Phase 45 Clone passing a clean signal (mostly)

[aleks_tedstone]

Hello there,

I have just built a PCB MXR Phase 45 clone from Fuzzdog effects (schematic available here: https://shop.pedalparts.co.uk/P45_Phaser/p847124_7971603.aspx) and it's the first time I've been stumped by one of these kits. I have put together a monosynth, univibe, analog bitcrusher and various dirt pedals which I was able to get working sooner or later from Fuzzdog, and they have all been great additions to my collection.

The problem: The pedal only passes a clean tone, until I (very unscientifically) held it in a certain place that must have provided just enough capacitance to make it do the phase sweep we all know and love. My fingertip was around R6, R7, R15 and R16, so I made sure to reflow all of those solder joints but to no avail.

I believe the LFO is operating correctly due to the voltage readings on the opamp of IC2, and the fact the JFET gate is also swinging between two values. I have adjusted the trimpot through its entire range to no avail, and probed the continuity of every trace I could see to check for bad joints or shorts. I have tried to check that the input is not shorting to the output, but that doesn't seem to be the issue. I guess all-pass filters can allow the signal through clean if they aren't oscillating to create the phase notch, but this is about the limit of my understanding.

The only mod is the univibe switch. Mix mod is left off, and bias is controlled by the trimpot. The JFETs are matched in house by Fuzzdog, although in hindsight I should have double-checked with my multimeter.

With a 8.82V battery, component values are as follows:
2N5457 JFETs are both:
D= 5.09V
S= 5.09V
G= osc between 4.25 to 4.60 V

IC1 TL072:
1= 5.09
2= 5.09
3= 4.89
4= 0
5= 5.09
6= 5.09
7= 5.09
8= 8.82

IC2 TL072:
1= 5.09
2= 5.09
3= 5.09
4= 0
5= 2.5 - 5.6 osc
6= 2.5 - 5.6 osc
7= 1.3-8.7 osc
8= 8.82

Z1: 5.09 V at cathode, 0V at anode
Power supply diode: 8.80V at cathode, 0V at anode

These seem to check out against the General Guitar Gadgets voltage list for their Phase 45 clone.

If anyone can help, I would be eternally grateful and this post may serve someone encountering a similar problem.

Aleks

[danfrank]

It sounds like a FET bias voltage issue, the bias voltage isn't optimal for good phasing sound. Also, the Phase 45 is a subtle sounding effect, 2 stages doesn't make for strong phasing sound. If you have the means to check for FET cutoff voltage, check to see how well matched your FETs are.
I've built a Phase 45 clone and was lucky enough to find 2 FETs that were matched to within 0.01volt. Once dialed in, it had a nice phasing effect. Another thing I noticed is that the bias trimmer on most of these effects/clones are really touchy... There is one tiny spot on the trimmer where good phasing occurs. I always try to use a 10 turn trimmer for the bias adjustment, it makes biasing more precise. Hope this helps.

[aleks_tedstone]

Thanks! I'll try and get the bias sorted before replacing the FETs, it's a (nominally) 25-turn trimmer so it might be that I just didn't find the sweet spot because I'm not familiar with their design relative to trimpots that look more like regular potentiometers.

[danfrank]

On most phasers it's usually between 1-2 volts, somewhere in that range.

[deft]

Had the exact same problem with this exact same kit! There is indeed a bias sweet spot, but I kept getting 'lost' on the supplied 25 turn trimmer and didn't have the patience to sit there twiddling it. Swapped it for a normal 500k single turn trimmer and dialed it straight in - there is an obvious window where it's biased correctly so it doesn't need to be that precise.

[aleks_tedstone]

Thank you everyone for your inputs - you were all right! It was the bias, and the 2N5457 JFETs are both:
D= 5.08V
S= 5.08V
G= osc between 3.47 and 3.75 V

The multi-turn trimmer required a lot more than its nominal 25 turns - I might have over-turned it in one direction when I first started adjusting it? Anyway, worth persevering to get some nice phasey swirls. Hope this helps future Phase 45 builders.