Bill and Will's Synth
MOTM 420 Construction
"MOTM-420 Voltage-Controlled Filter"

February 2009 -

We turned our attention to the MOTM 420 next for our recording of the "One Mad Track" album this winter.

We built our 420 from an old-style Synthesis Technology kit. For those of you who are building this as of "MOTM 2.0," you can download a Bill of Materials (BOM) we made for 2.0 users by clicking here.

Table of Contents

This page has become really long, so here's a table of contents that we hope will make it easier to traverse:

Background - presents an explanation and Paul Schrieber's initial description of the Module with a photo

Construction Phase 1 - Resistors, Capacitors, IC Sockets, Power Plugs, MTA headers

Construction Phase 2 - Trimmers, Panel connections

Set up / Testing

Background

Paul Writes:

The MOTM-420 is a switch-selectable LP/NF/HP (lowpass/notch/highpass) voltage-controlled filter. Using the same filter structure as the MS-20, the MOTM-420 has a unique "cutting" sound that is perfect for techno, acid, house, and other dance music. The VCF will over-drive/distort in all modes, and the resonance can be cranked into self-oscillation. The LP/HP slopes are -12dB/oct and the NF is -6dB/oct.

The MOTM-420 includes 2 on-board 3-channel mixers: one for audio and one for control voltages. This eliminates the need for a separate mixer module in most patches. Multiple MOTM-420's can be chained in series to exactly emulate the MS-20 filter (a HP followed by a LP).

Unlike the more common 4-pole LP filters, the MOTM-420 has a brighter, "glassy" sound. This is due to the selection of parts used, and the filter design. The VCF will alter it's resonance as the input amplitude varies. This was a "flaw" in the original design, but is the main reason for the sound!

The MOTM-420 has 3 separate audio inputs, that can take audio signals from any other synth, drum machine or sampler. By using a "direct box", you can run a guitar or bass through it. Use a voltage-out pedal for sweeps as you play.

Construction Phase 1

All the stuff in Phase 1 gets soldered using "Organic" Solder.  At every break in the action, we wash the board off to get rid of the flux.

Resistors

After we've sorted and counted the resistors (part of the first step in building a kit is to account for the parts), we bag them separately to make them easy to find.  To see our handy-dandy Resistor Color Coding Chart, Click here.

resistors are in

Capacitors - ICs - Misc

Via Holes

Misc, ICs, and poly caps are in.  A note - we solder one contact of the ICs first. Then we make sure it's down on the pcb correctly and solder the diagonal contact and finish the other contacts up.
Construction 1 done.

Construction Phase 2

All the stuff in Phase 2 gets soldered using "No Clean" Solder. 

Pots, Temco, Wires

the PCB mounted pots


time for the tempco resistor


just like that




so all the wires are in

Prepare the panel

ready for the jacks


the holes are a little too snug and the jacks won't fit so we're filing out the holes a bit


just like that

Bracket

the mounting bracket

   
 
We start at the bottom corner nearest the bracket - the bottom right

   
 
Then we do the top one near the bracket - the top right

     
now the bottom-left

   
 
then the top-left

 
we put nuts onto the pots - just finger tight

 

slide the pots and bracket into place and screw down the bracket


nuts now go on the front of the pots


and the pcb gets tightened to the bracket

 
the nuts on the inside get tightened back against the panel where necessary


and the pot nuts get tightened
for more detail, click here

Wiring the switch

switch

Panel mounted pots

 



 


wiring up the panel mounted pots

Mounting them to the panel

ta da!

Wiring the jacks

a bad jack - luckily, we've got spares

Knobs

Construction Done

Set up / Testing

Use Notes