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Please help with MXR Phase 90 distortion problem

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Please help with MXR Phase 90 distortion problem

Postby DC-Choppah » Tue Sep 22, 2020 12:08 am

I need help figuring out if my MXR Phase 90 distortion pedal can be made clean. Is it broken?

I just got it on the used market. Looks to be in fine condition. But perhaps it is defective. Or just not what I expected.

Plugging the guitar directly into the pedal, and then directly into my mixing board, I can verify that the mixing board is not clipping from the meters. Sound is clean with the pedal off.

The distortion comes from the pedal itself. Like the pedal is both a phaser, and a fuzz box.

I am using my DC power brick from my pedal board to power the pedal. The same DC power cord works with my other MXR pedal no problem.

Here is a clip of what I am hearing as I play and turn the phasor on/off.
https://drive.google.com/file/d/1oywYRw ... sp=sharing

This was recorded with my Tascam handheld digital recorder listening to what comes out of the Adam studio monitors. Nothing is clipping in the monitor chain. The fuzz you hear comes from the pedal.


I was hoping more for the sound of Grant Geissman on Chuck Mangione's Feel so Good. They say he used a MXR Phase 90.

Grant's solos starts about 5 1/2 minutes in...
https://www.youtube.com/watch?v=NDSBV0vTfTo
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Re: Please help with MXR Phase 90 distortion problem

Postby Watchmaker » Tue Sep 22, 2020 1:53 am

That's definitely not how mine sounds. God only knows what's causing it. Actually I bet one of the forum members will identify the problem post haste but I can't recall his name atm. Have you opened it up to see if someone monkeyed with it?
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Re: Please help with MXR Phase 90 distortion problem

Postby resistorman » Tue Sep 22, 2020 3:28 am

Have you tried it with a battery? Have you turned that trimpot on the board yet :D
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Re: Please help with MXR Phase 90 distortion problem

Postby DC-Choppah » Tue Sep 22, 2020 3:57 am

Sounds identical when powered from the 9v battery.
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Re: Please help with MXR Phase 90 distortion problem

Postby DC-Choppah » Tue Sep 22, 2020 4:11 am

Playing with the trim pot, does not help with the fuzz.

Turning the pot too much in either direction takes the depth of the phase away. A sweet spot in the middle gives it the full range of depth. This is like a weird depth control. Sounds like a phaser in one position only.

Unfortunately it is still a fuzz box at any setting.
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Re: Please help with MXR Phase 90 distortion problem

Postby DC-Choppah » Tue Sep 22, 2020 5:02 am

If I turn the guitar volume knob down to 3 or 4, then the pedal no longer distorts.

But then when I turn the pedal off, there is a big (15 dB?) jump lower in volume.

The pedal is acting like it has a big boost of gain in front of it when you turn it on. And this is too much, and causes it to distort badly.
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Re: Please help with MXR Phase 90 distortion problem

Postby forumuser931182 » Tue Sep 22, 2020 7:08 am

It doesn't look like anyone has tried to modify it. Last owner may have plugged in an incorrect power supply perhaps.
You can find details about the circuit here: https://www.electrosmash.com/mxr-phase90
but only of use if you are electronically minded.
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Re: Please help with MXR Phase 90 distortion problem

Postby Hugh Robjohns » Tue Sep 22, 2020 11:08 am

If it sounds like it's phasing properly when you lower the input level, then the chances are it's a problem either with the input buffer opamp, the output transistor, or the one of the power supply voltages (Vref or the 9V supply).

There's a good talk through the circuit and fault-finding process here:

https://www.circuitbenders.co.uk/forum/index.php?topic=1532.15
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Re: Please help with MXR Phase 90 distortion problem

Postby Watchmaker » Tue Sep 22, 2020 1:03 pm

No to be critical of your purchase decision...This is why I almost always buy anything with electronics new. You never know what the last person, or the one before them, did. Unless you have a drawer full of spare parts, some free time, and like to solve this kind of puzzle, imo the difference between 20 bucks for a used item and 100 bucks for a new one pays for itself instantaneously. :-)

Oddly, I have no problem buying used rack mounted gear and hate the idea of spending for new compressors. fwiw, I use the script version with LED (model CSP101SL) and mine has served faithfully for years.

Good luck with the project, I hope you find a happy, easy solution!
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Re: Please help with MXR Phase 90 distortion problem

Postby DC-Choppah » Tue Sep 22, 2020 3:33 pm

Thanks for the info!

The voltages all check out. Thanks for the link to the circuit walkthrough Hugh.

So I clipped the R28 resistor (feedback resistor) that they note many people do. This is known as the 'R28 mod'.

This helped considerably. Still a bit dirty. But if I roll back the volume to 8 it cleans up.


New audio file after the R28 mod and guitar on 8.
https://drive.google.com/file/d/12ak3R8 ... sp=sharing

I can work with this now.
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Re: Please help with MXR Phase 90 distortion problem

Postby Hewesy » Tue Sep 22, 2020 7:03 pm

The R28 mod is essential for any new Phase 90, no idea why they thought adding that boost in was a good idea.

What's the signal upstream of the Phase 90, just guitar or any other FX pedals?

What's the guitar, and pickups? If it cleans up when you dial back the volume I wonder if you're clipping the input.

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Re: Please help with MXR Phase 90 distortion problem

Postby DC-Choppah » Wed Sep 23, 2020 12:24 am

Hewesy wrote:The R28 mod is essential for any new Phase 90, no idea why they thought adding that boost in was a good idea.

What's the signal upstream of the Phase 90, just guitar or any other FX pedals?

What's the guitar, and pickups? If it cleans up when you dial back the volume I wonder if you're clipping the input.

Hewesy

Well, for the recording I posted, the Phase 90 is all that was in between my guitar and the clean mixer. Not clipping on the mixer.

Situation is the same for all my guitars. All guitars need some roll off to not distort this pedal. My main axe these days is my Gibson 195. But they all need a roll off to not clip this pedal.

Yes, it sounded like the input was clipped - and not in a good way to my ear. With the mod and guitar rolled off to 8 it is acceptable.

I am learning something here. I really wanted the Grant Geissman 'Feels so good tone'. It appears that to get that you have to have a different phasor pedal than the one I have?!?

Oh man. This music stuff can be endless sometimes.

I mean I read that Grant used a Phase 90. But now I realize there are lots of these pedals.
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Re: Please help with MXR Phase 90 distortion problem

Postby The Elf » Wed Sep 23, 2020 8:34 am

I've never understood why the Phase 90 has gone down in legend, while the superior Phase 100 is a forgotten gem!
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Re: Please help with MXR Phase 90 distortion problem

Postby Martin Walker » Wed Sep 23, 2020 2:54 pm

The Elf wrote:I've never understood why the Phase 90 has gone down in legend, while the superior Phase 100 is a forgotten gem!

Perhaps it's because the Phase 90 is easier for guitarists to use, whereas the Phase 100 is more a keyboard player's delight?

Runs for cover :beamup:


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