How to test / discharge capacitors when positive not accessible

Discussion in 'Glowing Bottle Tube Amp Forum' started by pikesville, Nov 7, 2019 at 2:51 PM.

  1. pikesville

    pikesville TDPRI Member

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    I was planning to do some work on my "Sweet Baby" JTA-05 Fender Champ clone but, having opened it up, I. can't see how I'm able to test or discharge the filter capacitors. The positive end of each is inaccessible, buried in the board below. See pic. I'd greatly appreciate any advice on how I can make safe this amp before starting work on it.

    As an aside, the mods I'm planning are a fixed bright cap across the volume pot (which I'm assuming I can get to around the back), a switch to remove/add negative feedback (R13), Rob Robinette's 10% power switch ( https://robrobinette.com/Generic_Tube_Amp_Mods.htm#10%_Power_Switch ) and a switchable 25uF capacitor over R4 (or C7).

    image.png image.png
     
  2. kbold

    kbold Tele-Holic Silver Supporter

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    You could (with power off, of course) pull the tubes and drain caps with a suitable resistor at the sockets. Insulate yourself suitably (of course).
    Perhaps give thought to fitting a drain resistor permanently (suitable high resistance) across the high voltage caps, so they drain in a few minutes when the amp is switched off.
     
  3. MuddyWolf

    MuddyWolf Tele-Meister

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    Turn off the amp with the standby on the operate position and let the sound fade out. This discharges the caps.
     
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  4. corliss1

    corliss1 Friend of Leo's

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    I'm still not sure how this myth gets around the internet, but it's 100% not true and not a safe way to verify an amp is okay to work on.

    How would you measure voltages on those caps if you can't get to them? They connect to *something* - a resistor, a rectifier, somewhere. Find that point on the schematic and use that location to bleed/test.
     
    Last edited: Nov 7, 2019 at 7:58 PM
  5. joeford

    joeford Friend of Leo's

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    .

    unplug the amp, turn the power on, and let it sit a few hours like that to be safe. when i come back later with test probes they're always drained. is this by some coincidence? I've worked on dozens of amps this way without an issue... what gives?
     
  6. corliss1

    corliss1 Friend of Leo's

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    Depends on the amp. I'm just saying it should not be used as a method to avoid testing the caps with an actual meter and verifying it is safe to use.
     
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  7. jman72

    jman72 Tele-Afflicted

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    Here's a great video that shows that it DOES work because the warm tubes are drawing charge from the caps as they fade out. I've done this myself many times to drain the filter caps (but I still always use a multimeter to check the voltage- better safe than dead). But I agree, always check the voltage somehow at some connection point in the circuit.
     
  8. pikesville

    pikesville TDPRI Member

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    Thank you everyone for your help - really appreciated.

    kbold - thanks, I'll definitely fit a permanent drain resistor as you suggest once I've made safe enough to do so. When you say take out the tubes and drain from there, I guess there is no harm in justing trying all points on each socket rather than targeting a specific pin?

    Corliss1 - With your approach, I think I have identified 3 suitable points, circled in orange that I can access - using Rob Robinette's Champ diagram which the JTA-05 seems consistent with in the most part (although I need to double check C6 position because it appears to be a 10uf 450v whereas C3 is 22uf 100v). Do you agree with my logic of choosing these three points?
    Champ layout.png
     
  9. corliss1

    corliss1 Friend of Leo's

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    Yup - those would be points in the power supply that I would expect there to be high voltage. The other side of those 100K resistors would also have voltage too.
     
  10. Wally

    Wally Telefied Ad Free Member

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    You can clip a grounding wire onto any of these pins...p8 on the r3ctifier, p3 of the 6V6, or p1 of the preamp tube with the amp unplugged and the power switch in the on position. The points on the board connect to those pins, but the pins might be more accessible.
    Draining the caps by unplugging or turning off with a chord ringing does drain the caps, but some amps’ caps can recharge. Always maintain the discharge.
     
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  11. pikesville

    pikesville TDPRI Member

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    Thanks corliss1 - I connected to R5, read 15v and drained down from there. Read 0-1v at each of the other points.

    Wally - thank you too. I only saw your reply after I'd tried the approach above but the pins you describe are very accessible and definitely a good option for next time.
     
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  12. pikesville

    pikesville TDPRI Member

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    I've updated the diagram to include Wally's suggestions in case of use to anyone else for reference

    Champ DV voltage checkpoints.png
     
  13. corliss1

    corliss1 Friend of Leo's

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    You'll notice those are the same points you have circled already, just on the other end of the wire :D
     
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  14. King Fan

    King Fan Friend of Leo's

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    Back at the earlier (but potentially life-saving) :) argument about draining caps. @corliss1 is right. Yes, caps will drain over a fairly short time if the tubes are 'hot' when the amp is turned off. *But*...

    1. That time is not as short as if you discharge the caps or use a proper-size bleed resistor. Folks working on amps get in a hurry. Zap.

    2. Caps won't drain if the tubes *aren't* hot. This can happen all too easily when we're testing an amp. Switch on, oh wait, forget something, switch off, go back to test. Zap.

    3. They definitely won't drain if the tubes aren't mounted, or only the rectifier is mounted. This is a routine step in startup, and sometimes useful in testing.

    It's possible to use hot tubes to drain your amp if you always remember to make sure they're hot *and* if you always remember to wait a minute for them to drain *and* if you always remember to test they're drained... Lot of 'ifs' though, and so simple to avoid.
     
    Last edited: Nov 8, 2019 at 12:56 PM
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  15. Wally

    Wally Telefied Ad Free Member

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    King Fan, ime, e-caps do drain en I unplug an amp or turn the power off while in standby switch play mode. You can witness t he drain. However, as I noted, unless a filter section has drain resistors as in the bigger Fenders or one establishes a connecti9; from the filter section to ground, caps can and will recharge. I sometimes watch that recharge with a meter just for grins....and as a reminder. Perhaps this recharge is why you are thinking that there is no drain that occurs when using the ‘dying sound’ method??? With an amp with drain resistors, there is no recharge that I have witnessed. I always check and maintain such a drain....as should be done with any other method.
     
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  16. King Fan

    King Fan Friend of Leo's

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    Thanks, Wally. We may be talking about different things re playing, and I may be talking unclearly re standby. :)

    I meant that *playing* a hot amp after you power it off isn't actually necessary to drain the caps. They'll also drain without playing; the playing lets you hear 'em drain.

    As for standby, I made it confusing with all that 'on' and 'off.' Yes, leaving the amp in 'standby' mode should keep the heaters hot, at least if the standby is standard standby. I'm gonna edit out my whole bit about standby. Apologies...
     
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  17. Dreadnut

    Dreadnut TDPRI Member

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    Give your friend a sharp xacto knife and tell him "poke it right there," LOL, guaranteed to discharge the cap. Then it will be safe to work on; you'll have positive confirmation of the discharge. This is another trick I learned in the Navy. (Couldn't wait to try it on the next unsuspecting newbie.) Be prepared to run...
     
  18. Wally

    Wally Telefied Ad Free Member

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    King Fan, I am not sure that caps will drain just by shutting an amp off...unless there are draining resistors in the filter section. If all caps drained as a matter of course, why would we be concerned with draining them?????
     
  19. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    Interesting discussion, I always leave standby on (and speakers connected when I shut off any amp in order to drain the caps, assuming that with standby off they don't drain as much or at all?
    But at the same time I never assume there is no voltage hiding in an unplugged amp, even if I drained the big caps manually.
    Don't assume is a good rule!

    I assumed we drain caps because we don't always know or remember the shutoff sequence?

    An answer to your above question would be helpful.
    More specifically, if caps do sometimes drain when power is shut off, what would prevent them from draining?
    Is leaving the amp on standby when shut off a way to keep the caps from draining?

    If the tubes use the current in the caps and everything is connected, how would the caps hold their charge when connected in running mode (not on standby) to hot tubes?
    Maybe really big caps can't fully drain before the tubes cool down enough to stop drawing a little power?
    I'm asking not arguing.
     
  20. charlie chitlin

    charlie chitlin Doctor of Teleocity Silver Supporter

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    I've seen this.
    It's pretty sure BillM knows his stuff.
    What's the deal with them charging themselves back up, though?
    Can this happen with the amp unplugged.
    I have a tech buddy who swears a PR recharged itself and bit him hard.
     
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