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Fender DRRI Vibrato Stuck On

Discussion in 'Amp Tech Center' started by Roadgnat, Apr 16, 2021.

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  1. Roadgnat

    Roadgnat TDPRI Member

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    Hi, all.
    I recently purchased a used 65 Deluxe Reverb Reissue. Sounds great, but there seems to be an issue with the vibrato channel. As I understand it, the vibrato is not supposed to function without the foot switch plugged in. That isn’t the case. The only way to turn it off is to drop the intensity and speed down to 1.

    It works like that without the foot switch plugged in at all. Plugging the foot switch in makes the oscillation of the vibrato much noisier and the foot switch does not turn the vibrato off. Reverb switches on and off just fine. Tested the FS with a multimeter and everything is fine on that front.

    I’ve seen similar threads to this, but no definitive solution. In the grand scheme of things it’s not a “bad” problem to have. I can turn the knobs down to turn off the vibrato effect. However, it would be nice to be able to turn it on and off within a song, or just simply understand why it isn’t functioning properly.

    Thanks for any insight!
     
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  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The way the 65 DRRI vibrato works.

    The vibrato is *off* unless the foot switch turns it *on*. There must be something with the contact of the jack/plug if the foot switch is ok.

    EDIT: Possibly a leaky C19 or open R38 would create this problem. The jack/plug should be OK. TP26 needs to have some negative voltage to turn *off* the vibrato. The negative voltage either is not getting past R38 or is being lost to ground after R38. R38 is a 2.2M Ohm resistance. From my understanding, resistors over 1M fail more often than resistors of 1M or less. A leaky C19 or a short to ground are also possibilities.

    I doubt there is a problem with the negative bias supply. A low bias supply would effect the power tube bias which would likely have the power tubes red plate.
     
    Last edited: Apr 16, 2021
  3. Roadgnat

    Roadgnat TDPRI Member

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    Thanks for the reply. I’ll pop it open and take a look at those components.
     
  4. Peegoo

    Peegoo Poster Extraordinaire

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    Also check the footswitch jack internals & solder tabs to ensure nothing wonky has occurred there.
     
  5. Boreas

    Boreas Friend of Leo's Silver Supporter

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    Probably fairly obvious, but make sure the center prong of the plug hasn't broken off inside the jack.
     
  6. nojazzhere

    nojazzhere Doctor of Teleocity

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    Have you double checked that someone before you didn't plug a shorted RCA plug into the jack on the back? I know this sounds like an obvious thing, but sometimes we all miss "the obvious". ;)
     
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  7. Roadgnat

    Roadgnat TDPRI Member

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    Yeah, I also thought maybe it’s the jack. Wishful thinking on my part it could be that simple. We shall see!
     
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  8. Roadgnat

    Roadgnat TDPRI Member

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    Nothing plugged into the jack. I can plug and unplug the foot switch as normal it just doesn’t affect the fact that the Vibrato is engaged no matter what. I mean, perhaps I should just be happy that it is in the on position and don’t need a mod to make it that way! Lol
    It’s easy enough to turn the intensity and speed all the way down and the tone isn’t affected.
     
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