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Thread: Power supply electrolytics

  1. #1
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    Default Power supply electrolytics

    Tried out a Tandy 1000 TL I got for $50 and got a bit of a smell, then a bang. One of the electrolytics blew out in spectacular fashion.

    I guess I'll replace all of them to be safe (luckily the computer itself wasn't connected to it).

    Do I have to worry about these discharging safely before taking them out? Is it the same level of worry as a CRT?

  2. #2

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    Normally the high voltage caps on the primary side have bleeder resistors, just leave it off for 10 min before touching it.

    The charge in a CRT bulb is only a worry if you have to remove the EHT cap and or remove the CRT for handling otherwise they are better not discharged.

  3. #3

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    Quote Originally Posted by falter View Post
    Tried out a Tandy 1000 TL I got for $50 and got a bit of a smell, then a bang. One of the electrolytics blew out in spectacular fashion.

    I guess I'll replace all of them to be safe (luckily the computer itself wasn't connected to it).

    Do I have to worry about these discharging safely before taking them out? Is it the same level of worry as a CRT?
    A CRT will make you jump. A full charge on a high side cap will kill you. As Hugo says, the bleeder will drop the voltage to a safe level in less than a minute. That is assuming that the bleeder resistor is still working. I've recently fixed two older switchers that needed minor repair but the bleeders were both open.
    Trust but verify.
    Dwight

  4. #4

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    It's always possible bleeder resistors can go open. They have a penchant for it as all resistors which operate at high applied voltages seem to. Fortunately electrolytic caps are intrinsically leaky and you will find, even with no bleeder resistor, most types under 1000uF will have discharged to a low voltage over a few hrs, you can always put a meter across them to check or a resistor of a few hundred ohms to be sure. Never short them out directly.
    The trick is just don't turn off the supply and immediately start working on it, give it some time for the stored energy to dissipate.

    It's charged non-electrolytics you need to watch out for , can stay charged for many days, just like the bulb on a CRT.

  5. #5
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    By the time I get parts for it I'm hoping everything will be fully discharged.

    Side note - when it blew, I was up close with the PSU out in the open to see if any bad smells, etc were coming up. Kinda blew up in my face. No projectiles or anything but you felt the 'puff' as it gave way. Hope I didn't poison myself.

  6. #6
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    Learn from the experience and keep yourself well away from the 'action'! If you need to be close, put a piece of Perspex/plexiglass between you and it.

    Dave

  7. #7

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    Quote Originally Posted by falter View Post
    By the time I get parts for it I'm hoping everything will be fully discharged.

    Side note - when it blew, I was up close with the PSU out in the open to see if any bad smells, etc were coming up. Kinda blew up in my face. No projectiles or anything but you felt the 'puff' as it gave way. Hope I didn't poison myself.
    It is not particularly poisonous but may have some salts in it that you'd not like to drink. It is mostly borate. People used to wash their cloths in it. You wouldn't want to drink laundry detergent, would you?
    When I worked at Intel, we made development systems. They had linear power supplies with large electrolytic capacitors. They would surprisingly work backwards for about 3 to 5 minutes before blowing up. At incoming inspection we had what we called the coffin. It had power and loads for several supplies. It was standard procedure to connect them up and let them run for 1/2 hour before rechecking them and installing them. Every now and then we'd send one back with the can blown off. They'd hit the inside of the coffin with enough energy to dent the wood frame.
    Do check the diodes for shorts before installing a new capacitor. For electrolytics to blow, it is often caused by a shorted diode and sometimes it blows the diode.
    Dwight

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