Joined: Dec 31, 2007
Location: Austin, TX USA
|Posted: Sun Jul 08, 2012 7:04 pm Post subject:
Mods for CGS75 VCS
|Posting the changes I made for a recent build of CGS75 VCS modules. A few of the mods may be relevant to the DUSG as well. Some mods have been posted elsewhere, while others are new. I am a bit surprised that no one has mentioned the reverse action of the Rise and Fall rate knobs as shown in the schematic and wiring connections.
Summary of changes to Serge VCS
1--) TRIG IN: Change second resistor to ground from 150K to 100K. Matches the CGS DUSG schematic.
2--) TRIG IN: Change series input capacitor from 10 nF to 1 nF. Provides more consistent triggering at high frequencies without stalling. Matches the CGS DUSG schematic.
3--) BIPOLAR OUT: Change pulldown resistor at TL071 pin 2 from 470K to 620K. Centers 5V range bipolar output signal around ground. 470K was off center.
4--) PROCESSOR INPUT: Add a 200K resistor from +15V to the processor input jack NC jack. This makes the processor a +/-5V source when nothing is plugged into the input.
5--) SLEW RANGE: Change resistor from IN jack op amp TL072 pin 7 to transistor array from 22K to 8.2K. Matches the CGS DUSG schematic. Before this change, the max slew rate was 400 seconds! The maximum self cycle frequency was 1.27 KHz. After the change the max slew rate is about 100 seconds and the maximum self cycle frequency is about 3.1 KHz.
6--) RISE RATE and FALL RATE: Swap the 10K log pots with 50K or 100K linear pots. Matches the CGS DUSG schematic. Before this change, the pot control was highly skewed towards one end of the scale. The linear pots provide smoother control over the entire range.
7--) RISE RATE and FALL RATE: Swap CW and CCW connections. Before this change, clockwise rotation made the self cycle waveforms lower in frequency, or slews longer. After the change clockwise rotation makes the self cycle waveforms higher in frequency.
8--) RISE CV and FALL CV: Swap the CW and CCW connections. After the change, a postive input voltage combined with the knob in the positive position (12 o'clock through 5 o'clock) will make the VCS cycle faster.