Symptom:
Odd acting low octave. Seemed to not work, or if it did it would sound after the note, almost like a slap back delay.
Problem:
Bad CD4011. Touching pin 1 with the probe would kick the low octave on (if a steady sine wave was fed into the input, 100mVp-p).
This same repair I also replaced all the electrolytic caps. A couple bad ones were causing low output. I pulled all of them, and about 1/2 measured bad (low capacitance).
Notes on the circuit:
The "mix" control blends straight guitar on CW, and octaves on CCW.
The "tone" control only affects the low octave sound.