SynthFrog » synth porn

Fixing the MC-909 rotary encoder

aris — Mon, 05 Jan 2009 22:40:00 +0000

And of course taking a peek at its guts to shoot some porn in the process:

This is the MC-909.. It is my favourite synth because it allows a hands-on approach to sound synthesis. Even if it is targetet to the DJ market (which I am not!), it is capable of very interesting textures (and it has a nice sampler even if it lacks the velocity and key multilayering of the high-end samplers)

I got it second hand from ebay and since day one, the rotary encoder had a slight problem. I had to turn it slow to register correct values. If I rotated it quickly it would jump 5-6 values behind every 5-6 values (so it didn’t go anywhere).

I decided that I would fix it and gutted the machine.. pictures follow:

The faulty encoder can be seen in the following picture:

The pads are velocity sensitive and this is how it works: there are two contact areas
and the rubber pads have the graphite layer that correspond to these two areas in different heights. So when you hit the pad, the MC measures the time it took to close both switches which corresponds to the velocity of your hand.

This is the encoder. Close ups:

The part can be found at mouser as pt.no. 688-EC12E2420802 (alps 12mm rotary encoder) and it costs only 2 bucks. It is very simple to replace it, you must have a solder pump though to cleanly remove it.

Things to remember:

*) when you unscrew the machine, make a note (NOT mental) to the whereabouts of each screw..
*) the order of removing the components is the following: 1. the PSU, 2. the IO board, 3. the sound engine, 4. the “DJ” slider (the big one), 5. the mains outlet and 6. the main board.

some connections (eg the one marked with x) need not be disconnected. You can put the IO board aside for a while.

Good luck if you have the same problem.

Good choice if you have an MC-909 instead of an MC-808

Now my MC-909 is like new.

Dissecting SY 77

aris — Tue, 28 Oct 2008 21:46:00 +0000

I had an SY-77 lying around in my old house so I decided to take it appart just to look how it is built and what is made of. It wheighs around 15 kg and it is a very solid built. You have to unscrew at least 14 screws just to remove the underside panel which reveals the electronic guts of the instrument. There are two main boards visible. Both have countless DIP chips and some surface mount giants. Of course, it is well over me to make out what each of the chips actually do so I was just staring at them in awe.

The SY was the flagship of Yamaha back in the 90s. It had 32 voices, 16 of them where AFM (an advanced kind of DX’s FM synthesis with more algorithms between the operators and the ability to feedback an operator with it’s signal) and the other 16 were AWM (a PCM like sample playback synthesis method). Each voice signal could have two resonant filters and the output of the AWM voices could be used as input to an FM operator. This, if you didn’t know what you were doing, was usually a noisy mess. I didn’t but the guys that programmed the presets did because it has some wonderful mininoog style sounds that became very expressive with the velocity/channel aftertouch, semi weighted keyboard the SY features.

Moreover, in its FM synthesis, a different envelope (both pitch and amplitude) could be assigned to each of the FM operators so you could program some evolving sounds. Its envelopes featured repeat points and so on.
The SY also featured an on board sequencer which I didn’t really like (It was the time of cubase on atari ST). But my SY grew older, some keys grew sticky, some sliders were in need of repair and it gave way to the VST stuff. The floppy drive died a horrible-sticky death (there is a rubber band that used to turn the disk but it just disintegrated in my hands leaving a sticky, tar-like residue. I cleaned it with acetone but I need to find a replacement now).

I put it back in its case, waiting for the time to move to a bigger house where it will take the place it deserves. A master controller maybe? Or maybe not?

For more pictures and closeups of the ICs you can always look here

QY Synth porn pt 2

aris — Thu, 23 Oct 2008 21:49:00 +0000

This is the bottom of the main PCB. Is it bendable?!?

Some synth porn

aris — Thu, 23 Oct 2008 21:45:00 +0000

Dissecting the QY 70. Nice piece of gear. Would really like to know what are those chips anyway! (filters/roms maybe ?) The big one in the middle must have the OS.