EVH Phase 90 Issues

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Ebulakowski
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Post by Ebulakowski »

I recently took on a friends EVH phase 90 pedal. Normally I steer clear of SMD stuff but first glance identified a blown zener. I replaced it and she powered up. A little tweak to the trim pot and there was phase effect. BUT.......

The effect sound is not even. It phases up and then abruptly down. Almost like it phases up and then resets like a saw tooth wave.

Am I SOL? what would I want to look for?

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yobakolobok
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Post by yobakolobok »

I know it's been 2 years, but have you figured out what was the case? I have the similar thing.

I've heard of mods that change the shape of the shift pattern, making it more random, but that involved those 4 same caps on amplifiers inputs, people added mismatching kit and it wobbled much more weird, maybe some of them was faulty?

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lolbou
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Post by lolbou »

I would check the LFO signal with a scope and read this (chapter 6) for reference first...
- Are you a mod or a rocker?
- Uh, no, I'm a mocker.

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yobakolobok
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Post by yobakolobok »

Thanks for suggestion. The wave that LSO was making looked more like human pulse than a square wave smoothed to a saw.
Image
That, or soldering old microphone cable to marked points and screenshoting Audacity could alter the signal with soundcard's filters.

Anyways, just to make sure, I checked all the connections i had in that block, rechecked resistor and capacitor values with tester, swapped zener, tried TL072 from another supplier since i had a few, nothing changed.
The 15uF cap, which is a b*** to find and i thought would be fine to leave at 25uF, was changed to couple of 10uF and 4.7uF in parallel - the wave did not change, just bumped up frequency.
Tried replacing resistors with different values to look what happens, only thing that changes is the wave length again.
All that's left out of spec is 500k pot which i changed to 50k because the LFO wave took half of minute to rotate with the same shape.

What else could I look at? Something drains the 15uF cap faster than it charges it, but i can't find what.

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Fender3D
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Post by Fender3D »

you sure LFO has 072?
'cause I never get it oscillating...
I usually use 062 or other "worse" opamps

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yobakolobok
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Post by yobakolobok »

Yep, pretty much any diy MXR Phase90 schematic out there mentions TL072. Could try 062, why not, everything is socketed anyway.

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yobakolobok
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Post by yobakolobok »

Welp, finally got my hands on bunch of different-branded TL062. There was so little difference from TL072, that i didn't even have to touch the trim pot, it sounded EXACTLY the same, the frequency, the wave form, the sudden drop. I guess I better go study how this op-amp oscillator functions in general, probably will give an idea what's the difference between the phases. Will add something useful later.

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Manfred
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Post by Manfred »

The 0.01uF capacitor between pin 6 and pin 7 of the operational amplifier is responsible for the edge slope of the square wave signal during switching.
Remove this capacitor and look at the output signal at pin 7.
I ran a simulation of this circuit, the oscillator works without it too.

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yobakolobok
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Post by yobakolobok »

You mean, short it or disconnect it? From what i found, that exact cap is partly responsible for square generation, being a feedback on negative input



The video describes the regular triangle generator layout, but in phase90 it looks like they made a single opamp do both blocks

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Manfred
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Post by Manfred »

You mean, short it or disconnect it? From what i found, that exact cap is partly responsible for square generation, being a feedback on negative input
I meant remove the capacitor respectively desolder it on a connecting wire.
The circuit shown in the video is a different one.
Here the 15uF electrolyte determines the oscillation frequency.

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