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Ibanez PT-9 Phaser Bias Adjustment

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  • Ibanez PT-9 Phaser Bias Adjustment

    Hi everyone,

    I've been working on repairing a Ibanez PT-9 Phaser, and have already made a lot of progress, thanks to help from this forum!

    The pedal is back up and running again, but needs bias adjustment. I have done this in the past with simpler circuits, but have trouble making sense of the instructions in the service manual (attached).

    I have just gotten a dual channel oscilloscope, so I should be able to produce some lissajous patterns (I'm using my phone as a signal generator).
    My question is: Where exactly do I have to put the probes? I can't see that mentioned in the manual. Does it just go to the output jack?


    Ibanez_PT9_service_manual.pdf

  • #2
    I've never heard of using a scope to set the bias . I use a white noise input, listen to the output and set it by ear.

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    • #3
      Originally posted by dmeek View Post
      I've never heard of using a scope to set the bias . I use a white noise input, listen to the output and set it by ear.
      I might give that a go (with input being looped guitar or something similar), until I figured out how to do it following the service manual. Thanks!

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      • #4
        Do you really need to change bias adjustment?

        For adjusting by ear using a 400Hz or lower frequency square wave input signal might work.

        To display Lissajous figures with your scope you need to set it to x/y mode.
        First channel probe goes to input, second channel to output (I think, as not otherwise noted)
        It typically takes a while to find the best scope settings.
        Last edited by Helmholtz; 05-17-2021, 06:05 PM.
        - Own Opinions Only -

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        • #5
          Originally posted by Helmholtz View Post
          Do you really need to change bias adjustment?

          For adjusting by ear using a 400Hz or lower frequency square wave input signal might work.

          To display Lissajous figures with your scope you need to set it to x/y mode. First channel probe goes to input second channel to output.
          Oh, that makes sense! I'll try connecting to input and output.

          When I swapped out the broken component, I didn't get signal at first, but regained the effect when adjusting the bias trim pot a little; just wanted to make sure I adjust it correctly.

          Thanks everyone, that should be enough to sort it out!

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          • #6
            Originally posted by clarisso11 View Post
            When I swapped out the broken component, I didn't get signal at first, but regained the effect when adjusting the bias trim pot a little
            If the bias trim is not a sealed type, you should probably give it a shot of cleaner.

            "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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            • #7
              Originally posted by g1 View Post
              If the bias trim is not a sealed type, you should probably give it a shot of cleaner.
              Thanks! Definitely wouldn't hurt..

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              • #8
                I managed to reproduce the Lissajous pattern from the service manual, using the method provided by Helmholtz, all looking good now! A word of warning to everyone attempting this, the correct trim pot range to get it right is tiny, even the slightest adjustment will throw it off.

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