Thursday, February 14, 2013

Korg Monotron CV Mods

Here are some simple mods to the Korg Monotron that allow you to use external Controlled Voltage to operate the monotron instead of the ribbon keyboard. I think Korg had a pretty revolutionary thought in letting its customers double as their R&D department to see which way to take the analog revival. Korg release the schematics, and printed all of the controlled voltage points on the PCB. By letting the hackers and tinkerers have at it, some pretty cool things resulted. In the years since this release, Korg has come up with the monotron duo and delay, the monotribe, and the new MS-20 Mini.

For my monotron, I simply connected the CV gate and CV pitch points to RCA jacks and mounted these on the right side of the keyboard. Since I am using my PAIA Fatman as a MIDI to CV converter, the RCA jacks were the easiest way to go.

 After connecting the two machines up, I did notice the pitch tracking seemed off. To remedy this, I adjusted the trim pot on the back of the monotron to get closer to the sound I was looking for. While this was better, I found that I could easily add a 100k potentiometer to my cable connecting the CV pitch, and get more adjustment to reach the lower octaves. The results are shown in the video below, where I used the CV controlled monotron as a bassline synthesizer, to create some acid style synth tweaks reminiscent of the mid 1990's stuff I used to DJ.

Whippany Rhythm Master modifications

A while back I modified my Whippany Rhythm Master analog preset drum machine from the late 60's/early 70's. This normally sounds like your typical cheesy rhythm box to accompany organs from this time, but with a few mods you can open up some possibilities. There are no chips I messed with here, or no markings on the circuit board to give you. I pretty much just replaced some of the trim pots inside and some of the resistors with potentiometers mounted to a panel and fit inside a wooden case.  

 You can check out the pictures here to see some of the wiring that I put in. The trim pots(now removed) control the output volume of the various instruments, including separate controls for the noise and the pulse of the snare sound. To find which other resistors controlled the sound parameters, I shorted out the resistors around the trim pots while the machine played.

 At some point, I will go back in and try to find the trigger points for the sounds. If I ever get the external triggering figured out, I will have a pretty bad ass analog drum module for less than $50. In the meantime, you can check out a video of how this sounds right now.

Thursday, February 7, 2013

Circuit Bending the Yamaha RX-120 Drum Machine

Here's a brief run through of how I modified my Yamaha RX-120 Rhythm Machine. By using a patchbay connected to the sound ROM, there are a lot of effects that you can achieve, including synth type tones, flanging, ring mods, bit crushing, and others. Using more multiple patch cords lets you stack these effects. The preset only RX-120 shares the same sound set (and chip) as the programmable RX-17. By using MIDI to trigger the RX-120, you have a ton of flexibility for programming. 

Additionally, by probing on the output opamps, you can come up with some crazy feedback filters too!