Modifying the MXR Phase 90 (M-101)
This post deals with the regular MXR Phase 90 (M-101). It does not apply to the EVH phaser, Custom Shop Script models (with and without LED), as they have a different type of circuit board. The 1974 Handwired model can be true bypassed with the help of this post. But for this excercise, make sure the circuit board in your matches the one pictured to the right – some boards are green and others red, but as long as the layout is the same, it’ll work.
Also, I’ve noted the difficulty level for each mod. Be advised that desoldering components from the M-101 circuit board is tricky – it has through-plated holes, and it is very easy to damage the board. I do not offer support or troubleshooting help, nor am I liable for any damaged pedals.
Before we start, I should also say something about the trimpot. The M-101 Phase 90 does have one, located at the top of the board. Do not adjust it – you will just make the pedal not function properly. Despite popular belief, it is not an effect mix or intensity setting. It is used by the factory to compensate for component tolerances, to make the oscillator work properly. If the pedal works, the trimpot is set where it should be. If it’s set anywhere else, there will be no phase effect…
R28 mod (also known as the ”script” mod)
The regular Phase 90 has a feedback resistor – basically, some of the phased signal is fed back into the phaser section, to make the effect stronger and more pronounced. Sadly, it also causes some distortion in the midrange, which isn’t very pleasant. Cutting the feedback resistor out makes the effect more mellow, making the pedal sound a little more like the original ”script logo” pedals from the ’70s. This mod is part of the Stinkfoot Stage 1 Phase 90 mod, by the way. You can hear the difference here:
Start by disassembling the pedal – remove the knob (on most pedals, it simply pulls straight off the shaft) and the jack nuts. Then remove the nuts holding the pot and switch in place. Gently push on the pot and switch, and the circuit board should pop out of the box. Make sure to note where washers etc goes, so you can put it all back together again.
Do not desolder the resistor! I can’t stress this enough – one of the through-plated holes also serve as a pathway between the two sides of the circuit board, and it is very easy to damage it (leaving you with a dead pedal). A pair of small cutting pliers is all you need, and will keep the pedal safe and working fine.
Then simply put the pedal back together. Don’t forget the washer that goes on the pot (inside the box) – without it, the circuit board won’t sit right. Once I have the board back in the box, I always start with the nuts for the jacks. Do them up by hand first, to make sure you don’t damage the threads in the plastic jacks.
This is a mod to the phasing section which makes the phaser a little more wobbly (more like a Uni-vibe – hence the name). A Uni-Vibe is at heart a four-stage phaser, just like the Phase 90. The main differences are that the Uni-Vibe uses a lamp/photocell circuit to produce the sweep, and the capacitor values for the phase stages. I initially tried replacing all four capacitors with the values from the Uni-Vibe, but eventually settled on replacing only two of them. Together with the R28 mod above, it was part of the Stinkfoot Stage 2 Phase 90 mod. You can hear it here:
This mod is a little more involved, as it requires you to desolder two capacitors, clean up the holes (without damaging them) and install new ones. This time, you need a soldering iron/station with a fairly small tip, solder and a desoldering braid.
Disassemble the pedal (as above).
Locate C1 and C3, desolder and remove them. Use desoldering braid to make sure you’ve cleaned up the holes as much as possible before removing the capacitors. Keep in mind that the holes are through-plated, so there will be solder on both sides of the board. I use the soldering iron to heat the soldering point from the back, and lift the legs of the capacitor straight up. After clearing as much of the solder as possible with a desoldering braid, I then push the capacitor legs through with the tip of the soldering iron. To clean the holes up, I sometimes add fresh solder and desolder it again.
What you’re left with should look like the picture to the right. Once you’re there, install the new capacitors: C1 = 0.015uF, C3 = 0.22uF. Use box film capacitors with 5mm spacing – caps with bigger physical size will be tricky to fit, due to the proximity of other parts.
Put the pedal back together (as above).
That’s it! And before you ask – yes, I might add the true bypass mod as well. But on pedals with this type of setup (where the switch is soldered directly to the circuit board, it is quite advanced work, requiring you to cut out a section of the board. And I am a little hesitant to provide such information – I already get enough angry e-mails from people who have messed their (simpler) mods up. It’s funny how some people can do a hack job, and then accuse me of ”damaging their pedal”, wondering ”what I’m going to do about it”… I guess time will tell how many such e-mails I get about this post, and I’ll decide from there
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