working safely on an amp... discharging caps and such

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by spaceboy, Aug 13, 2004.

  1. spaceboy

    spaceboy Member

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    books and internet resources always go on about how you shouldn't work on amps if you don't know how to do it safely what with the various dangerous/lethal voltages and all... but hardly anyone ever says how you SHOULD work safely on amps!

    is it basically just discharge all caps (or at least the big ones)? i read something about how to do this properly but can't find it anywhere now, so i would appreciate it of someone could explain it or give me a link or something...

    and how else am I supposed to go about not killing myself for the sake of my amp?

    cheers
     
  2. loverocker

    loverocker Member

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    Yes - discharge all caps. The most important ones are filter caps. If your amp has a Standby switch, you can discharge the caps by switching off the On/Off switch before turning off the Standby switch. Keep on playing and you'll hear the sound decay over a couple of seconds as the amp runs on the juice stored in the filter caps.

    When it's all quiet:
    * switch off the Standby
    * disconnect the mains at the wall socket (this is so easy to overlook but *very* important - trust me: 240V AC up one arm, across your chest and then to ground through the other arm feels very unpleasant :eek: )
    * open up your amp
    * do what you gotta do

    As it happens, I always use a meter to double check the voltage across the filter caps before touching anything inside.

    The other way of discharging them is to make a lead: an insulated croc clip at either end and a 10K 2W resistor in the middle. You're supposed to connect it across the filter caps to discharge them safely. I made one, but now I do the thing I mentioned first.
     
  3. spaceboy

    spaceboy Member

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    perfect! that standby trick looks great, and that aiken link is the one i read before and lost! so thanks guys - really helpful
     
  4. Scott_F

    Scott_F Member

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    I just unplug the amp while I'm playing and that's the start. Then I take the Insulated Aligator clips with the resistor soldered in-line and I attach it to Pin 1 of the first preamp tube. Then I wait a minute. Then I check with a meter.

    Then I go to work. I always leave the aligator clip frorm pin 1 to ground. Caps are known to kinda charge back up a bit on their own. Leaving that clips in place assures you that nothing will build up.

    YMMV, be careful.
     
  5. Sean Doty

    Sean Doty Member

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    Forgive me, but I want to be absolutely sure before I start tinkering....

    After I go through the dischrage procedures set forth above, I want to measure the voltage on the filter caps to make sure they are really discharged....is this simply by connecting my black lead from the meter cliped to chassis ground and then the red probe to the individual terminals on the caps? Or is it black lead clipped to the negative terminal of the cap and the red lead clipped to the positive terminal of the cap to measure "across" the cap as it were?

    Again, sorry for the very basic question.....but I don't want any surprises...
     
  6. Richard Guy

    Richard Guy Member

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    Either way should work. If you are real anal about it, measure directly across the cap. Make sure the AC power cord is disconnected and remove all jewelry... put your left hand in your rear pocket, then begin. Sometimes after the caps 'appear' to be discharged, a charge then reappears. If you connect a clip lead to the largest cap in the B+ from '+' to chassis ground and leave it on there while you work on the amp, you will be fine. Be very, very aware that this clip lead is on the cap as you would not want to turn the amp 'ON' with it connected. ;^)
     
  7. sstweed

    sstweed Supporting Member

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    One more point not mentioned. After you have done everything mentioned here - unplugged the amp and attached the drain resistor - take the amp out of standby (close the standby switch). Sometimes (ussually) there is a filter cap past the standby switch that will not be discharged by your drain resistor when the switch is open. You could then be messing around in the amp and drain it into a finger. Not pleasant.
     
  8. Sean Doty

    Sean Doty Member

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    Thank you both, very much, for your replys!
     
  9. sabby

    sabby Member

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    Would an insulated test/jumper lead from radio shack, with a 10w/50000 ohm resistor soldered inline, be suitable to drain the caps from pin1 of V1? A dumb question -- and redundant -- but I'd rather not fry.

    :eek:
     
  10. aleclee

    aleclee TGP Tech Wrangler Staff Member

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    Lord Valve sells discharge probes for like six bucks.
     
  11. sabby

    sabby Member

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  12. sstweed

    sstweed Supporting Member

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    50K would drain sort of slow, but not unbearably. 10K is a good value. Bear in mind that some amps drain pretty good on their own. It's just not a good idea to count on that happening. I have seen some amps hold a charge days after they were turned off and unplugged. My experience with my silverface fenders is that if I turn them off and remove the chassis to check something, there is rarely more than a couple volts left in the caps by the time I get it to the bench. But once I had the chassis out and decided to check the bias on the tubes, so I had to remove the discharge resistor. When finished I started to slide the chassis back into the cabinet. Of all the millions of times I have installed a chassis into these amps, this time I bobbled it slightly. I didn't drop it but my reflex was to grip the chassis and in doing so my thumb touched one of the pins on a preamp tube. Let me tell you! My thumb was burned, and the breathe was knocked out of me for several minutes. Like I had been kicked in the chest. Not a mistake I will soon repeat! Be Careful!!!!
     

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