MFOS Noise Toaster

My first kit, I bought the PCB from synthcube (they serve the orders from the MFOS shop as well).
It is a fairly simple affair and there is a stripboard layout posted on electro-music.com, but I would not
recommend that for beginners as you would need some troubeshooting skills.

Box from thrift store painted with Hammerite green paint and white posca marker. Sprayed on matte clear coat

Things to note:

• Transistor Q5 is supposed to generate noise, but not all transistors can do that reliably on a single 9 Volt
  battery. If possible, build this part on breadboard first and test, or put Q5 in a socket. You can also try other
  transistors. I ended up using a BC547. Not perfect, but it's noisy with a decent battery.
• This is more of a noise box than an accurate synth. There is a lot of interference in mine at least,
  but it is a lot of fun. You can however get semi-decent 1 V/oct tracking.
  • Q3 and Q4 should be matched. Use hFE on your multimeter if you do not have anything better.
    Transistors from the same batch should be pretty close.
  • Put Q3 and Q4 into thermal contact with each other.
  • Assuming your external CV input resistor is 100 Kohm R11 should be around 1,8 Kohm. Substitute R11
    for a 1 Kohm resistor and a 1 Kohm trimpot. Measure and adjust so that the voltage at Q3's base increases
    by 0,018 V for each octave you play.

Modifications

The PCB has room for adding an external audio input to the filter and a external CV source for the VCO. These
mods are documented on the MFOS site and in the book Make: Analog synthesizers. It's really just a matter of adding
another mixing resistor (plus a capacitor for the filter) on the inverting inputs of U1-C and U2-A.

I also wanted to use external gate signals. My solution is crude and not so good (might make a better one later).
I simply added a jack which I connected with a 1 Kohm resistor to the center pin of switch S8. The ground pin on
jack is connected to BN, not the virtual ground. My Keystep seems OK with this but it's on your own risk.

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Updated 2020-04-27