tonepad MXR blue box clone

[dfarrell]

I've built the MXR blue box clone from tonepad. When I use it, the octave effect oscillates. I've checked voltages on all the pins for IC1 and IC2, and they look ok. I was hoping that someone else might have built this and had a similar problem, and can give me a clue.

[The French connection]

Hi! What's dual IC in there? I've got the same problem with a 4558 IC and replace it with a TL072 and it works perfectly now...

Dan

[George Giblet]

There was a fix where you could add a cap across one of the ICs (which doesn't change the sound).  Use the search function.

[dfarrell]

i've tried replaceing the with a TL072, but it still won't work right. I am looking for the fix were a cap is added across the IC, but I can't find it. I'll keep looking.

[Mark Hammer]

It's normal behaviour actually, if I've understood/interpreted what you mean by "oscillate".

The tracking of the BB is unstable as portions of the signal fall above and below the critical threshold for producing a flip-flop dividing action.  There are different ways to address it.  One way is to use tight strings (heavy gauge).  Another is to develop a consistent picking style.  A third is to use a compressor or something to keep a more consistent level.  A fourth is to roll off as much top end as you can so that it does not trigger on harmonics. 

What the circuit really wants is an oscillator craning out a constant level triangle or sine-wave.  In some respects, what that translates into is a front end that provides heavy clipping followed by steep lowpass filtering to yield something vaguely sine-ish.

In the absence of that, you'll find it skipping around a lot, and occasionally flipping back and forth between a higher and lower octave.

[dfarrell]

Ok Mark,
you've pretty much described excatly what is going on. I've found that it dosen't like open strings, or any note lower than d (and that seems to be pushing it), it does seem to do better with the highs rolled off, and it dosen't like long sustained notes. I was hoping to just plug in and instantly get that Jimmy Page/Fool in the Rain solo sound, but maybe I need to exlpore it a bit more. I am also building a compressor, so hopefully that will help too. If anyone else has some insight they can share on this guy, I 'd love to hear it!

[oldrocker]

Yes rolling off the hi frequencies and using the neck pick up is how I tame the madness.  There are a couple of mods that work great too.  One is bypassing the low pass cap C9 .01uf with a switch to kick up the volume but also adds a little more fuzziness.  The other mod is the 1 octave to 2 octave down spdt switch.  When the octave switch is in one position it connects the 1M R11 to pins 3 and 13 of the 4013 IC for normal 1 octave down operation.  When switched to the other position R11 is diconnected from pins 3 and 13 and R11 is connected directly to pin 1 of the 4013 IC to give a 2 octave down effect.  I re-made my own schem for this project.  Then breadboarded it and tried the mods.  I pulled the 4013 and 4558 out of an old Tom Sholz portable Rockman and perfboarded the circuit.  It sounds great and is one of my favorite builds.
Check out the partial schem I posted below to see the mods.
I used a 4558 dual opamp and 3904's.
http://i218.photobucket.com/albums/cc93/ftrock/bb5.jpg
http://i218.photobucket.com/albums/cc93/ftrock/bb2n.jpg

[dfarrell]

that looks interesting, I might try the 1 octave -2 octive mod

[oldrocker]

I did both mods and it gave the pedal much more usefulness and versatility.

[jacobyjd]

I made one using various mods I found around the web--

-1 or 2 octave switch

-tone control

-decay control

I like mine a lot--I'm going to be rehousing it with some other effects soon.

Some of what you're running into is just the nature of the beast--the tracking is very glitchy, but with some tweaking, you can get 'fool in the rain' pretty easily, although not at a volume suitable for lead work. Lots of people complain about it sitting at just below unity gain...you can remedy this by adding a booster to the end of the circuit with its own volume control (and change out the volume pot on the blue box for a fixed resistor), so you can have tons of headroom without sacrificing any of the sound.

just some thoughts. It's a great effect!

[oskar]

SW2 shorts the signal...     

[Mark Hammer]

A potentially more useful tone-control mod would be something like this.

If you look at R20 in the Tonepad Caja Azul schematic, you will see that it adjusts how much relative attenuation occurs for each channel/side by "bleeding" signal to ground in complementary fashion.  R16, and the portion of R20 between R16 and ground, form a divider that reduces the fuzz signal level.  The same thing takes place for R17 and the other part of R20.  Turn the level up for one and you turn it down for the other.

Okay, let's say that you had a second 50k pot in parallel with R20.  That is, R20 remains in place and the two outside lugs of this third pot are tied to the outside lugs of R20.  Instead of the wiper going to ground directly, though, it connects to ground through a capacitor.  As the pot is rotated fully in one direction, the cap to ground is now tied almost directly to R16.  There is, however, 50k of resistance stuck in between the cap and R17.  So, the cap will have a treble-cutting action for the fuzz side, at that point, but little audible treble-cutting action for the octave side.  Rotate it fully in the opposite direction, and you get full buzz on the fuzz side, and a mellower octave sound on that side.  Obviously settings in between get you variations.

In the original schematic, C9 provides a treble-cutting action for the entire signal. You can omit it, if you wish, or reduce it a bit, supplement what our added control does.  The suggested additional pot and cap would produce a 6db/octave rolloff starting around 265hz, using a .01uf cap to ground from the tone-control wiper, when the control is fully rotated to that particular side.  Depending on your particular taste, you might prefer to leave the current C9 in place or make it slightly smaller, at 4700pf.  So, in sum, the suggested additional control lets you trim treble from the fuzz side, octave side, or both.  becasue a 50k pot set to the middle (25k per side) might not provide enough treble cut for the octave, consider paralleling the octave side of the pot with a 12k fixed resistor so that the resistance to the bleed cap for the octave side never goes higher than 9.6k.  You can also feel free to experiment with lower-value pots like 10k.

If you wanted something simpler, and have run out of panel space for another pot,  You can simply wire up a 3-position on-off-on SPDT toggle so that it connects a cap to ground from either R16 OR R17, or neither (full treble).  So, cap goes between the common of the switch and ground, and the two outside switch lugs go to the R16/R20 junction and R17/R20 junction.

Alternatively, since the point where you want the rolloff to begin may be different for octave and fuzz, consider using the same SPDT on-off-on switch so that it connects a 3300pf cap when in the fuzz-side/treble-cut position, or a .01uf cap when in the octave-side/treble-cut position.  This would provide a rolloff starting around 860hz for the fuzz side, and around 265hz for the octave side.

[oldrocker]

SW2 shorts the signal...     

Yes it sure does.  It bypasses the C9 cap.

[oskar]

SW2 shorts the signal...     

Yes it sure does.  It bypasses the C9 cap.

Oh brother... If that triangle under C9 suggests ground... then the switch shorts the signal to ground.
Else I have forgotten everything I've ever learnt about electronics ( possible ).
Put the switch in series with C9 to bypass...   Not?     

[Monotremata]

Thanks for the description Mark Hammer!

I just bought one this weekend after reading one of the guys in Neurosis uses one and man is that a wierd pedal.
I had other owners tell me 'Its not really a normal octave fuzz' but that wierd tracking it does was throwing me for a loop.
And yeah mine will fizz out and kill notes dead too if their not constantly feedbacking or singing.

Ive been playing with it at home on my 20 watt solid state Fender all weekend and tonight I finally get to try it on the JCM800 at band practice.
Maybe Ill put it right after my Metal Muff so its getting a nice constant gainy signal and see what that does..

[oldrocker]

oskar
You are right my friend.  when I made the schem I didn't watch what I was doing. (Won't be the last time.)  Yes it should be in series.  My mistake.  Sorry if I misled anyone.  Thanks oskar for pointing out my error.