Ad Widget

Collapse

Announcement

Collapse
No announcement yet.

Fender Deluxe Reverb Scope / Wave question

Collapse
X
Collapse
 
  • Page of 1
  • Filter
  • Time
  • Show
Clear All
new posts
Previous template Next
  • #1

    Fender Deluxe Reverb Scope / Wave question

    Having an odd issue with a SF Fender Deluxe reverb from the late 60s: http://www.prowessamplifiers.com/sch...b868_schem.pdf

    With a signal generator at input and reading on a scope, about 50% of the time, switching the amp on from standby mode results in a a curved, sawtooth-type wave when the volume is maxxed. I've tapped and chopsticked the board and doghouse connections all over, and nothing seems to make a difference. Flipping the standby switch off and on a couple of times will get the wave back to a normal clipped / square wave. Sometimes it's right from the get-go, but about half the time, it's the curved sawtooth wave.

    It's not the tubes (I've subbed in known good ones) and it's just had a recent filter cap job and had the board electrolytics and bias cap replaced (values of these are correct). I also put a jumper on the lugs of the standby switch itself to bypass it and rule out the possibility that's the problem.

    Any ideas on what could be the root cause? Could it be some sort of intermittent arcing occuring in either the PT or OT?
    Tags: None
  • #2
    Does the strange wave go away if you turn the volume down and back up again? Does it ever happen if the volume is down when you put the amp in play mode and then turn up the volume?
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

    Comment

    • #3
      No, it doesn't seem to - if the sawtooth-type wave starts, turning it up or down doesn't make any difference. Although, at lower levels, it looks pretty similar to how a normal wave would look. Flipping the standby on with the volume down still has about the same odds of getting the sawtooth-type wave once you go to turn the volume up.

      Comment

      • #4
        This only happens with the reverb up? Or does it go away of you turn reverb to zero?

        If you unplug the reverb cables will it still do it or not?

        What does it SOUND like coming out the speaker? Hum? A low WOOOOO? A high pitched SKREEEEE?

        Look at your scope, what frequency is this? Pull your probes off the work, and touch your finger to the scope probe tip. The waveform you see may be ugly, but will still be 60Hz. Now set the sweep for maybe two complete cycles across the scope screen. Now back to the amp, how many cycles of the noise do you see inthe same space at the same setttings.

        You want to know if it is your OT arcing? Pull the PI tube and power up. Still does it or not?

        Do ANY of the controls have ANY effect on the sound of this? Tone controls affect the tone of it? ANY volume control affect the volume of it at all?

        Look at the stages. With it ready to do this, pull V1, any change? Pull V2, any? Pull V3, any? Oh, have the PI back in the amp for these tests.

        Regardless of what you poked, resolder the ground ends of all the filter caps.
        Education is what you're left with after you have forgotten what you have learned.

        Comment

        • #5
          This only happens with the reverb up? Or does it go away of you turn reverb to zero?

          If you unplug the reverb cables will it still do it or not?

          What does it SOUND like coming out the speaker? Hum? A low WOOOOO? A high pitched SKREEEEE?

          Look at your scope, what frequency is this? Pull your probes off the work, and touch your finger to the scope probe tip. The waveform you see may be ugly, but will still be 60Hz. Now set the sweep for maybe two complete cycles across the scope screen. Now back to the amp, how many cycles of the noise do you see inthe same space at the same setttings.

          You want to know if it is your OT arcing? Pull the PI tube and power up. Still does it or not?

          Do ANY of the controls have ANY effect on the sound of this? Tone controls affect the tone of it? ANY volume control affect the volume of it at all?

          Look at the stages. With it ready to do this, pull V1, any change? Pull V2, any? Pull V3, any? Oh, have the PI back in the amp for these tests.

          Regardless of what you poked, resolder the ground ends of all the filter caps.
          Education is what you're left with after you have forgotten what you have learned.

          Comment

          • #6
            Spent some time with it again this morning -- Enzo, thanks for the detailed test recommendations. I traced the signal towards output and found nothing out of the ordinary in wave shape or voltages. PI was fine, wave on Pin 3 from each 6v6 was fine. Scratched my head for a bit, checked all electrolytics again for ESR, proper ground, etc. Tested choke, it read 156 ohms which is right in spec. Tested output jack. Wave looked right. Switched probe on scope and the problem was gone *facepalm*.

            Thanks for the help, guys. I gotta get some new probes!

            Comment

            Previous template Next
            Working...
            X