Bass going in & out on Deluxe Reverb

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by 1morepedal, Mar 7, 2015.

  1. 1morepedal

    1morepedal Member

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    I have an original Fender Deluxe Reverb from 1966. Lately, the bass is going in and out. The bass can be out for a few seconds or a few minutes. When it is out the sound is obviously very thin and the bass knob has no effect whatsoever. When it suddenly kicks back in, the bass knob operates normally. Any ideas on what might be causing this?
     
  2. slider313

    slider313 Silver Supporting Member

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    Try touching up the solder on the tabs of the bass pot and make sure the wires are making good contact.
     
  3. Diablo1

    Diablo1 Member

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    And you might try cleaning the pot with Deoxit.
     
  4. 1morepedal

    1morepedal Member

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    I've tried cleaning. I'll touch up the connections. In viewing the schematic, I see a 0.047 uf capacitor (however, I see a .033uf in the layout drawing, hmmm). Anyway, If that capacitor was shorting out wouldn't that shunt all my low freqs to ground thereby bypassing the bass pot? How do capacitors usually fail? Short or open? My other guess is a bad pot.
     
  5. myersbw

    myersbw Member

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    When you cleaned...did the problem exaggerate or get slightly worse? If you noticed any change in it...swap the pot out. Often a cleaning of an older pot will reveal the defects more significantly.
     
  6. 1morepedal

    1morepedal Member

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    I really didn't notice any change after the cleaning.
     
  7. pdf64

    pdf64 Member

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    Same on both channels?
    If one only, it may be that its slope resistor is intermittent, eg hairline crack, more prevalent on such longitudinally mounted resistors, perhaps due to board flex.
    What happens when the treble control is swept, when the amp in the fault mode?
     
    Last edited: Mar 7, 2015
  8. 1morepedal

    1morepedal Member

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    I can't remember what happens on the other channel or how the treble pot reacts. I'll try this next time. Is the slope resistor the 100K ohm in the tone stack circuit?
     
  9. swiveltung

    swiveltung Member

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    Try the other channel. If it's ok it could be the pot has gone bad. Doesn't happen often But I have had old BF pots go bad.
     
  10. swiveltung

    swiveltung Member

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    That is pretty common, the layout says .one thing but all I ever see is the other version in reality.
    I think it's an AA763 thing vs AB 763 thing. Not sure many AA's were ever made...
     
  11. pdf64

    pdf64 Member

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    I can't think that it's likely a bad bass pot would cause this issue?
    For it to cut bass it would have to short along the track somehow, or short to ground.
    It's possible that the track is flaking off or a solder blob has got into it, but that's uncommon.
    A bad connection, bad resistor seems more likely, after that a bad cap.

    Yes.
    You may be able to see how if that goes very high resistance / open, then the only route that signal could get through to the volume control and next stage is via the treble cap / pot high pass filter (250pF and ~250k makes its break frequency around 2k5Hz http://www.sengpielaudio.com/calculator-RCpad.htm).
     
    Last edited: Mar 8, 2015
  12. zenas

    zenas Member

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    By original do mean all original ? Or has it had the electrolytics replaced?
     
  13. 1morepedal

    1morepedal Member

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    So it happened again (only for a minute or 2). I quickly moved to the Normal channel and the bass was just fine so this seems to be localized to the Vibrato channel. I'll get my chopstick out and poke around the bass circuit to see if I can instigate the problem manually and hopefully identify the bad component.

    As far as the electrolytics, I replaced the filter and bias capacitors. I also replaced the Vibrato volume pot with a push-pull so I can bypass the bright cap (it is now a "pull-bright"). This was completely non-destructive and I saved the original pot if I ever want to restore it to original. Note, that the bass issue I'm experiencing was happening prior to this mod.)
     
  14. tlpruitt

    tlpruitt Member

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    I suspect a cold solder joint on one of the tone pots in that channel.
     

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