MXR phase 90 (101 3001 305) - Needs Schematic And Parts List

smilan · Jun 9, 2019

A friend of mine gave me a non working MXR phase 90 for repair.
The PCB version is 101 3001 305.
After googling this PCB number I found this photo that proves that some of the components on my PCB have the wrong values.

Does anyone have the original schematic and prats list of the 101 3001 305 PCB so I'll be able to put the right components and make this unit works again?

PRR · Jun 9, 2019

> Does anyone have the original schematic and prats list

I don't understand. Your image surely is the official MXR documentation. Which parts on yours are different? Have you tried putting the right parts in? Have you gone over the jacks and switches and wiring for faults?

smilan · Jun 9, 2019

PRR said:
I don't understand. Your image surely is the official MXR documentation. Which parts on yours are different? Have you tried putting the right parts in? Have you gone over the jacks and switches and wiring for faults?

Since I don't have the complete official MXR documentation I can't check all the components.
From what I could see R30 is 24k instead of 68k that appears on the picture I found in google , also it seems like the original op amps has been replaced, instead of 3 TL061 I have two TL082 (U1 and U2) and TL072 (U3).
When I received this unit there was a short circuit on CR2 (zener diode) so it doesn't work at all.
Now after replace CR2 with new zener the pedal turns on but not operating well.
The speed pot doesn't work at all and while playing with the bias trimmer (R27) I can get some kind of manipulation on the signal but it surely doesn't working well.
Also in this schematic
https://www.electrosmash.com/images/tech/phase-90/mxr-phase-90-script-logo-schematic-parts.png
the bias trimmer (R27)is 250k while in my PCB I have a 25k trimmer.
This is why I would like to get the complete official MXR documentation so I'll be able to know what's going on.
Here's my PCB with the circuit board component placement (that I took from google) on top:
https://i.ibb.co/PWS4vrC/mxr.jpg

Rob Flinn · Jun 9, 2019

also it seems like the original op amps has been replaced, instead of 3 TL061 I have two TL082 (U1 and U2) and TL072 (U3).

That seems rather unlikely since TL061's are single op amps & TL082 & TL072 are dual op amps ?

smilan · Jun 9, 2019

Rob Flinn said:
That seems rather unlikely since TL061's are single op amps & TL082 & TL072 are dual op amps ?

My mistake, there's other PCBs with 6*TL061 while in this version supposed to be 3* TL062

Bo Deadly · Jun 9, 2019

Probably need a scope to watch the CV and understand what's really going on. I would also want to look at fft plots.

Maybe C10 died short? That would certainly break the oscillator. Also check C8. Maybe just replace them both with electrolytics (15uF is a strange value - maybe 10uF would be ok).

smilan · Jun 10, 2019

squarewave said:
Probably need a scope to watch the CV and understand what's really going on. I would also want to look at fft plots.

Maybe C10 died short? That would certainly break the oscillator. Also check C8. Maybe just replace them both with electrolytics (15uF is a strange value - maybe 10uF would be ok).

Thanks Squarewave,
C10 was faulty, I've replaced it with 10uF electrolytic and the pedal is working now!

smilan · Jun 10, 2019

One more thing, after setting up the trimmer to get the right phaser there's there's some kind of overdrive on the signal when the phaser is on.
Any suggestions what should I check?

Bo Deadly · Jun 10, 2019

You're probably just putting too much signal in. You want to adjust the bias just so that the sweep is continuous. If the bias is not right, the sweeping will flatten at one end of the other. But if you set the bias too high, the JFETs will spend more time at lower impedance which could cause more distortion. But generally, the circuit can probably only handle 100mVpp or so (which is about the output of a typical e-guitar). Anything above that and the JFETs could start clipping.

MXR phase 90 (101 3001 305) - Needs Schematic And Parts List

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