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Yamaha CS01 combo synthesizer mods

The CS01 was a monophonic mini-key synth released in 1982 (without MIDI - that arrived the next year) and was part of a "combo" range that Yamaha designed to be complementary - I think there was a mixer, mini amplifier, and electronic drum pads as well. It had a tiny in-built speaker, ran on batteries, and had lugs for a guitar strap, so you were completely mobile and self-contained, even if no one more than a metre away could hear you playing! However, when you plugged the thing into a serious sound system, you discovered it was capable of some gorgeous and versatile tones. A Breath Controller input was provided (the BC1 controller was sold separately) which coaxed surprising expressiveness out of the machine (my first exposure to the BC1 was with a DX7, and I was distinctly underwhelmed on that particular synth). The tone generation was widely thought to be analogue, but if one checked the schematic, it showed that the "oscillator" was actually a clever Yamaha proprietary chip, a DTG (Digital Tone Generator) that scanned the digital keyboard matrix and then output any one of several waveforms, as well as noise and PWM, along with a voltage "gate" signal that triggered the envelope generator. The waveform was then fed to another Yammy chip, a VCF, which featured variable resonance, and then on to the VCA. The DTG could also do "glide", but not true portamento, it stepped thru intervening notes in a glissando, the rate of which was adjustable. The DTG required a master clock input of about 566 khz which was provided by a twin transistor oscillator. Interestingly, it was by varying this frequency that Yamaha enabled pitch bend (UP only for some reason) and LFO modulation.

Breath Control Input

I commonly hear of people doing mods on this machine for external CV control of the VCF, but Yamaha have provided one already! The simplest mod one can do doesn't even involve opening up the machine. The breath control input takes a stereo miniplug (3.5mm), the tip of which expects to see about negative 9 volts supplied by the synth. The ring of this connector then provides a CV of between 0 and negative 5 volts (from the tests with my BC1) which goes to the two breath control pots on the synth, one for the VCF, one for the VCA, which allows scaling of the CV independently for those sections.
So all you have to do for external control is solder up a cable with a stereo plug at one end with the tip NOT connected, but with the ring connected to the centre wire, the other end of which connects to a mono plug which goes to your midi - CV converter (connect the earths to shield as normal). I'm using a Kenton Pro2000 converter. The auxiliary outputs can generate +/- 12 volts approx. Perfect, as the converter needed to be able to generate negative CVs.
You may find you have so much fun with this mod alone (I sure did) that you may not need to do the more complex stuff! The key to the extraordinary expressiveness is the separate scaling control of the VCF and the VCA. Pull the VCF cutoff down a little, as well as the VCA EG slider, and feed a quantised CV to the input while holding a note, and moving the breath pots... oooh! Acid!
(See my demo vid coming soon)

Midi mod

I purchased the Highly Liquid midi kit for the CS01. I didn't want to stuff around with an external control box, I wanted it to be self-contained, so I was encouraged to see a couple of people had done this, by removing the internal speaker.


Above: after removal of the speaker and it's mounting brackets.


I populated the PCB without soldering anything, and immediately saw that the dip switches would have to go. Switch 1 wasn't used by this configuration anyway, and switch 2 was just for selecting the midi input channel, so I made an "executive decision" on the channel number, and just jumpered the relevant connection (switch info supplied on the Highly Liquid site).

Unfortunately, there still didn't seem to be enough room, so after examining the circuit diagram, I worked out I could remove a few components to get more space - namely the headphone amp chip, a couple of associated big electro caps, and the phones jack assembly. Removal of this last one had the added benefit of giving me somewhere to put the midi input socket (after a bit of enlarging of the hole).


Components removed: At 9 oclock to the hole in the main PCB, there is the outline of a large electro cap, and the battery spring just above that. Below this you see the mark for IC5 - the audio amp that drives the speaker and the headphones. To the right of this a jumper has been moved, and a smaller electro removed. Below the line output jack housing, the phones housing has been removed.


Components removed.


Phones port enlarged for a midi socket.


The circuit board was lined up and two holes drilled for the LEDs so that they could be mounted directly onto the reverse side of the PCB (see pic). One of the speaker mounting screws was very conveniently located so it could be used to hold the edge of the PCB. I traced out a path from the voltage regulator for a point to connect the power to the circuit (where I took it from, the power to the midi board will not switch off from the front panel, you have to remove the power plug. Highly Liquid suggest a different point). The speaker connect cable was recycled for the power cable, so I could unplug it if I had to remove the main synth PCB for other work.


Holes for the LEDs


Former speaker mount screw just holding the edge of the PCB.


PCB in place, ribbons run to the other end of the synth to connect as per the Highly Liquid install instructions.


Insulation tape on the keyboard frame, and on the PCB for good measure.



Anyway, thinking I was very clever, I put it all back together, and guess what? It didn't work!
I opened it up again, and eventually found that the plastic insulating tape on the metal frame holding the keys, on which the midi kit rested, had been pierced by the sharp points of the soldered IC socket, shorting them to ground. I nipped off the points, re-insulated the area, and put it back together - it worked!

But then I noticed... the buckle in the case! DOH!! The midi kit was still a bit too big, it was pushing out on the main pcb, flexing it. Damn!


Nothing for it but to remove the IC socket and solder the chip on to the board directly, to decrease it's height profile. A 40 pin socket. And I don't have a professional desoldering tool!!
Sh%t. It took nearly two hours of painstaking work (I hate desoldering - I rigged up a nozzle and connected it to an old vacuum cleaner which helped a bit, but it was still an almighty PITA).
I didn't want to risk lifting or cracking the traces on the PCB, which is double-sided.
Anyway, finally, it worked! Phew.


Note the brown wire, replacing a jumper on the other side, removed for extra space for the chip.


Resonance mod

My CS01 is the Mark 1, which has a switch for high or low resonance. Messing with the internal res trimpot while playing a note showed that the filter could be a really mean beast if it wanted to, so I thought this should be accessible. I simply removed the trimpot and brought the control out to a 20k minipot on the rear panel of the synth. To make room for this I had to drill out an area of the battery support. I left the connecting wires long to allow the main synth PCB to be flipped up in case any other work was needed.



Video: Yamaha CS01 does 303 emulation

I know, I know, you're probably heartily sick of everyone banging on and on about the sound of the bloody 303.
However, when discussing an "analogue" (or nearly!) monophonic synth, it can be a convenient shorthand to get an idea of the tonal shaping options on offer. In this video, a simple 16th note pattern with ties and rests, typical of yer average 303 acid riff, is programmed into an Ableton clip, triggering the CS01 via the new midi input (Later at 3.54 you see the midi activity LED flashing). I adjust the synth to play a square wave initially. Then, the VCA slider is brought down to allow the effects of the CV input to be heard on the volume - as I turn up the VCA breath control you hear certain notes being "accented" above others. Then same thing for the VCF. The extra resonance is then tweaked, some envelope mod added to the filter, and more tweaking. A little glide is added at times - not quite the 303 effect here! But you get the idea.