How can I tell if my caps are discharged, and safe?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Relicula, Mar 8, 2006.


  1. Relicula

    Relicula Supporting Member

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    I recently bought some mustard caps to restore a few changed parts in my Marshall, the 0.68 cap, and a few output coupling caps.

    I have a mixer resistor wired to a couple of alligator clips that should drain off the caps, how can I test them to make sure I wont get zapped?
    I was wondering if I could put my multimeter on them somehow to see if they are completely discharged, any help would be appreciated before I blow myself up:jo
     
  2. brad347

    brad347 Member

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    after you're pretty sure you've got them discharged, use a plastic handled screwdriver (or other insulated thing) to touch both the + side of the caps and some ground point (I use chassis).

    Is there a big, scary spark? No? Then they're discharged.

    That's my method, but there are probably better ways! :D
     
  3. Washburnmemphis

    Washburnmemphis Member

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    "That's my method, but there are probably better ways!"

    Better way: Substitute your multimeter for the plastic handled screwdriver, much safer.
     
  4. Chiba

    Chiba Gold Supporting Member

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    Coupling caps aren't the ones that store a charge - you're thinking about filter caps. The mustard caps you're talking about you shouldn't have to worry about.

    --chiba
     
  5. Blue Strat

    Blue Strat Member

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    Yes, using your multimeter set for DC voltage is how to tell if they're discharged or not. Note that this DOES NOT DISCHARGE the caps, only measures the voltage on them.

    It doesn't matter that you're replacing in the amp, it's still a VERY good idea to make sure the power supply caps are discharged before poking around in there.
     
  6. donnyjaguar

    donnyjaguar Member

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    Color me cautious, but when I work on a tube amp I wear my gummy soled shoes and take voltage readings with my right hand. A jolt of 450Vdc hurts like hell.

    I wouldn't recommend discharging your cap's with a short. This usually causes some damage to the contacts or the tool. Simply measure the voltage on the first capacitor and when its down nice and low you're good to go. In my designs I always put bleeders over the capacitors anyway. The only reason some amp's don't have this is probably for cost-reduction reasons. By my calculations a 470k resistor will drain 100uF to a safe level after one minute while adding less than 1mA of load.

    I usually pull the plug on the amp too. This way there's no danger of touching the 120Vac (240Vac for limeys) by accident or introducing ground loops.

    I'm a radio amateur too and there are plenty of stories of "silent keys" (dead radio amateurs) who killed themselves while working on vacuum tube linear amplifiers.

    DJ
     
  7. dbx

    dbx Member

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    :dude thanks for the laugh...this is my biggest fear with working on amps...everything's cool then all of a sudden "SPARK"and Jimi's asking me what key we're in....

    :YinYang
     
  8. thid

    thid Member

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    With the older Marshalls you can monitor the DC voltage between the HV fuse and chassis ground.

    And another way to drain the filter capacitors on these old JMP Marshalls* is thru the power supply resistors:

    Clip a jumper (with insulated clips) from the first preamp tube socket pin 1 to the chassis and wait for the voltage to drop.
    Leave the jumper until your work is done so the caps don't collect electrons from elsewhere in the circuit.

    And don't forget to remove the jumper before re-energizing.
    Mine have yellow and black safety tape on them.

    *Edited
     
  9. Rod

    Rod Tone is Paramount Silver Supporting Member

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    Hi Ricky...thid's way of using a jumper from pin1 of your first 12AX7 to the chassis is a good one...you measure w/ your voltmeter between ground and the HT fuse...The 1 handed technique is helpful....new tunes at www.motorkings.com check out Cold Sweat.....
     
  10. chuby galoso

    chuby galoso Member

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    any one who sticks his hands in a guitar amp without enough basic electronic know how to ask this question is in need of a good shock. it only hurts for a second. as long as you dont hit your head on something when your ass is on the floor.
     
  11. scottywompas

    scottywompas Member

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    There are plenty of sites out there with this info. I'd say, do some googling, then do some reading.

    We don't want any Obotuarys on TGP.

    Scott:dude
     
  12. John Phillips

    John Phillips Member

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    Been there, done that. I was 18, and had plenty of experience working on solid-state stuff (which typically has voltages in the few tens at most), and I knew to unplug from the wall. Then I did some work on my first 50W Marshall, and these are one of the dangerous amps with no bleed resistors on the main caps. The amp had been off (and unplugged) for a LONG time, but it still threw me halfway across the room. I think I touched something like one of the big 10K resistors on the board, which are directly connected to one or more of the caps. I think I had my other hand or arm resting on the chassis too, since I thought there was no risk.

    You tend to remember that type of lesson. The really scary thing is that the voltage on the cap still measured over 200V afterwards, which I discovered while trying to work out where this very frightening and unexpected shock had come from... and it hurt for a lot more than a second, BTW.

    My prefered method now in amps with no bleeders is to clip an old-fashioned analog voltmeter (they're quite low resistance) across the caps and watch the voltage fall, and click down through the ranges to speed things up as it gets lower.
     
  13. 1968

    1968 Member

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    Classic :D;)
     
  14. brad347

    brad347 Member

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    i ain't dead yet!

    Okay so I won't do that anymore! :p
     
  15. claptonisgood

    claptonisgood Member

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    John! We are brothers in (twitching) arms! I had the exact same experience at 19 with a 100W Marshall. When I regained my senses, I wondered why I was sitting on the floor across the room looking at my workbench. I developed a profound respect for filter caps that day.
    :angel
     
  16. brad347

    brad347 Member

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    hearing these stories will make me more careful. I've always been nervous around filter caps though. I've worked on only about 10 amps and always used a resistor to discharge the caps but I did use the screwdriver method to "check" that they were discharged (stupid, I have a meter sitting right there....) They always were luckily :D

    Actually I did get shocked by a champ once when I knew even less about what I was doing than I do now. The on-off switch failed and I was going to replace it for a friend (simple enough) I knew enough to drain the caps but I forgot to and I got a barely unpleasant little jolt. No big deal. Luckily it wasn't a bigger amp...

    A 50w or 100w Marshall, I'm sure that would pop ya. Glad I never tangled with one of those.

    I'd like to learn more about amp repair but I'd also like to live to tell the tale!
     
  17. TheArchitect

    TheArchitect Guest

    If it doesn't knock you on your ass and/or kill you, you are probably ok. :)
     
  18. mbratch

    mbratch Member

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    I was quite shocked (literally) today to discover that this method is not sufficient. On my BFDR it doesn't discharge the capacitor holding 420V on the rectifier. So I also put a jumper in that discharges this particular cap through one of the resistors going to ground.
     
  19. John Phillips

    John Phillips Member

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    It should if you left the standby switch in the on (play) position. That cap is upstream of the switch, but if the switch is on it will still discharge through the B+ chain like all the others.

    You really need to know how the power supply is configured to tell which of the ways of discharging is safe and which isn't.
     
  20. mbratch

    mbratch Member

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    Ah, that makes sense because I actually clipped one end of my jumper to one side of the standby switch. So I could have achieved the same effect by turning on that switch.
     

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