Mutron (Nutron) II Phaser question

[sl8v]

Hello all iron gurus!
Another "soldering by numbers" guy here. I'm stuck in debugging this great pedal. Layout is from bangerang101, pcb etched by local tech. Its (almost) all working fine except of bad oscillation if feedback knob turned about 3/4 CW  (7.6K between 1 and 2 lug). I use icl7660scpa inverter, 4558 chips (also tried  tl072).
Readings on ICs 1-4 are about these values:
-0.5mV  9.22V
+0.5mV  50mV
0           2mv
-8.23V    0

IC 5:
50mV   9.23V
40mV   3.3-3.8V
40mv    200mV
-8.25V   200mV

IC 6:
-6/+6V     8.55V
0             -3/+3V
-2/+3V     3mV
-8.25V      0

ICL7660s:
9.23V    9.23V
4.65V    7.46V
1mV     5.21V
-4.01V   -8.26V

I tried to change feedback pot, change values of r7 and r26 with no big success.
One finding while debugging is if I change R7 with variable resistor (10K iirc) i can tune some kind of "deepness" of the effect. Very nice mod for my ears, so i plan to make additional external pot for it.
I also read somewhere that lt1054 is a better option, but i can't buy for now.

Your help would be greatly appreciated.

[hatchet]

i can add photos if needed...

[Bret608]

Hi,

I built this last year and like it a lot. Hopefully we can help get this sorted out! My first question would be: did you follow the set-up procedure for the on-board trimpots in Bangerang101's original thread? Not sure if that makes a difference; just thinking out loud.

[midwayfair]

The trims are just to adjust the oscillator, they don't affect the dry signal behavior in any way.

This is not totally abnormal. The feedback pot really is feedback, so turning it all the way up is sending an in-phase signal from the output back to the input. I'm guessing your feedback is a fairly high pitch.

R7 in the stock unit limits the feedback to a little under half of the volume, and ALSO forms a filter with C5 at about 6KHz. If R7 gets too small, it starts letting in more and more high frequencies, which results in more and more oscillation even without playing anything, just like pointing a microphone at a speaker. So what you probably want to do is filter it a little more strongly, and leave R7 as the stock value. Replace R7 with the 8k2 again and if it still oscillates, increase C5 to 4n7 (4KHz) or even as high as 10nF (~2KHz). Use a socket if you like. It will affect the sound a bit (it alters where the "notches" appear that creates the phase shift effect) so you probably only want to make the cap as big as you absolutely have to in order to kill the oscillation.

Of course, if it sounds good to you before you reach the point where it oscillates, you can leave that cap alone and simply make R7 10K or 11K.

The other possibility is that your charge pump is creating the whine, but as long as pins 1 and 8 are connected it should be just fine.

[sl8v]

thanks for your help, Jon and Bret. yes, "whine" is the better word here. when i try change r7 (Vr=25k) with feedback knob off - there is no whine or other bad sounds, just like intensity of effect is changed. maaaybe i have a mistake in pcb (it was rewritten to .lay format) - need to check it third time. with all values stock on powered pedal and effect in "Off" there are some values readings:
fb - pin1<- 7.62k-> pin2 <- 2.2k-> pin3 (border of whining)
r26 - 3,20k
r7 - 4.87k
when pedal in ON - r7 increases to 6.3k . i changed r26 to 12k but that also changed feedback usability. will try cap values soon.
Only increasing r7 doesn't help.
One more question - is difference of ~1v (+9V against -8.25v) could be a reason?