Trying to build an MXR Phase 90 but I'm not hearing any effect at all

[Joe_Shroe]

I'm trying to breadboard the MXR Phase 90 from Electrosmash but the output sounds clean and not at all affected by the circuit. I'm using the same TL061's, but for the FET's I'm using PF5102 instead of the original 2N6962. I'm also only making 2 stages of the phaser stage instead of 4 as I don't have enough breadboard space. Here are the voltage readings:

    1:
    2 (IN-): 4.2-5.6v (oscillating at ~1 Hz)
    3 (IN+): 0.30-0.45v (oscillating at ~1 Hz)
    4 (GND): 0
    5:
    6 (OUT): 0.7-8.6v (oscillating at ~1 Hz)
    7 (VCC): 9.0V
    8:

I'm also not sure what Vbias should be set to either, but it's not affecting the sound at any setting. Do the PF5102's not have the right specs for this circuit and should I get different ones?

[duck_arse]

hey joe. [assuming you mean 2N5952 and not 2N6962 ....] you have clocked the fact that the PF5102 is pins-reversed to the 2Nx9x2, haven't you?

[Joe_Shroe]

Yes I've made sure that I connected the right pins on the FETs. The one I'm using does have a much lower cutoff voltage than the ones on the plan though, so I think I should get the right parts before I try anything else.

[Mark Hammer]

Verify that you have the zener oriented properly.

The trimmer needs to be adjusted right to hear any phasing whatsoever.  Although the biasing of the FETs could be done without the zener diode, the zener is there to provide a stable basic voltage to the trimmer input so that you never have to rebias the FETs, once set.  The trimmer divides down a known voltage to feed the FET gates exactly what they need.  By the time the battery has run down to 5.1V, it wouldn't have enough power to run any part of the circuit.  So chopping down 9V to 5.1V gets you what you need without getting in the way of anything.

[Joe_Shroe]

I see that the zener would give me 5.1V from any source voltage, but for the purposes of this build I'd be fine with just making a voltage divider to get the 5.1V wouldn't I?

Also how important is the cutoff voltage for the FET? Would I need a cutoff voltage that's at least -3.5V or lower which the 2N5952 has, or are there any other specs I should be looking at?

[antonis]

I see that the zener would give me 5.1V from any source voltage, but for the purposes of this build I'd be fine with just making a voltage divider to get the 5.1V wouldn't I?

It depends on voltage divider specific impedance..
(for a couple of tens Ohms it should be OK but for a couple of hundredfold thousands Ohms it shouldn't..)

[Mark Hammer]

I see that the zener would give me 5.1V from any source voltage, but for the purposes of this build I'd be fine with just making a voltage divider to get the 5.1V wouldn't I?

If you are powering it from a battery, it will likely matter at some point, since dividing down 9.6V gets you something different than dividing down 9V, 8.5V, 8.0V, etc.  The zener is there to mimic an unchanging supply voltage.  Now, if you are using an external power-supply ONLY, then you can likely get away with skipping the zener.

Incidentally, there is nothing magical about 5.1V.  If you had zeners of slightly higher or lower voltage, that would work too.

[Joe_Shroe]

For anyone with this same problem, I forgot to mention that the solution was to adjust the trimmer pot for the phasing to happen at all as Mark Hammer suggested. Or more importantly, making sure the Vref is somewhere near what you need, in my case 5.1V. If the Vref is the same as your 9V source voltage then your zener diode is kaput and you better hope you have extras.

[Mark Hammer]

There are instances where trimmers are there to pin down a "best" setting out of the possible settings.  For instance, some modulation pedals, like phasers or flangers, will include a trimmer set to the maximum feedback possible before oscillation sets in.  I set up a friend's Deluxe Electric Mistress the other week to get more intense feedback than he had been getting. 

But there are other instances where trimmers are included to pin down a now-it-will-work setting.  Phasers, chorus/flanger/delay pedals rely on elements that require a specific bias voltage to actually work, rather than merely work "better".  Tweaking the trimmer is not always going to be the magic button that makes a pedal-build work.  But it is often a very good place to start in one's trouble-shooting, simply because of how critical it is to the successful functioning of such pedals, and because it requires no removal or modification, or even a scope, just a small screwdriver and a decent ear.

Congrats on getting your pedal up and running.