The RXCV was the 2nd module i built. It's now succeeded by the EFM 3503 midiport.
The circuit is designed to be powered by a central modular
supply of either +/-12V or +/-15V. Actually it refused to give 1V/Oct until i used +/-15V. The datasheet says the LM324 only goes up
to +V - 1.5V. That would mean it can just about reach 10V on a 12V supply, but it's very close to its maximum at that point, which
usually suggests it's not linear anymore.
There is an on board +5V regulator for the PIC, DAC, inserter and opto coupler. Midi data is
read by the PIC. The PIC interprets the data and switches it's outputs to comply with the midi messages. It will respond to pitch,
pitch bend, modulation, key down and more that one key down messages all others are ignored. Pitch and pitch bend control
voltage outputs are obtained by switching a dual 7528 DAC. Gate, Glide (Slide), and Mod controls are generated by the PIC.
The delay generator is built from a complementary fet pair in a 4007. It's normal state is high. When
triggered the output from the 4007 goes low and then rises. The time it take to do this is set by the Mod Delay pot and is variable
from immediate response to almost a full second. First draining then charging the 10uF capacitor C10. When connected to a VCA
used to control a LFOs output level, it has the effect of delaying the Mod and then gently applying it smoothly and slowly much
like you would do naturally with a mod lever. You can elect to have automatic mod after every note played by plugging the Key
Gate output into the Delay Input .
If you setup the module forget about
the -10.56 at pins 4 & 18 of the DAC!
This might be the value that tom needed
for his setup but the calculated value
would be -10.66V (for a 8bit DAC we
have 128 steps with 1/12V each, that
gives us 128*0.83=10.66). For setup
just play the highest C on your
keyboard and adjust the scale trimmer
T1 until you get the exact voltage
(i.e. 7V for C7).
To bring the gate to an usable value i lowered the value of R16: