Is this sound of phaser FETs hitting cut off voltage?

[disorder]

I am trying to dial in a Maestro Phaser PS-1 that I rescued from the landfill. The work I did to it included replacing all components in the power supply after the transformer, and also replacing the trim pots. Here is the schematic. I am following the biasing instructions listed on the schematic. I noticed that even after I set the JFET bias to the "middle" of the range where I can hear the sweep, once I start bringing the oscillator amplitude up, it will cause the JFET bias to cross over a certain voltage (seems to be about -1.8V) and I get this wonderful thump noise... see video below. Is this the cutoff (Vgsoff) of the FETs? Perhaps causing some DC shift in the opamp/allpass circuits which makes it's way to the output?

https://youtu.be/2flDo77Tk9E

[Kipper4]

There's  phaser like that over on Beans site.
Someone is bound to be able to help.

[disorder]

Well it looks like I should address another problem first... I noticed my +12V and -12V rails shift up and down by about 500mV in sync with the oscillator. Check this out... top and bottom traces (purple and yellow) are the voltage rails, and the middle trace (blue) is pin1 of the oscillator opamp IC4.

[GibsonGM]

I don't even know if that's a problem, but instinctively I'd think that maybe a cap in the power supply or LFO section is faulty (old electro?) and no longer doing its job as a reservoir to stop power supply fluctuation.  At that age, all the eletro's should be changed out anyway.

Just a guess.

[disorder]

Yeah the shifting of both rails and the ripple on the positive rail has me concerned. I get a nice thump even with oscillator grounded as the rails bump up and down. I replaced all the electros in the entire circuit, same capacitance as schematic and most of them got increased voltage rating. I also replaced the zener regulator diodes and series resistors. The series resistors (230ohm) are getting real warm, which has me concerned. I think my next step will be to disconnect internal power supply and feed this thing +9 and -9V from batteries.

[GibsonGM]

That wouldn't be a bad idea, I think.    That thump is weird....on the decay??  Hopefully someone will have heard something like that before and get back to you - I am mystified.

[disorder]

I disconnected the internal power supply and the phaser still gets that weird noise when JFET bias gets to about -1.8V... Anyone care to offer advice... I'll take anything that might stick at this point

[robthequiet]

There is a note on the schematic, with a "3" in a triangle, referring to trimpot R1, which I take to mean the output offset -- does such trimpot exist in the circuit? It doesn't seem very clear as the output offset is noted in the calibration but I can't find the label as "R1" . The word "thump" is involved there.

Otherwise, the LFO bleeding into the supply rails suggests a lack of isolation or insufficient power regulation, as GGM said. Hm. I was voting on the zeners until you said you replaced them. Just for kicks I would check that the switches are clean -- corrosion can cause issues.

My three guesses are up. GL and let us know anything new.