Bill and Will's Synth
MOTM 300 Construction
"Ultra Voltage Controlled Oscillator"

November 2008 -

Heading toward our work on the "One Mad Track" record, and punctuated by my travels, Will and I have been building the MOTM440, YU Saw Animator, Cat Girl Synth Tube VCA / Timbral Gate (CGS65) , and now the intimidating MOTM300 Ultra VCO.  We're going to build three of them and include some modifications to boot.

There are some great photos and modification info on the Larry Hendry site. 

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 couple photos from Larry Hendrey

Modifications - presents details of Larry Hendry's Fine Tune Modification and Paul Haneburg's Tracking Adustment

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-300 Ultra VCO represents a milestone in analog synthesizer design. Using only the finest available circuitry, the Ultra VCO has unmatched performance where it's needed most: stability and tracking. Using 4 separate temperature compensation servo loops, the average drift over 12 hours is typically 0.01% or about thirty times better than a Moog 921B VCO! Why do we call it the Ultra VCO? Because the tracking circuitry uses an ultra-matched differential pair transistor array that will hold over a minimum of 15 octaves! That's twice the range of a piano.

"Although we put the focus of the design on stability, the MOTM-300 is indeed a 'full-featured' VCO. Sine, Saw, Triangle, and Pulse waveforms are all available simultaneously. PWM (Pulse Width Modulation) with initial pulse width and depth creates complex spectra. Other features include:

• Dedicated 1volt/octave input with military-spec 25ppm 0.1% summing resistor
• 2 separate Frequency Modulation (FM) inputs, each with it's own attenuator
• Linear FM with switchable AC or DC coupling
• PWM (Pulse Width Modulation) from DC to over 15Khz
• Hard and Soft sync option. Master Sync I/O jack.
• Low power consumption, only requires +-15VDC @ 60ma.
• All audio signals wired with coaxial cable for lowest noise and crosstalk
• Low-drift op amps and precision resistors used throughout
• 10 volts peak-peak audio outputs
• Frequency range from 0.2Hz to over 38Khz
• 2U panel width

photo from Larry Hendry

Modifications

1.  Fine Tune Mod.

Larry Hendry wrote:

"The fine tuning has never been "fine" enough for my personal taste. I have always felt the range of the fine tune control was greater than what I liked. The fine tune control seems too touchy when trying to zero beat oscillators to me. So, I have modified one of my VCOs (and will soon do the other four) so that the fine tune control is more fine.

"On my stock MOTM 300s, the range of the fine tune control is about 11 semitones. I wanted something in the 2 to 3 semitone range on that control. The modification is simple and virtually non-destructive. It requires adding one 270K resistor to the circuit board and cutting one trace. Since the trace resides between two via holes, the modification is easily reversible by removing the resistor and inserting a wire jumper.

"You can size the resistor so that the range of the fine tune control suits your own needs. If you go much above 390K, the fine control will have too little effect and be useless. 270K produced the results of 2 to 3 semitones I was looking for. Something around 100K would produce between 5 and 6 semitones. The smaller the resistor, the less reduction from the current range of around 11 semitones.

"Here is the photo of my modification. The changes are very simple:"

photo from Larry Hendry

2.  VCO Tracking Mod.

Larry Hendry presented a procedure for tweaking the VCO tracking developed in 2002 by Paul Haneburg. The procedure involves determining a value for R50 that provides the best tracking for the specific VCO and you can download Paul's detailed procedure instructions here.

So for now, we're going to leave R50 out.  We may put a trimmer in there - or maybe some kind of clip so we can clip resistors in and out as we follow the procedure.

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 sorted, ready to begin

PCB front

PCB back

Here's where the modifications will be

5% Resistors - Part 1

Whereas we are vigilant about orienting all the resistors, caps, etc. consistently so their values can be read easily (in case we need to trouble-shoot them later), we oriented the resistors with the "tolerance" stripe on the left (relative to the text on the pcb).  Why did we do it this way?  'Cause when we started out doing these builds, we thought the gold stripe is so pretty and easy to see... and we put it on the left - well - just because.  But now, we do it so all our modules are consistent with each other <shrug>.  You might want to do it the opposite way - with the "tolerance" stripe on the right - it kinda makes more sense.

5% Resistors - Part 2

We've left the 2.2M R out of R50 - that's for the tracking modification

Precision Resistors

Fine Tuning Modification Resistor

cutting the trace

the 270K resistor

Capacitors

Capacitors are in - except for the 4700uF poly cap

ICs - Misc Stuff

ICs and Transistors - power header and ferrite beads

Transistors, trimmers, big cap

 
Via Holes

Construction Phase 2

All the stuff in Phase 2 gets soldered using "No-Clean" Solder and the PCB doesn't get washed off from here on.

Tracking Modification

Here's how we've decided to handle the tracking modification - months ago, Gino Wong sent us these very cool little pins and sleeves.

We're going to use them to make a situation where we can easily change the tracking resistors (R50) when we need to.

the idea is to solder them together like this so that R50 can be swapped out when we want to.  We're starting out with a value of 1.5M which is the value Paul Haneburg ended up with for two of his VCOs - we're betting ours will be the same but we'll test later to be sure.


We soldered the resistors solder to the pins


and the wires to the sleeves


like this


Then we soldered the wires into the pads at R50.  We figure that if we want to change the resistor value, it'll be a lot easier to unsolder the 1.5MR and solder a new one into the pins rather than the PCB.

Tempco Resistor (R42)

Now for the tempco resistor...

PCB Pots

Coax & Wires

soldered in

 

 
tying them down


the triple wires


double wires - phase 1

the double wires all in

Jacks & Switches Into Panel

Jacks




Switches

Panel Mounted Pots

We're going to use a "chicklets" to help wire the panel mounted log pots.  Chicklets can be gotten from _____.

Then we mounted the Pot to the panel along with the ___.

PCB Mounting Bracket

the parts are ready


fitting the spacers and screws


the nuts are still loose


nuts go all the way down on the pots


ready to install


the pots go through their holes - the nuts are brought snug against the panel


the pot nuts screwed down - the bracket/PCB nuts are tightened


affixing the bracket
for more detail, click here

Wiring the Jacks, Switches, and Panel Mounted Pots

the pulse jack - ground soldered


pulse jack done - and the jumper on the FM2 jack


all the jacks done


the sync switch - we put some heat shrink on there - we've got plenty of it around


the resistor gets put on the sync jack




sync jack done



the rest of the switches and pots

Construction done!

back


All done front

Set up / Testing

Use Notes