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 (http://www.sugardas.lt/~igoramps/article16/fx91.gif)
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 (http://www.diystompboxes.com/smfforum/index.php?topic=92004.0))
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...
interesting... I didn't even know DOD made this. I'll keep an eye out for one, as I can never have enough fuzz pedals for my bass lol.
Sorry to bump an old topic - just scored one of these guys cheap, and it does indeed have the wrong value in r3.
haven't plugged it in yet, but i suspect I'll be restoring it to factory spec. also keen to try out the diode swap. would a Ge (1n34a) have a similar result?
J
IMHO, I'd find a spot ti put in a toggle and make it so you can toggle in the 200R or 1k. The 200R gives it a certain scream that I really started digging on once I got my Phaser back at the end of the chain (it has an "always-in" tone control). If you swap back to the 1k and want to run diode switching, Schottky diodes would give something close to the scream, Ge's would get close, but would probably lack the top edge sound. But they are close enough in forward voltage that it would probably be a nice one to do. Nice soft fuzz, or a hard-edged scream. I think I may do that to mine... that sounds like a better idea than the crap I have going on now, lol.