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Fender Red Knob Champ 12 with 100Hz Hum

Discussion in 'Shock Brother's DIY Amps' started by jimi00, Feb 9, 2013.

  1. printer2

    printer2 Poster Extraordinaire

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    Could you measure across the power transformer secondary, the two red wires where it meets the two diodes?



    I do not think we want to start redesigning the amp at this time. Just get it back to original operation.
     
  2. jimi00

    jimi00 TDPRI Member

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    PT secondary: 192 VAC / 0 VDC

     
  3. jimi00

    jimi00 TDPRI Member

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    Thank you for all the advice.

    I'm not sure if my reasoning is right but I tried eliminating that possibility by using an ungrounded wall socket plug, but it has no effect.
    I tried removing the screws holding the power transformer in place against the chassis so as to have the PT "floating", and that doesn't change anything either.

     
  4. printer2

    printer2 Poster Extraordinaire

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    Well that is right. I do not understand why you have such a high AC voltage on the caps. The 192V AC gets rectified by one diode and charges up the one capacitor while the other diode charges up the other capacitor. The one capacitor sits on the other so you get double the voltage that you would normally get by rectifying the voltage coming from the transformer.

    The peak value of an AC sine wave is about 1.4 times the AC voltage reading. So 192 x 1.4 = 270V DC given no losses (there are always losses). So your 246V and 249V readings makes sense.

    The maximum AC voltage in the circuit should be the 192V, you should not be able to get more than that. I would expect to see up to 25V AC on the stacked pair of capacitors, would like to see the value down to half that though.

    Your 545V AC translates into a peak DC voltage of 750V on your capacitors should be turning into smoke and you should get arcing in your tubes at this level. So I am at a loss what is going on.
     
  5. printer2

    printer2 Poster Extraordinaire

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    Had a thought. Do you have this AC voltage on the power supply caps if the power tube is pulled?
     
  6. jimi00

    jimi00 TDPRI Member

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    Yes.
    C102 616 VAC
    C103 623 VAC
    C101 630 VAC
     
  7. bdgregory

    bdgregory Tele-Afflicted

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    there is a voltage doubler in that PSU (C101) that takes the B+ up to 496 Vdc.
     
  8. dsutton24

    dsutton24 Doctor of Teleocity Ad Free Member

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    Many meters, when used this way, do not measure only the AC component. They measure the DC plus the AC peak component.
     
  9. printer2

    printer2 Poster Extraordinaire

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    Yes, caught that.
     
  10. printer2

    printer2 Poster Extraordinaire

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    I thought of that but I have a half dozen meters and even the cheap $6 one reads them separately so I was going on the assumption the meter did not.


    Now to rule that out, do you have any high voltage caps? If you put one in series with one of the meter leads it should block the DC when taking AC measurements.
     
  11. Obsessed

    Obsessed Telefied Silver Supporter

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    Very interesting thread. I seem to have the same hum, so I'll try to follow every step and hope for your success. BTW, I just started the Champ 12 Owners Club.
     
  12. jimi00

    jimi00 TDPRI Member

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    As a matter of fact I do have HV caps, the same ones I used to replace the power filter caps in the amp (47/350V). My meter is an "El Cheapo" Digital piece of c**p. I tried the experiment with a 12V adaptor I had lying around.
    DC 12V
    AC 24V
    AC with cap 20V (and dropping)
    Also this might not be strange to you but I can only take AC measurements one way, with the red probe on +.

     
  13. jimi00

    jimi00 TDPRI Member

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    Welcome to the club.
    The first rule of the Champ 12 Owners' Club is you do not talk about the Champ 12 Owners' Club.
    The second rule of the the Champ 12 Owners' Club is you DO NOT talk about the Champ 12 Owners' Club.

     
    czech-one-2 likes this.
  14. dsutton24

    dsutton24 Doctor of Teleocity Ad Free Member

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    I have a Craftsman DVM that has this 'feature'. The trusty old Simpson 260 would also do this if you simply set the selector switch to AC and tagged it across a composite voltage. That's what the 'output' jack was for, it just put a capacitor in series with the meter leads and the rest of the meter guts.
     
  15. spacerock777

    spacerock777 TDPRI Member

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    This amp is supper finicky on 6l6 some do some do not I had mine ever since new and the original tubes out of the crate hummed like crazy as well. What drives be bonko is the pops with Sovtek 12ax7wa again I go ape over this
     
  16. spacerock777

    spacerock777 TDPRI Member

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    mine since new always even right out of the box hummed with stock Fender NOS glass I hate the pop Iget with Sovtek 12ax7wa and so on other than this I am pretty uesed to this things hum deal and feel nude without it
     
  17. Racing

    Racing Tele-Meister

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    FWIW.
    As far as i know it was the same team that came up with most redknobs.
    My old Super 60 suffered from the same dilemma. That went away as i adhered to the old saying that the grounding of the bridge/CT vs the negative of the first lyte is the forbidden zone.
    I regard this as a design flaw tbh,and having modified that with a simple piece of wire run directly for the bridge neg the amp turned into studio quality silent in idle.

    [​IMG]

    The grey wire in the pic made all the hum run for the hills.

    As it according to legend was the same team that developed the Champ...might be worth looking into.
    Again.
    Absofrigginlutelynothing is let to be installed between the bridge neg/the CT and the grounding of the first lyte.
    If so...the amp WILL hum.
     
  18. printer2

    printer2 Poster Extraordinaire

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    I hate to say it but I do not understand your post. Any chance you could draw out a quick schematic of your change?
     
  19. printer2

    printer2 Poster Extraordinaire

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    The capacitors are pretty large and will take some time charging up with the DC voltage where then the AC voltage can be read. A sub 1uF would be more convenient and safer as they are not polarized. You don't want the electrolytic cap to get hooked up in reverse to the DC voltage. A big bang and shrapnel would be the result. Also remember once the cap is charged it is charged with the same voltage on your amp and it can give you the same shock.

    If you know anyone you can borrow a meter which will read true AC in a DC circuit that may be the best way to go.
     
  20. jimi00

    jimi00 TDPRI Member

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    Sorry Racing,
    The link to the photo is dead.

     
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