# Phase 100 internal trimpots



## cdayo (Jan 28, 2014)

I popped open my phase 100 the other day and noticed some internal trim pots inside. While I am still learning my way around schematics, I haven't found much info online about what these control, or if they should even be tampered with. 


The schematic is here, although it isn't official and I am not sure how accurate it is. The pedal is a newer model.
(sorry image i found was too big to post)


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## mhammer (Nov 30, 2007)

That schematic is for the original P100. I can't speak to what design changes may have been implemented in recent issues. The single trimpot shown sets the basic illumination level of the LEDs in the optoisolator modules. 

The P100 has 6 swept phase shift stages, and 4 fixed stages, for a total of 10 stages. Sweeping all stages is not critical to getting a phased effect. As long as some of the stages are sweeping, you're in business. The total phase shift is what matters, and that is summed across all the stages. The point where the phase shift is 90 degrees for that stage is given by the formula F = 1 / (2 * pi * R * C), where C is the .01 cap going to pins 3 or 5, and R is the resistance to ground from pins 3 or 5. The fixed stages provide 90 degrees each at 1/(2*pi*.01uf*.02Megohms), or roughly 800hz and above. 

The swept stages add to that total phase shift by varying the resistance of the photocell in parallel with the 100k resistor to ground. You don't want that parallel resistance provided by the photocell to be too low (such that the combined parallel resistance is always in the <10k zone, or too high (e.g., the combined resistance is always in the 50k+ zone). Since photocells can vary in their resistance, even within the same part number, the trimpot allows you to set the LED glow onto the photocells to a suitable level, so as to achieve the optimum resistance range.

To some extent, you can move the range of phasing around via that trimpot. Extremes of adjustment will result in no audible phasing, but there are likely to be positions just adjacent to the factory-set "sweet spot" where the resulting range of sweep might be more to some players' liking. I'll note that the same is true of the Phase 90 and any other FET-based phasers.


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## cdayo (Jan 28, 2014)

OK cool I am going to try it out and see what the results are.


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## mhammer (Nov 30, 2007)

Before you do, be sure to mark where it started out at.


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