old Ibanez/Maxon PT9 phaser problem.

[served]

Hi.

I got a phaser. Its PT9 but the old one with 3403AD chips.

It has a problem. The LFo sweeps fine. The biasing works and feedback is set to the last point before sqealing.
But it just acts strange.
For example, if one sweep lasts for 4sec, the phasing pattern is only audioble for the first 1 sec, and the rest is just clean signal.
I have only one channel oscilloscope, so I can't do the pattern biasing. But no matter how I bias it, it will not phase the whole cycle.
The other strange thing is that the background noise is making the full sweep (hissing).

Is there a good explanation for this? I have had a failed phaser before but it didn't pass any signal then and it was a failed CA3080 in Small Stone.

If I look at my oscilloscope screen then the LFO is sweeping triangle the whole time, but the phase sound will stop following it at one point.

[greaser_au]

I do not have oine of these so I'm just looking at the schematic.

Assuming this is a failure, and nobody has been in there twiddling trimpots, I'd be looking around D101 as a first step (what is the voltage at CP3?) before changing anything else (given that some of the sweep works).

Your CRO needs an X input rather than dual channels for the pattern biasing,  but you should be able to set it up by ear if the static voltages are correct.

good hunting!

david

[armdnrdy]

I would check the FETs (between source and drain) for changing resistance. Make sure that you don't have one or more "dead" JFETs.

Also check C112 which will affect the sweep.

[Mark Hammer]

It sounds to me like the LFO is not providing a proper waveform.  You will note that the LFO uses a back-to-back pair of electrolytic capacitors instead of a single non-polarized cap as found in most dual op-amp LFOs.  One or both of the caps might be dried out and not up to spec.  Might be worth replacing them.

[served]

I messed around with it yesterday.

Replaced the Feedback trim and Bias trim.
It might be something else. I managed to get the sound do the sweep in a way. Well it doesn't sound perfect but it sweeps.
It can be a dry cap issue actually. The more I think about it the more it sounds like that.

Another interesting thing is that it squeals like hell in bypass, if I turn the feedback knob down, then it stops, but if I play it in "not sqeualing position", its just booring and there isn't enough wet in the signal.
So I will start changing caps, maybe the LFO signal is not wide enough.


EDIT:
So I changed all the elCaps in the pedal. Sounds completely different now. The bias is working differently and it sounds like it should.
Still the sqeal is there in bypass. But it can be reduced by the Feedback trim. And now its sounds alot better!


Thank you for your thoughts!

[Mark Hammer]

Thanks for the progress report!  Nice to know that all of our musings are not too far off base.

happy phasing.