mxr phase 90 won't work

[gotta'58burstonmemind]

I've got an MXR phase 90 that I've been modding recently so I can get a more vintage sound out of it. I have already done the r28,c11,c12 mods that I hear so much about, and they sound great, but I had ALSO heard somewhere that by tweaking the trim pot on the pedal you could fine tune the phasing or something like that. So I tweaked the pot by about a 1/4 of a turn, plugged it in to my pedalboard, and it wasn't working. I turned the pot back to where it originally was but all I hear is a loud crackling when I strum my guitar.

Does anybody have a clue as to what the problem could be?

[Mark Hammer]

The trimpot sets the bias voltage needed by the JFETs to be swept in the range required to produce the intended phasing. It is possible to "mis-tweak" the trimpot so that you get no phasing at all, but it should be possible to re-tweak and restore phasing.

At this point, your best bet is to plug in, strum, and tweak until you hear what you're supposed to hear.

[gotta'58burstonmemind]

It seems I've done the best I can for now. It still makes a bad crackling noise when I play. I can't hear any individual notes or chords unless I pluck/strum hard. Thanks for the info though. Any other suggestions would be appreciated.

[Mark Hammer]

Is this a P90 of recent manufacture, or an older one?

[gotta'58burstonmemind]

It's fairly recent. I bought it new earlier last year, which is why it's bugging me so bad. It really seems like turning the trim pot right back to where it was would have worked. Could it be a faulty solder connection? Or maybe the glue on the solder side of the PCB is messing with the pot somehow?

[Steve Conner]

If turning the pot back to where it was didn't make it work again, then you have something else wrong there. Most probably a loose connection or short circuit related to one of your previous mods, or the trimpot is bad, or you zapped it with static or something.

Try twiddling the pot back and forth a few times. If it has a loose contact that may help restore it.

[Mark Hammer]

Steve's right on several counts:

1) There can often be discontinuities that aren't immediately visible to the naked eye, like pot and trimpot wipers that aren't making contact with the resistive strip, or wires that have fractured but the last little bit of insulation softened and stuck to the pad during soldering so it looks like the wire is still connected.

2) Contacts can become dirty with age. A decent cleaning, or perhaps outright replacement isn't necessarily a bad idea.