Synthesis Technology MOTM-101 Mods

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Last updated: February 28, 2004

1. Increase Random Output Level

Changed R11 from 100K to 27K to get 10 Vpp on Random output. Random output level is now similar to Pink output level.

2. Use Blue LED For TRIGGER

You'd think that changing from a red LED to a blue LED would be trivial. Not so with this module. The problem with blue LEDs is that they tend to be extremely bright compared to the more common red/green/yellow varieties. To prevent them from burning holes in your retina, the current has to be reduced substantially. In general, I've found that replacing the current limiting resistor with one that is 8 to 10 times greater produces a pleasant soft blue light that is easy on the eyes. Changing the current limiting resistor is easy to do in circuits that use a dedicated LED driver. But this is not the case with the MOTM-101. You can try to reduce the current by increasing R28 and R29. But as R29 is increased, more of the vibrato signal gets fed back to the sample and hold circuitry which adversely affects the operation of the module. Instead, I chose to drive the TRIGGER LED directly off U7B with a resistor to ground. This allows me to customize the LED brightness without affecting the remainder of the circuitry. The procedure is simple:

Replace D3 (the red LED) with a 1N4148 diode on the MOTM-101 motherboard.
Connect the new current limiting resistor (I used 10K) to pin 7 of U7B. I made this connection by soldering a 10K resistor on top of R28 (the side that connects to pin 7 of U7B).
Run a wire from the other side of this new resistor to the anode of the LED.
Run a wire from the cathode of the LED to ground.

I wasn't able to find a blue LED that matched the one supplied with the MOTM-101 so I made my own. I bought a blue 5 mm LED (Digikey #67-1751-ND) and used a belt sander to remove the dome (a dremel also works). I then put this LED into the plastic shell that housed the red LED. I had to remove some tiny standoffs in the bottom of the plastic shell to get the blue LED to sit as low as possible in the shell. Finally, I attached some heat shrink tubing to the LED leads to keep it from sliding forward in the shell. The end result looks (and works) great.