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Always remove the old electrolytic from the circuit. As a quick repair, putting the new electrolytic in parallel with the old one _will_ work most of the time. But it's not a good idea. The only reason it can work, is that generally, as electrolytics age, they lose capacity. Paralleling the new cap to the old adds the capacity of a new cap with that of the old. But then there is still an old, dried up electrolytic in the circuit. It will continue to lose capacity, and can eventually fail shorted, or partially shorted - acting like a resistor. You're just asking for problems doing this. Disconnect the old cans. Wire new replacements under the chassis. Also, when possible, avoid replacing only one section of a can - yeah, the others may still be good, but for how much longer? One failed already. Also, it is possible (although unlikely) for a really bad cap to develop leakage between sections.
To test old paper capacitors using a multimeter, do the following. Note, that this only applies to the old wax/paper or molded paper capacitors (like the black stripey ones, or "MicaMold" paper caps) :
Disconnect the cap completely from the circuit.
Switch the meter to the resistance scale, and hold the probes, one in each hand.
Balance the old capacitor across the two probes.
Hold the probes over a garbage can.
Move the probes apart until the capacitor falls.
If it makes a <clunk!> noise as it hits the garbage can, or any variation thereof, it's bad.
<grin>
But seriously, a multimeter won't be of a whole lot of help, because you really need to test the capacitor under working voltage. Even newer meters with cap checking functions only supply a tiny voltage/current, and bad caps won't always show leakage this way. You need something like an old style HeathKit Cap Checker that can supply full working voltage to the capacitor under test.
-Ian
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