Fatman Synth, Part 1



(All the parts right out of the box.)

Back in the 80’s, a company called PAiA came out with a hobbyist line of build-it-yourself analog synthesizer kits. I bought the smallest one, a little ribbon-controller unit entitled The Gnome, for about $35. It had pretty much the same functionality as Gakken’s SX-150, but with a soft vinyl ribbon strip. It was fine for playing with for a few hours, but after a while I put it in my closet, and eventually gave it away. It’s now a collector’s item on E-Bay. Anyway… I have something of a soft spot for PAiA, and recently I’ve been looking for a cheap DAW I can hook up to the Roland A-300 Pro for making music independent of a PC. Generally, a DAW (digital audio workstation) is software running on a PC, but there are a few hardware instrument synth boxes on the market, in the $1000 range. I wanted something cheaper, and I kept coming back to the PAiA Fatman kit.


(Everything excluding the case. Glasses not included.)

Finally, I ordered one off the PAiA website, about $260 including the main kit and desktop case, plus $60 for shipping to Japan. More than I wanted to spend, but what the heck. The kit has an optional 12 VAC power adapter that you can decide to not get for international orders. Because the west side of Japan runs on 110 60-cycle VAC, I decided to get the power adapter with the kit. Shipping weight was about 4 pounds. Finished package size is 29cm x 14cm x 7cm (roughly 11″ x 6″ x 3″). Everything is included except solder and tools (required tools include soldering iron, clippers, some kind of pliers and a small screwdriver. A ruler and wire strippers are recommended.)


(Circuit board with the 43 jumper wires and all of the resistors.)

There’s no suggested assembly time. Reported times are between 6 and 40 hours. It took me about 16 hours, not including that for calibration and tuning. The only way I can see to get a 6 hour time is to pre-cut all the wires and pre-sort all the resistors before starting the clock. The circuit board is single-sided, which means that there’s a need for adding jumper wires that normally would be plated on a 2-sided card. The kit can be mounted in a rack, or put in a desktop case, which changes the wiring needed between the knobs and the board. I picked the desktop case, with its larger surface area for getting at the knobs. The instructions are pretty straight forward, although I did have to flip back and forth a lot between the main manual and the desktop supplement when it came to the knob wiring. You start out by adding the 43 jumpers. Then, all the resistors. Although the instructions don’t warn you, you’ll end up with 2 extra resistors when you’re done, because they’re used later when you wire the knobs.


(All the capacitors installed.)

One power resistor is actually two 1-watt resistors in series and raised off the board to allow for air flow. Next, you add the caps, followed by the diodes and transistors. One power transistor needs to have the heat sink clipped on before being wired into the board, but there’s no diagram of the heat sink installation and I was left guessing as to what I needed to do for that. Fortunately, it wasn’t that big a deal to clip on. This is followed by the trim pots, the 2 IC sockets and the back panel connectors (3 RCA jacks, and 2 MIDI jacks – one IN and one THRU).


(Trim pots and sockets.)

The main EPROM and the 8031 microcontroller are the only two chips that go into sockets. The rest are soldered in place. This is the trickiest step in my opinion, because there’s no way of knowing if you’ve screwed something up until too late. The EPROM and the 8031 get plugged in at the very end. Then comes the case wiring.


(ICs)

The wires running from the board to the pots in the case are called the “flying wires”. Installation is in three steps. First, cut the flying wires, tin the ends and solder them to the board. Next, cut the second set of wires, depending on whether you have the rack or desktop case, and wire up the connections between the pots. Third, solder the floating ends of the flying wires to the pots. Steps one and three are where I made my first real mistakes – I soldered the wires so they pretty much stand up and down on the board and the pots. Bad move. This prevents the board from fitting in the case when it comes time to fold the wires in half and put the board in place. I had to go back and resolder all the pots so the wires run mostly flat against the inside surface of the case. Even so, the wires still don’t like being folded to bring the board up to the pots. Note that the DIP switch S2 is mounted on the solder side of the board for the desktop case.


(Steps 1 and 2 for putting in the flying wires.)

Put the EPROM and the 8031 in their sockets, check your work, then plug in and turn on the power switch for the all-critical smoke test. There was no smoke, and the power LED lit properly. Nothing overheated abnormally. The Fatman doesn’t have its own input system, so you have to plug in a MIDI keyboard to the MIDI IN jack. The MIDI Note On/Off LED would flicker properly when I pressed the A-300 Pro keys, but I couldn’t get the Gate LED to light when I set S2 for MIDI Port 1. That’s when I spotted the big solder splash shorting 2 pins on IC 7, a smaller solder bridge from one pin to an adjacent circuit path, and a big ugly bridge between pins 2 and 3 on the EPROM. I take a fair amount of pride in my soldering skills, so it really depresses me to think that I completely missed all three problems at the time. Especially the ugly bridge, because I don’t know how that could have happened unless it was loose solder on the table and it welded itself in place to the board when I set the circuit card down. Anyway, there doesn’t seem to have been any permanent damage caused by the bridges or splash, and the Gate LED lights properly when I send MIDI signals to port 1. So, that’s good.


(Step 3.)

The next step is tuning and calibration. This requires either using an o-scope, a frequency counter, or comparing the sound out against another musical instrument. And I have none of those things. I do, though, have the Japanino with the LCD shield, so I’m in the process of making myself a cheap and dirty signal tracer/freq. counter. When that’s done, I’ll return to tuning the Fatman.

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