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Capacitors, a can of worms!

Discussion in 'Home Brew' started by mechanic, Sep 1, 2010.

  1. mechanic

    mechanic Well-Known Member

    Mar 4, 2009
    Likes Received:
    Doing replacement electrolytic capacitors in several older short wave radios and a few CBs I have come across some interesting things, mostly by accident. :pop:
    I am not fortunate to have a capacitor checker and if I suspect a rig with one or more bad caps, I replace them all. Most of the time the symptoms tell about bad caps. :headbang
    Tonight I restored a Cobra 148 GTL to working condition! No receive but good transmit with modulation.... I wanted to find out what electrolytic was causing the problem, so I replaced them one at a time and checked each new cap for the rig coming back to life. This time it was the fourth cap I replaced, a 220µf @ 10 volts. I replaced it with a smaller one in size but it is a 220µf @ 16 volts.... The radio came back to life! (y)
    Just wondering what happened I checked the cap with my Fluke DVM and compared it to another new cap. Here is what I found... Checking the new cap with the leads of the DVM in correct polarity and set in ohms scale charged up the cap, the bar meter showed a build up and then it went to about 2675 ohms and slowly fell about an ohm per second. I tried a different value new cap and it did about the same. The bad cap I pulled from the 148 did something different, not a short but the meter went to figures right away of about 60 or so ohms and slowly built up. :glare:
    So I was wondering if this is a good way of checking caps the cheap and dirty way? But the Cobra is alive!! (y)

  2. Captain Kilowatt

    Captain Kilowatt Professional Amateur
    Staff Member

    Apr 6, 2005
    Likes Received:
    An analog meter is by far the better choice for checking electrolytic caps than a digital. You can see the meter swing much better than watching the numbers dance. You have the right idea about checking for good/bad but it will not give you value of course. A good cap should look like a near short for an instant and then the resistance should slowly increase as the meter is connected. A bad cap will not show a near short when the meter leads are first connected.

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