MXR Blue Box - octave finicky

[lowell]

Have an original blue box that works but the octave is acting weird.  It doesn't always play the octave, the octave changes pitch sometimes depending on pick attack and string bending.  any ideas?  Schem here http://www.simplesmus.xpg.com.br/esquemas/octave/octave.pdf

[jacobyjd]

Is this taking into account the glitchyness that the blue box is known for?

[audioguy]

That sounds EXACTLY like what the Blue Box is kinda known for... just unpredictable octave fun.

[wavley]

Yep, that's what they do... kinda just have to learn to control it or enjoy the glitchy ride

[jacobyjd]

--if what we're all thinking is correct, then your solution will be to switch to your neck pickup, roll down your tone control, play single notes above the 12th fret, and enjoy the ride

[Mark Hammer]

The Blue Box comes with no adjustment or compensation for what it is that is supposed to be tracked.  I contrast this with some of the various Octave Multiplexers from EHX that had filtering adjustments.

Now, while there is a gain stage at the front end that could have its gain altered (e.g., via R5), this stage unfortunately feeds both the envelope follower that sets the gating action of the two transistors and the octave division circuitry.  The is means the only way to adjust the triggering sensitivity of the circuit separately from the envelope is to do so after IC1a, namely at IC1b.  Even here, I am wary since that same stage does both the amplification to prepare the signal for the 4013's requirements, and the production of the fuzz tone at pitch.

So here is a suggestion.  Do not change the gain anywhere.  Rather adjust the relative sensitivity of certain pitches by changing the value of C2.  Presently, it is .05uf (.047) which, in conjunction with R22, yields a 60hz low-end rolloff at 6db/oct.  Try subbing a .012uf or .015uf cap for a 234hz or 190hz rolloff.  That may tackle some of the issues of wonky triggering in the lower notes (which is where it tends to happen).

And if you try it, do report back and let us know.  Other than that, it really IS the nature of the beast, and one of the reasons why many folks prefer something like the Rocktave instead, for more consistent tracking.

[lowell]

rockin' explanation, thanks for that Mark.  Good to c u over here too, c u at ampage quite a bit.