21-07-2011, 06:10 PM
I go along with David. We're talking about paper caps here. If a capacitor is in a high impedance circuit such as grid, feedback or AVC, leakage could have a serious effect. If it is in a low impedance circuit such as a cathode bypass cap, then it will have little or no effect. Also it will depend on the voltage across it during normal operating conditions -the higher the voltage the higher the leakage current. Because of this, I tend not to rely on a test meter as the voltage applied to the cap during the test is very low - 22.5V IIRC with an AVO 8 for example. My favourite test is a neon in series with an HT power supply as David described - if the neon flashes or glows after the initial blink, the cap goes.
One point I must make about decoupling caps, the caps which are used to decouple the heater chain in a TV are under quite a strain and I change those on site if they are paper caps, ceramic caps are usually OK. You can imagine, if the last valve in the chain, usually the CRT, goes open circuit, then all the decoupling caps get a dose of full mains as little or no current is flowing.
One point I must make about decoupling caps, the caps which are used to decouple the heater chain in a TV are under quite a strain and I change those on site if they are paper caps, ceramic caps are usually OK. You can imagine, if the last valve in the chain, usually the CRT, goes open circuit, then all the decoupling caps get a dose of full mains as little or no current is flowing.
