Bill and Will's Synth
MOTM 830 Construction
"Dual Mode Mixer"

The first module we built was a MOTM 900 power module - kind of makes sense - it's one of the simplest (Paul Schrieber rated it as a "1" by  on his scale of 1-5 difficulty) and it could provide power to other modules as we build them.  Later on, we modified both our MOTM 900 & 950 modules so that our synth only needed one power cord and so that the power was distributed nicely - click here.

Now - the 800 EG Module is also rated a "1," but we'd decided to add Scott Juskiw's daughter boards and LEDs to our 800s, so we figured it's no longer a "1" at all. 

So The MOTM 830 looked like a relatively simple next module to build.  We figured it'd give us experience at all the essential elements - PCB with resistors, capacitors etc., coax, wire, jacks, switches, and knobs and stuff.

For those who are constructing an 830, please check out Larry Hendry's MOTM-830 construction photos by way of cross-reference.

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

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-830 is a dual mode (audio and/or control voltage) mixer. Using a clever switching scheme, the mixer can configure itself into either 6:1 or dual 3:1 mixers. The mixer is "split" when a patchcord is inserted into the OUT 2 jack.

The MOTM-830 is unique in the modular world: it is optimized for both audio signal quality and DC stability. Other mixers are generic, TL072-type mixers which are OK for audio but suffer from input offset voltage drift over temperature. The MOTM-830 uses special circuitry to provide superior audio specs (less than 0.005% THD, greater than 90dB SNR) while having superior DC specs (less than 1uV/C drift). Other features include:

• Shielded cables on ALL signals
• DC Bias generator: module can act as DC source!
• Mixer #1 (IN1 - 3) uses linear pots, while Mixer #2 (IN4 - 6) uses audio taper pots
• All pots are sealed, conductive plastic for long life and low noise

photo from Larry Hendry

Construction Phase 1

ready to begin

stuffing resistors... for a table of resistor value markings click here.

soldering resistors

As usual with us, 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 the gold stripe is so pretty and easy to see (of course)... and so we put it on the left - well - just because.  You might want to do it the opposite way.  (For the table of resistor value markings click here.)

washing the PCB

drying the PCB

the caps are in!

via holes

integrated circuits

cleaning the board

the board so far

It should probably be noted that whereas we have been 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).  (For that table of resistor value markings click here.)  You might want to do it the opposite way.

OK - time for a break

Construction Phase 2

In the couple days it took for us to actually get back to the project, we had decided to apply Scott Juskiw's 830 modifications to our module.  So we unsoldered R8, 17, and 18 and replaced them with the values Scott suggested.

ready to go with the board-mounted pots

we solder in the coax

We didn't know it yet, but we had made a mistake in how we did this. We soldered the wires kind-of sticking up because we missed Paul's instruction about tying them down.  They should look like the photo on Larry Hendry's site.  As you'll see, we went back and corrected the wiring later - but for now -

we've moved onto the next step of soldering on the other wires.

The photo above is a great shot of our mistake - the coax is sticking straight up - it should be lying down and be tied.  Here's what we suggest for others.

Paul has stripped the coax so that the length of the non-braided wire is pretty near perfect for how he intends the coax to be soldered.  We suggest you insert both the non-braided and braided wires into their respective holes, solder the non-braided first, then press the coax down against the board so that the braided wire sticks completely through the PCB - then solder it.

We start on the jacks


jack orientation

bridging the top and angled jack contacts

standoff hardware


the PCB mounted to the mounting bracket
for more detail, click here

bracket attaches to the face plate


looks great... but then we realized our coax mistake

So we took the mounting bracket back apart and corrected the coax.  Not as pretty as if we'd done it correctly first thing, but - <shrug> -


It really wasn't difficult.

switch soldered

shrink wrap down

switch done

wiring the jacks column 1

wiring the jacks column 2

wiring the jacks column 3

wiring the jacks column 4

the blue pots didn't fit the holes - so we filed the holes just a tiny bit with our rat-tail file.  Then it worked fine.

soldering the wires

construction is done

Set up / Testing

Use Notes