The Rotund Robot (3) – more info and research

I ended up leaving this this one for quite a while as a few other projects seemed to just flow out pretty quickly. I also spent a bit more time perfecting my painting process. One day I got an email from another DIYer called Filipe Alberti (many thanks for your help pal!) who was also working on the BOSS OC2 circuit, which was very helpful. It gave me a kick up the arse to get back to it as well.

I also came across some more info including this schematic for a modded chopped OC2 (minus oct 2, plus mids boost) which might come in handy. There’s a second version of this with the addition of a push me pull you for an analogue octave up floating about too if you care to search for it.


Here’s some more reading:

stompboxology-freq-div-sub-synth this one from my new DIY pal Filipe. Very useful for theory stuff.

boss_octave_oc2_guitar_effect_pedal_sch I can’t remember where this one came from (sorry). It includes various bits of information.

I also found this very useful thread on talk bass. This link will take you to page 7 where you’ll find  a great looking wee mod from Skrogh. Obviously I haven’t tried it out yet, but it looks interesting and simple enough to play around with.

There are plenty of threads on DIY stompboxes worth reading too, but I’m not going to list them all.

So, now we have a load of information, possibilities and ideas. So before getting lost in it all I thought it would be best to have a plan. So here it is. I’ll be following this up in further blog posts with my findings.

version 1.0 – original BOSS OC 2 schematic.

version 1.1 – synth mod (discussed in part 1 and 2)

version 1.2 – Skrogh synth mod

version 1.3 – experimentation – (tweaks and hacks) trying different values and jumpering various bits

Once I’m happy enough with version 1 I’ll do a small run of prototype PCBs. I’m not entirely convinced about the second octave down. It would be nice, but I’m a little concerned about board space, because for version two I’m thinking about adding a CMOS fuzz, and a synthy CMOS octave up. I might come back to the mids boost as well, but I’d like to stay as faithful as possible to the original synth octave idea before adding anything.

So, now I have a pretty definite plan, the first step is to breadboard the original circuit and go from there.

To simplify things I redrew the original BOSS schematic in KiCAD without the JFET switching. This way I could annotate the schematic myself (R1, R2 etc), the annotation on the original is not sequential which could get confusing. If everything works out, I’d need to draw up the schematic in KiCAD anyway to design the PCB. Here it is:

Rotund Robot V1.0
The Rotund Robot version 1.0 (BOSS OC2) – schematic

I should also point out here that I’ve included the first JFET buffer that was omitted from the BOSS schematic (it was taken out along with the JFET switching) found on the Sabrotone site. I’ve included the 1u cap and 1M resistor just before the last transistor on the output (found in the Stellan Lehrberg schematic). I’ve connected pin 7 of the CD4027 to ground instead of 9V. All of the above were suggested by Filipe from his own experimenting.

From this I drew out the breadboard layout

Rotund Robot - breadboard
The Rotund Robot version 1.0 (BOSS OC2) – breadboard layout

In the past I’ve printed out a blank breadboard layout and drawn everything in by hand, but it’s much easier to move components around in a graphics package with layers than redrawing the whole thing every time you make a mistake or find a cleaner layout.

I’ve decided to do a couple of blog posts about breadboarding and KiCAD alongside this project. Feel free to contact me with any suggestions or requests.


In the next Rotund Robot blog post, I’ll report my breadboard findings for version 1.

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