Bill and Will's Synth
MOTM 485 Construction
"GX-1 Voltage Controlled Filter"

February 2009 -

We turned our attention to the MOTM 485 next for our recording of the "One Mad Track" album this winter.

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-485 is a recreation of the Yamaha GX-1 LP/HP filter.

"The GX-1 was a massive organ/synth hybrid, costing over $60,000 in the late ‘70s. The best example is the ELP song ‘Pirates’, but was also used by Stevie Wonder and Led Zepplin..

"The actual GX-1 used a pair of these filters to form a bandpass response (the same method used in the MS-20 and the CS-80).

"There is a HP filter set to "half tracking" followed by a LP at "full" tracking. However, you can still use a single MOTM-485 as yet another distinct tone in your setup.

• Faithfully captures the unique GX-1 timbre
• 2 audio inputs
• Voltage-controlled resonance
• Switchable HP/LP response
• Switchable half/full tracking of the 1V/Oct input
• Fully temperature compensated using TEMPCO resistor and matched pairs
• Not a ‘clean’ filter: has higher distortion than other MOTM filters

the PCB

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 and ICs 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


in go the caps


the pcb-mounted pots


all the wires are in

Prepare the panel

ready for the jacks

Bracket

the mounting bracket


fitting the pcb onto the bracket


looks like this


now to fit it onto the panel
for more detail, click here

Wiring the switches

switches mounted in the panel


and wired up

Wiring the jacks

three done - three to go


jacks wired and tied

Construction Done

Set up / Testing

Use Notes