Help Fixing MXR PHASE 100

[Atodovax]

Hello everyone! I bought a MXR Phase 100 the other day and the effects is not working.. I have clean signal when bypassed and when i engage the pedal i still hear the guitar clean but with a slightly modified tone. When i turn the left knob i can hear how the tone is affected by rolling some highs/ lows but the speed does nothing. Its almost as if the LFO part would have died. Any ideas where i should Audio probe / meassure voltages? All the Ics have signal at pins 1 and 7.. I still havent tried desoldering the transistors. Any help would be apreciated. Thank you

[Kipper4]

You know the drill by now right.


https://www.diystompboxes.com/smfforum/index.php?topic=29816.0

[Rob Strand]

To help talk about the unit follow the naming on the schematic and layout from here,

http://www.generalguitargadgets.com/effects-projects/phase-shifters/mxr-phase-100/

FYI there's two PCB versions of the Phase 100.  Off hand I think this one is for the later model with the 8-pin opamps.

I have clean signal when bypassed and when i engage the pedal i still hear the guitar clean but with a slightly modified tone. When i turn the left knob i can hear how the tone is affected by rolling some highs/ lows but the speed does nothing. Its almost as if the LFO part would have died. Any ideas where i should Audio probe / meassure voltages?

Given the tone changes the LFO would be a very good place to start.  If it's not oscillating that will stop everything working.  It could simply be the Speed pot is is broken or has a broken wire.

Set the speed to slow and measure the output of IC1b.   Then perhaps the emitter of Q4.   At this point you are looking for a fact the voltage is changing; it's pretty slow say 10 seconds or more.   Change the speed control to a slightly faster setting to prove to yourself the speed is changing.

With the Speed on slowest setting,
- measure the  *minimum* and *maximum* voltages on the base and collector of Q3.
- Next,  measure the  *minimum* and *maximum* voltages on the base and collector of Q2.  Repeat for switch settings 1 and 3.

[Fender3D]

I put my money on this...

In phase 100 the 3 Vactrols are connected in series, hence if one fails, noone works anymore.
Lift one LED pin and see if you can measure some kind of resistance (test it like a diode).

[Atodovax]

Thanks for replying! Is there any other way to test the vacrols? On the LED side. I put leds in parallel with crocodile clips and all of them lighted up and fluctuate. But on the other side (3 pins) the fluctiation from middle pin to the others is very small