlike any I have ever seen and I have seen literally dozens of them. All the ones that I have seen are in sleek plastic cases with a removable light box. Was this reworked?

    PE

  9. #79
    polyglot's Avatar
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    Quote Originally Posted by ic-racer View Post
    Doesn't seem typical of an establishment like EG&G to use problematic components. Have you had the cover off yours? Certainly NOT Made in Taiwan
    Casual racism doesn't change the properties of your capacitors. The insulating oxide layer degrades with no charge applied (cap go bang one day), and the electrolyte itself tend to dry out, which means that the capacitance reduces gradually with time. If you've measured the capacitors and found the values to be correct, that's one thing but showing a picture of a hand-soldered point-to-point circuit proves nothing except that it's damn old and that any electrolytic capacitors in it have probably gone bad.

    If they're not electrolytics, they may not have degraded much at all. The capacitors in question are C1, C2, maybe C3 and C4. We can only see the back of them here, the bodies seem to be on the other side of that sheet of metal.

    I reckon the blue things are power resistors though - note that all six of them seem to be in series and they have only 2 terminals each. The bolt-style mounting in the heatsinks are reminiscent of power rectifiers though.
    Last edited by polyglot; 07-15-2012 at 07:49 PM. Click to view previous post history.

  10. #80
    Leigh B's Avatar
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    Quote Originally Posted by Photo Engineer View Post
    Was this reworked?
    I can't speak authoritatively, but...

    All aspects of the product indicate that it's an original factory item.
    The component imprints on the chassis (C2, L1, etc) were done by machine, not by hand.
    This was not built by somebody in his basement or garage.

    Many of the details, including
    1) method of wire harnessing,
    2) mounting type and style of the electrolytic capacitors,
    3) use of an oil-filled paper capacitor,
    4) fiber-insert anti-vibration nuts (modern a-v nuts use nylon inserts),
    5) selenium rectifiers,
    6) style of the octal socket for the flash tube,
    7) FR4 glass-epoxy board with swaged terminals (made by Keystone),
    8) phenolic-sandwich push button switch (made by Cutler-Hammer) next to the relay,
    9) epoxy end-fill ceramic capacitor (black tubular) on the terminal board,

    are consistent with a date of manufacture around 1964 or 1965.

    The imprinted wire-number sleeves would likely not have been used before ~1962.

    This indicates that the capacitors are about 50 years old.

    Just an observation.

    - Leigh
    Last edited by Leigh B; 07-15-2012 at 08:10 PM. Click to view previous post history.
    “Wise men talk because they have something to say; fools, because they have to say something.” - Plato



 

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