EVH MXR Phase 90 not phasing

[burger8]

Guys,

I have an EVH Phase 90 that only produces the phasing effect when the pot is set to max.  As soon as you back it off a little the phase effect stops and you get pretty much a bypass signal.  So far I have used my DMM to check that the Zener is dropping the 9VDC supply to just a little under 5V, which I think is correct.  The internal trimpot also seems to be working - I have adjusted this up and down and it's already in the sweet spot (which I can test when the main pot is set to max).

Any ideas?  Maybe it's a faulty pot?

[Mark Hammer]

If there is just the one pot, and it only yields an audible effect in a small portion of its range, then yeah, I would say it's the pot.

[jimilee]

Any other changes that happened before we got to this point? Trouble shooting in the dark and needlessly desoldering  is no fun for you.


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[burger8]

Thanks for the replies.

I should say that when the effect actually works (when the pot is on max) it's actually very fast oscillation.   Just like it should in a normal Phase 90.

I'm not aware of any other changes before this point.  I picked this up from a guy at work who said "it never worked properly".  On inspection there are absolutely no signs of modifications to the board, all looks stock.

[jimilee]

I didn't realize is it was Mark Hammer in the first reply, my bad. I'd go there first.


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[burger8]

Cheers, will test the pot tonight and let you know progress.

[Kipper4]

I'm probably way off base. It probably is the pot but.
I'd double check the lfo cap and resistor values just as a precaution. it could be that the lfo is stalling.

[burger8]

Yeah, the cap was the other component I thought of.  This was useful reading that pointed my to that, http://www.electrosmash.com/mxr-phase90
See section 6 on the LFO.

Also, regarding a possible pot issue this pointed me to that line of thinking also.  http://www.diystompboxes.com/smfforum/index.php?topic=107884.msg982704#msg982704

[burger8]

It's the pot.  It goes from 0 ohms maxed, straight to 800k+ after the slightest turn counter-clockwise.  I've got it phasing nicely with a 190k resistor  in parallel for now.  I need to try and source a reverse log 500k pot now!

[Mark Hammer]

You can always just wire up a regular log 500k pot backwards, so that clockwise gets slower.

[tubegeek]

You can always just wire up a regular log 500k pot backwards, so that clockwise gets slower.

When I do this I mark it "Sloth" instead of "Speed."