MXR Script Phase 90

[GuitarFreak302]

Hello,

I have a 1973 MXR Script Phase 90 and it is not currently phasing. The sound coming out sounds similar to a vibrato. I tried removing one JFET at a time to see if this made a difference and that didn't change anything. I also made sure I had both wet and dry signal at the PNP amplifier which I do. All of the opamps in the phasing section have similar voltages to each other so I figured they were working properly but I could be wrong. I am not really sure what else could be wrong with this pedal or what else I should try to make it work. Anyone's help would be appreciated.

Thanks,

Ryan

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[GibsonGM]

1) can you post a schematic?

2) what are the actual voltages on the FETs?

[Mark Hammer]

If it sounds like vibrato, that would suggest something amiss in the mixing of wet and dry.

[GuitarFreak302]

Here is a link to the best schematic I could find.

http://www.electrosmash.com/images/tech/phase-90/mxr-phase-90-script-logo-schematic-parts.png

I replaced the PNP amplifier and the two 150k resistors that are part of the signal mixing but that did not change anything. I will give voltages when I get a moment to check.

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[GuitarFreak302]

Here are the voltages I found.

U1

2) 4.789
3) 4.589
4) 0.00
6) 4.788
7) 8.79

U2

2) 4.804
3) 4.842
4) 0.00
6) 4.823
7) 8.79

U3

2) 4.925
3) 4.802
4) 0.00
6) 4.863
7) 8.79

U4

2) 4.868
3) 4.799
4) 0.00
6) 4.820
7) 8.79

U5

2) 4.868
3) 4.799
4) 0.00
6) 4.820
7) 8.79

FET 1 (2n5952)

G) 2.838
S) 4.802
D) 4.799

FET 2   (2n5952)

G) 2.838
S) 4.801
D) 4.799

FET 3   (2n5952)

G) 2.838
S) 4.801
D) 4.799

FET 4   (2n5952)

G) 2.838
S) 4.801
D) 4.799


2N5087

E) 4.801
B) 4.213
C) 4.764



Thanks,

Ryan

[GibsonGM]

If it sounds like vibrato, that would suggest something amiss in the mixing of wet and dry.

What Mark said.  Can you make an audio probe and see what it sounds like at the junction of R8, R16 (input to wet/dry mixer)? 

[GuitarFreak302]

I have an audio probe already made so I will try that when I can.

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[GuitarFreak302]

I get very quiet sound at that junction and I can't tell if it is oscillating or not

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[MrStab]

if it's an actual 1973 model, is it possible the caps in the phase shifters have aged and drifted (or died)? just something that popped into my head given the pedal's age and how no specific event seemed to make it happen.

[GibsonGM]

If they are electrolytic caps, then yes, that's a very good assumption!    Any electro in there should be changed.

[GuitarFreak302]

They are disc caps. The electrolytic/tantalum ones are for the LFO and power supply filtering

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[MrStab]

ceramic? i think they have noticeable ageing effects, too, and can also be quite brittle. if just one or two caps have a large enough difference, maybe some "UniVibe" type effect is causing the vibrato-like sound. it's been a while since i've read up on phaser theory, but aren't two 90-degree stages with the same(ish) corner frequency required to get each notch right? maybe the cap in one of the 90-degree stages has drifted too far or has failed.

...but then, you would expect a Phase 45 sound, not a vibrato, unless at least 2 caps in either half of the whole phase stage had failed

[GuitarFreak302]

I just replaced those caps and nothing changed.

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[MrStab]

at least the pedal is now future-proofed! from what you say, it sounds like the LFO is working, but what kind of voltage range are you reading on the FET gates? maybe it's too weak or there's some other problem there. on your list of voltages, you just showed a fixed voltage.

[duck_arse]

2N5087

E) 4.801
B) 4.213
C) 4.764

this. you can't have both collector and emitter higher than the base and expect it to werk, even if it is npn. I think.