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Vintage threads from the first ten years
Discharging Filter Caps
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 5/31/1999 7:47 PM |
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EmScott |
Discharging Filter Caps Question(s) from a long-time "lurker": Ritchie Fliegler's book about guitar and amp maintenance talks about discharging filter caps by shorting them to ground (the chassis), potentially causing a BIG SPARK. Other sources talk about draining out the charge by running test leads from certain specific pins on the preamp and on the power tubes to ground. Still other sources say that leaving the standby switch "on" when the power is off will help drain the stored charge in the filter caps. The question(s) are: 1) Do both the screwdriver-shorting method and the tube-socket-running to chassis-ground method work ? Do they only work in amps with certain kinds of circuits ? 2) If you use the screwdriver shorting method, must you touch the positive end of the filter cap (and not the negative) to the chassis ? Or the other way around ? 3) If leaving the standby switch "on" helps, how much does it help, and how long must it be on before the charge is usefully dissipated ? 4) Do experienced techs test that the filter caps are indeed discharged after draining them ? Can you ever be deceived about whether you've drained them properly ? Thanks for any help offered. Surely someone else early on the learning curve will be interested in the answer(s): the safety implications are real... |
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| 5/31/1999 9:07 PM |
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| Bruce |
Yes and the screwdriver method can blow the cap. So don't do it. Self draining through a tube occurs only if the tube has been on long enough for heaters to cook the cathode hot enough to emit electrons. Dont trust this, but instead, build yourself a test clip lead with a 10K to 50K resistor in it to short out the power supply to ground. A 1 foot 18g stranded wire with a 1 watt resistor in series and an aligator clip on each end would do the trick.
It works in every power supply that has voltage stored in the power supply's capacitors. Does it need to be done everytime? No, especially if the amp has a self biased power section. But don't trust it. Transistor amps use a much lower voltage supply and usually have a bleeder resistor in the supply that drains the caps for you. Don't trust those either.
Don't use the screwdriver method except as a last ditch effort to make sure there is no voltage left in the power supply. Don't leave anything to chance here.
Usually this takes just a few seconds but allow 30 to 60 seconds to drain it all off. And then DON"T trust it to do so.
Yes...Of course! There are 2 kinds of electronic techs: those that have been shocked and those that will. First (of many) commandment of electronics: BEWARE of the unknown lightning the lurketh in undischarged capacitors, lest it cause thee to be thrown on thy fanny, and thus sitting in thy own pee. Bruce Mission Amps | |||||
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