2000 Twin Reverb RI - Constant Static V4 issue

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by msl5078, Feb 3, 2015.

  1. msl5078

    msl5078 Supporting Member

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    I have a 2000 Fender Twin Reverb, and am having some trouble with the reverb part of the amp. The amp is producing a constant static, and when the reverb knob is turned up the static volume increases.

    So far I have tried the following:

    * I tested the power tubes using a tube tester and found all power tubes to be within spec.

    * I pulled the phase inverter, and had no noise.

    * I removed each preamp tube individually, until I pulled V4 and the noise went away.

    * I than tapped on V4 with a pencil, and discovered it was microphonic, so I removed and replaced the tube with a 12AT7 I had that I knew was good. The noise still was there.

    *I removed V4 and played the amp through the normal channel and no static was present.

    *I used the footswitch to turn the reverb channel on/off, the static was still present no matter if the reverb was on or not.

    * I than took my pencil to the inside of the amp, and got noise when pins 6 and 7 were touched. I also noticed some oxide buildup on the power tubes and took care of that while I was there. I cleaned up the solder on the pins on V4 and when the amp turned on the static was still there. Pins 6 and 7 still make noise, maybe the circuit board connection needs cleaned up? Also several of the pins were moving, so I am in the middle of re-tensioning the tube sockets.

    * I than thought maybe the RCA cable might be bad, so I unplugged the RCA cable from the chassis. Static noise still there.

    I'm not sure what else it could be? Please Help!
     
  2. msl5078

    msl5078 Supporting Member

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    I would like to add when the reverb is turned on the amp produces a hum in addition to the static.
     
  3. donnyjaguar

    donnyjaguar Member

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    I *think* you've nailed it down to the vicinity of V4. And I *think* what you are describing is a bad solder joint or loose tube socket contact.
     
  4. msl5078

    msl5078 Supporting Member

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    I used a sine wave generator and went through the circuit using fenders Test points and think I've narrowed the problem down to a combination of a bad preamp tube in V4 and also a bad socket. While I could do the repair myself, I'm taking it to a tech to take a look at it. I just don't have the time to keep messing with it at this point. So we will see what happens.
     
  5. donnyjaguar

    donnyjaguar Member

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    That's too bad, because I think you've got it. Now I've repaired one of these amplifiers and its a beast of a job to get to all the components with that PCB and all the spade connectors. I suspect this is what you're finding daunting.
     
  6. TimmyP

    TimmyP Member

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    Replace the plate resistor and the cathode bypass cap.
     
  7. Blue Strat

    Blue Strat Member

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    Yep, a lot of work that could end up breaking something else (cheap connectors) in the process. And if the first component change doesn't fix it...take it apart again.
     

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