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I gave my Fatman to my son. He's used it quite a bit but wanted
more functionality. We spent the day working on it and it now
really sounds quite nice. I added separate volume and pitch
controls for each VCO. I improved the glide circuit. I added
sub-octave dividers on each VCO, each with a separate volume
control. I also added an XOR function with a separate volume
control.<br><br>
I replaced the offset control with the pitch control for VCO2. I
replaced the 1/2 mix control with the XOR volume level. I added
four small potentiometers below the pitch, glide, offset, and mix
controls (shifted to the right). I added a hard sync switch next to
the mix control and a hp switch next to the punch switch.<br><br>
It really has a lot more functionality and sounds really great.
Here's the complete list of mods:<br><br>
<font size=3>The following changes required for correct operation:
<ol>
<li>Correct ADSR: Add 4700 ohm pull-up resistor from output O13 to +5
volts. This corrects ADSR operation by fully turning on Q11.
</ol><br>
The following changes improve performance:
<ol>
<li>Improve ADSR performance: Change R99 from 470K to 320K.
Implemented by parallel 1M resistor on bottom side of PCB with R99.
<li>Improve VCO Soft Sync: Replace R38 and R39 with LED.
<li>Lower Frequency Range: Replace R73 with jumper.
<li>Improved Glide: Replace IC10 and IC13 with LM347 (lower input bias
current prevents detuning when operating the glide control). Re-adjust
R42.
</ol><br>
The following changes add functionality:
<ol>
<li>Independent VCO Pitch Controls: Remove R33. Change R36 to 56K.
Change R40 to 10K potentiometer and wire in parallel with R34.
<li>Independent VCO Volume Controls: Remove R46 and R55. Replace
R57 with jumper. Wire R56 in place of R46. Add 10K volume control
and wire in place of R55. Wire the wiper of each potentiometer
through 100K resistor to point G.
<li>Sub-Octave / Square Wave Outputs: Wire clock of each flip-flop
(4027B) to pin 3 of each VCO. Wire each Q-1 output through 10 uF
(negative to Q-1 output) and 30K resistor to one side of 100K volume
potentiometer. Wire other side of potentiometer to ground.
Wire the wiper of each potentiometer through 100K resistor to point
G. Wire J and K inputs of each flip-flop to pin 16. Wire SET
and RST inputs of each flip-flop to pin 8. Wire pin 16 to GND and
pin 8 to -12 volts. Bypass pin 8 and 16 with 0.1 uF capacitor.
<li>Ring Modulator: Wire Q-0 output of each flip-flop to inputs of
XOR gate (4030B). Wire output through 10 uF (negative to Q-0 output) and
30K resistor to one side of 100K volume potentiometer. Wire other
side of potentiometer to ground. Wire the wiper of the
potentiometer through 100K resistor to point G. Wire unused inputs
to pin 7 (or 14). Wire pin 14 to GND and pin 7 to -12 volts. Bypass pin 7
and 14 with 0.1 uF capacitor.
<li>HP Filter: Disconnect positive side of C23 from pin 9. Connect
positive side of C23 to center of SPDT switch. Wire one side to pin
9. Wire other side to pin 8.
<li>VCO2 Hard Sync: Disconnect pin 4 of VCO2 from ground.
Wire pin 4 of VCO2 to center of SPDT switch. Wire one side to
GND. Wire other side to pin 3 of VCO1. </font>
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