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Old 3rd Feb 2023, 11:46 pm   #11
trobbins
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Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
Default Re: Testing for leaky caps

If this is for a coupling cap in a valve circuit then you may be able to do a test in-situ. For a common circuit with a coupling cap between stage A to stage B, and the cap going to stage B's input grid, with just a grid leak resistor to 0V on that grid, then remove stage B's valve and measure the DCV across the grid leak resistor. Caveats are that stage B's valve can be removed safely and without adverse consequences (eg. the supply rails don't rise beyond safe levels), and that stage A has the coupling cap connected to its supply rail (eg. via a plate load resistor). The leakage current is then a calculation that includes the input resistance of your meter. Typically the measured voltage is 0.0V, but increases with cap leakage current.
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