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ampage archive
Vintage threads from the first ten years
Draining filter caps
:
 5/31/1999 9:35 PM |
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EmScott |
Draining filter caps Thanks ! So you'd drain the caps DIRECTLY from one end of a capacitor to the chassis ground THROUGH the wire with the resistor in series: NOT using the preamp-socket-pin to ground method ? Do you mean to imply that you can't do it by the preamp-socket-pin to chassis ground method ? And does it matter--I bet it doesn't--which end of the filter cap you shunt to chassis ground ? Also, by "Do they only work in amps with certain kinds of circuits ?" I meant "Does draining the filter caps INDIRECTLY by attaching a wire--presumably with a big resistor in series within it--from a designated preamp socket pin to chassis ground work in ALL circuit types ?" Or do some preamp circuits have other circuit elements between the stored charge in the filter caps and the preamp socket pin making this method impossible ? Would it blow up the average digital voltmeter to test that the stored charge was gone from within the filter cap by touching one end of the cap to the other THROUGH the DVM ? I really appreciate your taking the time to help us here: I hope I'm not belaboring the obvious... |
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| 6/1/1999 12:17 PM |
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| Don Symes |
You _can_ do it that way, the issue is whether there is a big enough series resistor between pin and cap to keep from sparking. That spark is an instantaneous (a few hundredf nanoseconds) discharge of huge gobs (technical term) of energy stored at hundreds of volts. That comes out to some insanely high current that can blow holes in the cap's dielectric (and anything else in the discharge path INCLUDING YOU). These caps literally store enough energy, at high enough voltages, to put a LETHAL current through you. The advantage to the resistor-on-a-clip is that you KNOW the resistance is there, and you KNOW where the discharge path is. You don't necessarily discharge from cap+ to GND - the bias supply is negative. Instead, discharge from cap+ to cap- (if the capacitor plates are at the same voltage, no energy is stored). PS - try to keep the thread together - [reply] to the responses you get so the discussion is easy for everyone to follow. Take care, ask questions, and good luck! |
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| 6/1/1999 1:41 PM |
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| jason |
It does matter. Look at it like this, if the negative side of your cap is already tied to chassis ground and you try to discharge the cap by placing your shorting probe (wire with resistor Bruce described) from the negative side of the cap to chassis ground you really haven't accomplished anything. Might better stick with Don's suggestion and stick your shorting probe from the negative side of your cap to the positive side.
No, and I highly recommend that you do it that way. It only takes a few seconds and you know for sure that the cap has discharged all of it's voltage. Safety first, jason | ||
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| 6/1/1999 10:41 PM |
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| EmScott |
Thanks to you both for the input...yes, I saw that I'd inadvertently started a new thread...the reply "radio button" was off the screen to the "north"... |
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| 6/2/1999 7:54 AM |
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| Don Symes |
Now THERE's a first-rate tip! |
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| 6/2/1999 8:44 AM |
|
R.G. |
At the risk of even more fully polishing this... er... clod of earth... If you're in your amp a lot, you can improve safety by adding a 10K 10W resistor that goes to ground through the NC contacts of a SPDT relay. The relay needs a 120VAC coil and is connected across the transformer primary. Whenever power is applied to the transformer, the relay *opens* the contact and prevents the resistor from sucking current. When the AC power goes off, the relay drops out and the resistor now runs the capacitors down pretty quickly. The power caps are held safe all the time the power's off. Since the relay contacts never interrupt high currents, they should last a LONG time. |
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