One quick and dirty solution I have for testing electrolytic
capacitor polarity is to connect a 15-volt supply such as a little
wall wart through a 10 or was it a 20-Kohm resistor.  The actual value
does not matter as long as it is high enough so as not to draw more
than a milliamp or less if completely shorted to ground.

        I then pick off the voltage at the positive end of the
capacitor and low end of the current-limiting resistor and feed that
to a voltage-controlled oscillator such as a NE566 whose own RC
constant is set to deliver a tone of about 1,000 HZ when the
modulation input is near ground.  This modulation input is relatively
high impedance so it just monitors the voltage at the capacitor.

        When I connect a test capacitor, the tone from the oscillator
goes high and then sweeps down as the voltage rises.

        This little circuit works fine for many electrolytic
capacitors.  If you are testing a big filter capacitor, the time
constant created with the series resistor means that you have to wait
20 or 30 seconds to see if you are going to get a charge.  If the tone
starts to slide down, then it is correctly polarized.  If it stays
high, then it is either shorted or backwards.

        Of course, remember to discharge the capacitor after testing
or you will zap something later when installing it or touching it
against some other part of the circuit.  It will eventually charge to
near VCC if left in the tester long enough so keep that in mind.

        Any other VCO with a high-impedance modulation input should
work as well or better than the NE566 which is kind of an old chip
that I just happened to have on hand.  The square-wave output of the
566 will drive a pass transistor and small speaker just fine.

Martin McCormick WB5AGZ  Stillwater, OK
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