Such a lengthy heading…   I’ve been waiting to find out what was going on with this one, and it has recently been traced on the Pedal PCB forum.  There’s also a great back story here, with some info on the Zener diode.

The schematic can also be found via the Pedal PCB link above. 

There are other earlier versions, including a PNP version using an OC75.  The OC75 version is very close to the OC140 version – although there’s no diode involved in biasing the collector, just a 4k7 resistor (and obviously some different transistors on Q1 / Q2).  A couple of odd value resistors, like a 10ohm on the emitter of Q2.  I’m pretty sure I’m reading it correctly, but…  

The PCB is a Jumbo Tone Bender board with modified parts/values.   

COLORSOUND HYBRID TONE BENDER (2018 EDITION) – STANDARD VERO LAYOUT

I’ll get around to cleaning up the connections at some point, as they are a bit rough and will also add another diode for polarity protection – this was a quick solder job for testing.

It works….  sounds like a Tone Bender too.  Not a direct replication of the sound of a MKIII / MKIV, but the flavour is there.

I had to make a few substitutions as I did not have all the parts as per spec. 

• 5.1V zener diode, instead of the 4.7V (bear in mind the actual value has yet to be confirmed)

• AC127 instead of the OC140 – it’s quite leaky, with a gain around 100

• AT426 instead of the BC107

• 100n instead of 10n, just because I wanted to, and it has been seen in earlier versions
  

• B25k pot instead of W20k
  

TONE POT SWEEP

ON THE SCOPE / FFT

Bass

 Treble

 Tone 50 / 50 Mix