Princeton Vibrato problem, Help!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by twangbanger, Jan 18, 2008.


  1. twangbanger

    twangbanger Supporting Member

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    The other guitar player in our band has a blackface princeton AA964 amp and the vibrato doesn't work on it. When you turn up the speed or intensity pot there is no difference in sound. tried using a pedal switch in the pedal jack w/ no results. Haven't started checking components yet,was wondering if there are common things that are more prone to failure ,I've got the schematic and a multimeter and cap meter, just looking for some input from the techs on this one. Thanks Rob
     
  2. Blue Strat

    Blue Strat Member

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    First, check the bias on the power tubes. If they're set too hot the intensity of the tremolo is severely compromised. Shoot for about 20mA.

    It's probably easiest to swap all caps in the trem circuit as it's faster than actually troubleshooting and caps are cheap. Also check the values of all the resistors in the circuit.

    Half of the PI tube drives the tremolo and could be dead, but it's kind of unlikely.
     
  3. Richard Guy

    Richard Guy Member

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    This is a long shot, but I have witnessed the following; the 'roach' (component inside the black shrink-werp) had the two conductors shorted to each other on one end inside the black shrink-wrap. I recall the controls acting very strange as a result in addition to the vibrato not working. I'd follow BlueStrat's advice first before looking for this anomally.
     
  4. twangbanger

    twangbanger Supporting Member

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    Thanks! I'll see if I can get those caps locally!
     
  5. mark norwine

    mark norwine Member

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    Princeton's don't have a roach.

    My guess: the cathode bypass cap is old & dead. No ac bypass, no oscillator.
     
  6. Richard Guy

    Richard Guy Member

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    Hello Mark,

    Thanks for the clarification regarding 'no-roach' ;^)
     
  7. twangbanger

    twangbanger Supporting Member

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    Changed the bypass cap and one of the ceramic disc caps and all is well!Those are real sweet sounding amps.
     

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