Ibanez PT-9 Phaser Bias Adjustment

[clarisso11]

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

[dmeek]

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.

[clarisso11]

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!

[Helmholtz]

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.

[clarisso11]

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!

[g1]

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.

[clarisso11]

If the bias trim is not a sealed type, you should probably give it a shot of cleaner.

Thanks! Definitely wouldn't hurt..

[clarisso11]

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.