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Replace tantalum caps with aluminum?

Discussion in 'Electronic Components' started by Walter Harley, Dec 2, 2003.

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  1. I'm replacing the electrolytics in a nearly-40-year-old HP 654A test
    oscillator. This is a 10Hz to 10MHz sinewave oscillator. I have the
    service manual and schematics for it.

    Many of the caps are specified in the service manual to be tantalum. Some
    are quite large values, up to 390uF. The caps are all axial (cylinders with
    leads at each end), which is a relatively uncommon package these days.

    Can I replace these large-value tantalum caps with aluminum caps? I'm
    thinking that in 1967, the ESR, tolerance, and stability of aluminum was
    pretty lousy, but maybe now it's as good as tantalum was back then?

    Thanks for any input.
  2. Al

    Al Guest

    Check the operating temperatures. The tantalums may have a higher
    operating range.

    Tantalums are also made from slugs. Aluminum electrolytics tend to be
    wound; they may have some peculiar resonances.

  3. It's a low power device; basically sits pretty close to room temp.
    Hmm. I'll take a look at the mfr spec sheets for the aluminums. Thanks.
  4. Ian Stirling

    Ian Stirling Guest

    If they are mostly working, you might measure the ESR now, and get an
  5. Jim Yanik

    Jim Yanik Guest

    I suspect good low-ESR al. caps would work fine,and be much less expensive.

  6. May also be worth checking out the "Solid Aluminium" caps. These are
    different from the aluminium electrolytics. They have very low ESR
    (comparable to tantalums) and very long life.

  7. Ed Anderson

    Ed Anderson Guest

  8. Jim Adney

    Jim Adney Guest

    If these are in the metal cans with the glass seal around the positive
    lead, then these tend to be VERY reliable and I would not replace them
    with aluminums under any circumstances. They are quite pricey, but if
    you should actually have a bad one, you should replace it with another

    These are a much better cap than the teardrop tantalums.

    I've actually never seen one of these good tantalum caps fail, but
    I've increased the reliability of a lot of my gear by replacing failed
    (and commonly failing) low voltage alum electrolytics with these. If
    you keep your eyes open you can often get good deals on these at swap
    meets. I've been picking them up for years and now have a good supply
    whenever I need one.

  9. CWatters

    CWatters Guest

    Electrolytics can have low lifetime specifications. I once saw a some with a
    spec of only 1000 hours. Thats about 41 days! Must have been a mistake?
  10. No, just cheap caps. You need to select electrolytics with a higher
    temperature rating to get a longer life. In other words, the closer you
    run it to its rated temperature, the quicker it fails. Cheap caps are
    rated to 85°C, better caps are 125°C. You have to consider the operating
    temp of the equipment, plus the heat generated by any AC passing though
    the cap. A low ESR cap will generate less heat and last longer. Both
    higher temperature grade and low ESR cost more money, so lots of bean
    counters use the cheapest piece of crap they can sweep off the floor at
    the factory.
    22 days!

    Michael A. Terrell
    Central Florida
  11. I believe I've read that lifetime doubles for each 10C under the rated temp.
    So if they were 1000 hours at 85C, then when run at 25C they'd be good for
    64k hours. That's 7.5 years of continuous operation.

    So they probably put them in TV motherboards at 35C, and then the TV dies
    after 3 years and the owner buys a new one. The cap not only saves you
    money, it *earns* you money!
  12. Jerry G.

    Jerry G. Guest

    This has to be a mistake. Can you think of the hundreds of electro-caps
    used in each of every TV set and appliance???



    Jerry Greenberg GLG Technologies GLG

    Electrolytics can have low lifetime specifications. I once saw a some with a
    spec of only 1000 hours. Thats about 41 days! Must have been a mistake?
  13. Jim Yanik

    Jim Yanik Guest

    Look thru the Digi-Key catalog,the list the typical life of some of their
    electrolytics.I think that's a worst-case spec though.
  14. Jeff

    Jeff Guest

    Hi Jerry,
    No mistake but the rated life is done in hours over time
    at a given temp for so much change in value at a max.
    If ran that way most would still work longer if the circuit
    allows it but it would be a poor choice to use in that case.
  15. Boris Mohar

    Boris Mohar Guest

    Only if the customer is stupid enough to buy your brand again.
  16. Boris Mohar

    Boris Mohar Guest

  17. ....Unless the customer figures that all the other brands suck just as much.

    Far as I can tell, practically everything out there these days is made to
    the same low standards. It all seems to be a race to pinch every last $0.01
    out of the retail cost, regardless of whether it halves the useful life of
    the device, makes it less usable, or what. Except for "audiophile" audio
    gear, which is just as absurd in the opposite direction.

    That's part of why I'm fixing the old HP 654A that triggered this thread -
    because it was built to be fixed, and to work and last a good long time. I
    could buy some new Chinese-crap-factory test oscillator for less than the
    time to fix the HP is going to cost me, but I don't wanna.

    (Sorry to offend any Chinese engineers out there. I'm sure there are
    companies in China making superb, high-quality, reliable, robust products.
    Unfortunately, there don't seem to be any USA companies that want to import
  18. Boris Mohar

    Boris Mohar Guest

    Are those HP tantalums bad? You would be surprised how long tantalums last if
    not abused.

  19. This morning I measured their ESR, and they all seemed fine, though one or
    two have a bunch of crystallization around the terminals.

    Is that the definitive test? Is there a better test? (I suppose I should
    also measure their capacitance, which I've not yet done.)

  20. ctsbillc

    ctsbillc Guest

    In the military world, we treat reliability extremely seriously. We use
    Tantalum for most of the high C applications.

    There is an attempt to cut the price of military electronics by using
    commercial off the shelf components (COTS), but the reliability is not
    there. Apart from electrolytics drying out, we have issues with plastic
    packaged ICs which are hygroscopic, and even the in-built tendency of
    semiconductors to die after a few year due to diffusion of the metalization
    through the Silicon, and the migration of the metal across the surface of
    the die.

    In summary, the electrolytics are just one of the components which, for
    consumer level electronics, tend to die more often than we would like, but
    believe me, you do not want to incur the cost of going high reliability on
    your next television.


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