I was modding my FX91 and had been having troubles. I rewired the tone control, all frequencies calculated correctly. I still, as always, had just too much gain.
After many hmm's and ha's, I finally spotted it... factory mistake!!!
R3, in my pedal (Serial #670478), turned out to be 200R instead of the 1k that's supposed to be there. The stripes make sense... SHOULD be Brown - Black - Red - (Gold). What was in there (and I checked after I pulled it... 195R) was, if read backwards, (Gold) - Brown - Black - Red.
Schematic w/o switching
The FX91 is a Bass Overdrive with Gain, Tone, Blend, and Volume, built on the same board as the Mystic Blues Overdrive. It is essentially a Tube Screamer with a blend knob and a half-assed tone section (as I noted
HERE)
Is this correct? Should be. R3(1k) and C5(.22uF) would have a corner freq. of about 720 Hz, while 200R and .22 yields a corner frequency of about 3600Hz. Why did my ears bleed with the blend to full wet? Because I was getting distortion at about 500x gain at 3600Hz and up!! well, needless to say, one resistor change and I think I would've had a super sweet pedal.
BEFORE YOU MOD IT, CHECK R3!!! Try it first.
MOD AWAY! If you want something a little different, change the diode closest to the edge of the board to a schottky. Gives a nice, low volume, 1:3 clipping ratio (roughly) with a bit more dynamic feel and more tonal possibilities with the blend knob. Blends well.
To shave off a bit more of the highs, change R47(1k) and C31(.022uF) to 3.3k and .033uF, it changes the low pass to about 1400Hz, or about 1 octave above the boost frequency in the gain stage, rather than the 7200Hz. Gives it a little more of a farty distortion, sweet blend with the dry signal, and requires less filtering downstream, which is good... like I said, the tone section is incredibly lacking, however I would have to get another pedal to check for sure, considering the boost I had. I modded the tone section in hopes of taking the high end off, and maybe I just had too much signal to begin with and the tone section works great, in "correct" pedals.
Info for you bassists who may have one of these sitting on the side staring at you with it's great big gold and black puppy dog eye...
