MXR Blue Box -1/2 octave mod (dunlop reissue)

[rumblefuzz]

Hi there 

This will be my first post here, so here goes:

I searched my butt off trying to find a mod for the Blue Box that switches between -1/-2 octaves. I suppose there are already a bunch of those out there, but honestly I wasn't able to find one. I decided to put in a little extra time and come up with one myself. After about 2 hours of staring at the circuit and reading malekko's posts here on this forum, I figured it out! As it turns out it's a really simple mod, and results in some tasty additions to an already cool sounding box... The mod involves a switch, three wires, desoldering a resistor and one really tricky soldering point... In conjunction with the switchable "C11" mod I did I'd almost call this box versatile. 

Before I start writing the whole story, taking pictures and such, I'd like to know first if there's anyone even remotely interested in this. If you'd like me to post a complete walkthrough for dummies (like I am...) please PM me. Who knows I could even record some sound samples...

Please note that I only tested this on the new Dunlop reissue, the script logo one with an LED...

[Mark Hammer]

You've got pins 3 and 11 which are the relevant input pins on the 4013, and 1 and 13 are the relevant outputs.  Depending on the layout, you might have f/2 output coming from 1, going to 11 for f/4, or you might have 13 going to 3.  To select between f/2 or f/4, one basically leaves the inter-stage connection intact, and wires up a toggle that selects the final divided-down output from either pins 1 or 13.

[oldrocker]

This is a schem I made showing the 1/2 octave mod.



And don't forget the high frequency volume switch.  By taking C9 .01uf cap in and out of the circuit gives some more versatility to the pedal.



And my finished Blue Box.
http://i218.photobucket.com/albums/cc93/ftrock/bb5.jpg

[Mark Hammer]

The better spot for your treble-cut option is at the junction of R17 and R19, where it can smooth out the octave sound to something less square-wave-like.  There IS something to be said for a universal treble cut, but I imagine most folks want control over the octave-down tone rather than the fuzz.

[oldrocker]

Yes removing C9 from ground does add some fuzz to the treble boost.  I guess it's not a great mod but it's easy. LOL  The high freq switch mod I believe was intended to get back some unity volume to the pedal when the effect was engaged.  It was made simple so one could add it to the small MXR enclosures.   Thanks Mark.  I was wondering about a better tone mod for it so I'll try experimenting with it. 

[rumblefuzz]

Thanks for the replies, the schematics are great! In the dunlop reissue pedal this means desoldering the bottom lug of resistor r23 and connecting it to the common lug of a spdt switch. Run a wire from the hole to one side of the switch (stock mode) and another one to pin 1 (square hole) of the chip that's underneath the output pot for the -1 octave sound and presto...

The high cut cap is C11 in this pedal... I might try and find the junction you (Mark Hammer) mentioned. As I understand it putting in a cap with the same value as C11 at this point only cuts the highs off the octave signal?

[Mark Hammer]

The Blend pot attenuates, in complementary fashion, the fuzz and octave signals, such that it bleeds fuzz and octave to ground in equal proportions, or one more than the other.  Technically, they are mixed at the junction of R18 and R19.  By having your treble bleed at the junction of R17/R19/R20, you end up bleeding treble only from the octave signal.  If you wanted to, you could try installing a tone control that did more or less what R20 does, but only for treble.

So, imagine a second 50k linear pot, but with a .01-.022uf cap between the wiper and ground.  When centered, it would shave some treble off both fuzz and octave signal, but not very much.  Rotate it in one direction and it leaves the fuzz un-shaven while smoothing out the octave.  Rotate it the other way and you get smoother fuzz but raspy octave.  Naturally, by adjusting the Blend and the suggested Tone control, you could get smooth fuzz only, or raspy octave only, as well as raspy fuzz with smooth bass, etc.  Between the third control, and the f/2-f/4 switch, you could get a bunch of tones generally unattainable.

If you happen to be building on  perfboard and have the flexibility to do so, it may be a smart idea to make R5 (1k) a 5k or 10k trimpot to set the sensitivity of the unit for your guitar.  The stock 1k value may well be perfect for your guitar, but if not you can consider  turning the gain of that first stage up or down a little to get best performance.

[ddpawel]

Hi,
I'm working on MXR Blue Box to get pure fuzz effect.
I know I can just connect pins 1 and 11 of CD4013 to get straight fuzz, but is there any way to get rid blend knob and Q3?
Please, can someone explain me what to do with red part (on picture)?

[slacker]

You can just remove Q3, R11, R14, R17, R19 and the mix pot. Then replace the mix pot with a resistor of the same value, one end connected to where lug 3 was and the other end where lug 2 was. That will be the same as setting the mix control to just fuzz.

[ddpawel]

Thanks,
I probably connected 1 and 2 not 2 and 3 and there was no signal when I cut Q3.

[oldrocker]

If you're going for fuzz only also remove C9 for a brighter fuzz.  You can use a switch as in the schem above to take it in and out.

[ddpawel]

Thanks for help.
I really likes this fuzz (it really pushes tube amps and other effects even) and I just try to build combo pedals = Blue Box + Shin-Ei Octave. I have hope that give me really good analog synth-like tones.

[ddpawel]

This is link to Shocktave and my post at last place:
http://www.diystompboxes.com/smfforum/index.php?topic=75546.0

[marselb1]

Anyone have any idea how to put a fuzz/gain pot on the blue box?
Essentially i want to be able clean up the pedal so it is a little less crazy. I want a knob that would function like a gain control on a fuzz or overdrive pedal

[ddpawel]

When you're looking at Tonepad layout try to make R5 =1k into 470ohm res. + 10k pot. Another way is to change R8=1M into 100k res. + 1M pot. Or combine both of them. Be careful, I don't know how much low the values of R8 and R5 can be. MXR Blue Box have somehing called a Schmitt trigger around ic1b which needs a good gain.

[marselb1]

switching out the R5  resistor worked great! it functions more as a sustain knob then a gain knob. But it serves the purpose i wanted it to; it tames the mighty blue box even if only slightly =)

[ddpawel]

Another great mod I've found. Change R15 - 56k resistor going to ground. Change it to 1M log pot. It will give you - STARVE MOD, like in other fuzz boxes, because c7, c8, d2, d3 and R15! are working as a rectifier. Try this.

[diyguy]

Sounds like a great mod.  So am I understanding that you just run lead from the solder pad to a 470ohm res, which is connected to lug 3, then out of lug 2 back into the other solder pad?  I assume lug 1 & 2 are jumpered?

Thanks!

diyguy

When you're looking at Tonepad layout try to make R5 =1k into 470ohm res. + 10k pot. Another way is to change R8=1M into 100k res. + 1M pot. Or combine both of them. Be careful, I don't know how much low the values of R8 and R5 can be. MXR Blue Box have somehing called a Schmitt trigger around ic1b which needs a good gain.

[redbagy]

I am posting in this thread since there seems to be a good number of mod discussions going on about the MXR Blue Box. Currently I am trying to reduce the glitch on a clone that I built. I am tinkering with the feedback resistors of the Schmitt trigger (in general I am increasing the feedback from the output to increase hysterisis). Does anyone else have a better idea on how to reduce the glitch?