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Another amp blowing fuses... but why?

Discussion in 'Amp Tech Center' started by MrCoolGuy, Jul 10, 2020.

  1. tubelectron

    tubelectron Tele-Afflicted

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    Well no, @tubegeek : why the PT would be more endangered that way ? It would rather be the opposite situation :

    1 - if the SBY SW is open and the shunt cap proves to be defective o_O, then at worse your are in the situation of a "permanently closed" SBY switch. No possible short. Your SBY switch malfunctions because of the leaky/shorted cap across his terminals, but that'all. So you replace the defective cap and voilà... :cool:

    2 - most of the time, you play your amp, right ? :lol: So the SBY SW is closed, then shorting the capacitor itself, which de facto has no more voltage accros his terminals... :cool:

    I hope I'm clear in my description... :oops::rolleyes: I apologise in advance ! ;)

    -tbln
     
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  2. tubegeek

    tubegeek Friend of Leo's

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    I'm saying the OP's setup makes him liable to get a short on the power trans (just as it did). Ultimately possible PT damage.

    The way you have it is AOK.

    I was being sarcastic.
     
  3. tubelectron

    tubelectron Tele-Afflicted

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    Ah, sorry @tubegeek : I always miss some subtleties of english language... o_O:D

    -tbln
     
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  4. tubegeek

    tubegeek Friend of Leo's

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    Sarcasm fails about 20% of the time via written comms.

    As a chronically sarcastic person I am well used to backtracking.

    Is Français your native language? You do just fine in English.
     
  5. tubelectron

    tubelectron Tele-Afflicted

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    Well, thanks ! :)

    Yes, French, absolument. I try to maintain a reasonable training to English language - at least in writing / reading - since I do not work anymore in export... Not easy ! :confused:

    -tbln.
     
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  6. jhundt

    jhundt Doctor of Teleocity

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    so the problem was this cap which doesn't show on the schematic drawing?

    and the purpose of this cap is to make the standby switch function 'better'?

    there's a good reason to not add a standby switch and a bunch of 'improvements' to a good old simple circuit.

    glad you finally got it sorted!
     
    Last edited: Jul 20, 2020
  7. jhundt

    jhundt Doctor of Teleocity

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    It was right there, on this schematic; but I kept looking at the other one, and missed it. If we all had been referring to this drawing someone would have spotted it earlier, I guess.
    http://www.lynx.net/~jc/sobSchm.gif
     
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  8. tubegeek

    tubegeek Friend of Leo's

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    That's the one I had been looking at! But I was expecting to see the two-caps-plus-two-resistors cluster and I just didn't see it in the pics.
     
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  9. D'tar

    D'tar Friend of Leo's

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    Well shoot ! No wonder it failed..... It's only 60.volt!!!:)


    upload_2020-7-21_14-41-56.png
     
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  10. jhundt

    jhundt Doctor of Teleocity

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    yes; I am so used to seeing a schematic laid out in a certain way, this one tricked me. It is a lesson for us, I guess.
     
  11. tubegeek

    tubegeek Friend of Leo's

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    That's not it. That's the cap that was supposed to have B+ on it. 47uF 600V.

    Look on the other side of the standby switch, the two cap two resistor combo to ground. They mustv'e swapped in the single cap (which failed) as a production change/random decision/repairman's kludge.

    (The line & box symbol is Euro for 'electrolytic cap.')

    20200721_164331.jpg
     
    Last edited: Jul 21, 2020
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  12. D'tar

    D'tar Friend of Leo's

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    If those(both) failed the fuse would blow even in standby.

    Any way... I was just playing... It looks like they wrote 60.v but it's actually 600v
     
    Last edited: Jul 21, 2020
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  13. tubegeek

    tubegeek Friend of Leo's

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    Wow. You're right!
    How come the amp in the thread didn't I wonder? The shorted cap must've been on the Node D side of the switch, in parallel with 47uF 600V.

    Therefore its only purpose was to provide a failure mode, and it did its job perfectly.

    SMH. Someone screwed up bigtime.
     
  14. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    No, those two caps and resistors are there too. This is an additional cap. For stand-by pop. They just mounted it at the intersection of stand-by, choke and OT instead of on the stand-by switch.
    It is that 47mf/600v cap shown on this other schematic. Just didn't think of it. Brain fart.
     
  15. tubegeek

    tubegeek Friend of Leo's

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    That brown turd cap is many things, but 47uF/600V is NOT one of them.
     
  16. D'tar

    D'tar Friend of Leo's

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    nF

    [​IMG]
     
    Last edited: Jul 24, 2020
  17. King Fan

    King Fan Poster Extraordinaire

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    Poll. Which of the following is *most* prone to cause user error?
    1. uF, nF, pF
    2. Ohms, K, M
    3. Color bands on resistors, allowing for 4 vs. 5 bands
    4. Tiny, oddly labeled settings on DMM dials
    5. The 1mm-tall 'K' or 'M' hiding next to the 1-inch-tall (often decimal) reading on DMMs
     
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  18. D'tar

    D'tar Friend of Leo's

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    Um,

    I choose, number 1

    No, 3

    Maybe 5 or is it 4

    No,no, 2 for sure!

    Ahh..... 6 it is!
     
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  19. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    My bad... .047 uf / 600v.
     
  20. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    6. Microscopic print on diodes and small caps.
     
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