Variable-symmetry LFO for Paia Fatman

[The Old Crow]

  Heya folks,

    With the new year I decided to get off my tuckus and get a few things
done, the first of which is a Paia Fatman mod to provide an LFO function to
the filter.  This mod requires no trace cutting, just some component
swap-outs plus a few extra parts.  John [Simonton] originally sent me the
preliminary info on how this sort of mod might be done in June '95, and
after only 7 months it is here. :) A detailed web page will be provided
later, but this post should contain enough info to get it working without
difficulty. Happy soldering!

 ** LFO for Fatman VCF with adjustable symmetry **

  Additional parts required:

  Resistors:      3.9K, 4.53K, 4.7K (2), 15K, 27K
  Semiconductors: 1N914 diode (2), MPS-A12 NPN Darlington or eqv.
  Hardware:       SPDT or DPDT on-off-on (center off) miniature switch
  Misc:           10" #22 AWG insulated hook-up wire, spaghetti tubing,

  Procedure:

  1) Change R85 to 15K, change R103 to 27K.
  2) Unsolder wire from point "S" on board.
  3) Remove jumper wire parallel to R85 (between R85 and IC8).
  4) Solder 4.53K 1% (or any value 3.9K-4.5K) in empty jumper location.
  5) Solder wire taken from point "S" to 4.53K resistor, on the end
       closest to R88.
  6) On solder side of board, solder 3.9K resistor between pins 9 and 14  
       of IC8.  Point "S" next to pin 9 is convenient for one lead. 

  7) Unsolder connections to AR/ASR switch, remove switch.
  8) Mount SPDT (or DPDT) on-off-on (center off) switch.
  9) Reconnect switch wires.
 10) Solder cathode of a 1N914 diode and one end of a 4.7K resistor to
       unused lug of switch (If DPDT, leave one side unused.)  Insulate
       diode and resistor leads if necessary.
 11) Solder 10" length of insulated wire to "+" lead of C33.  It may help
       to first remove C33, attach the wire, then remount it.

 12) Remove Q7, replace with MPS-A12 NPN Darlington.  (Same orientation.)
 13) Remove R79.  If possible, enlarge hole closest to Q7 with a #60 bit.
 14) Mount second 1N914 in location of R79, with the cathode pointing
       away from Q7.  Solder cathode end only.
 15) Mount one end of a second 4.7K resistor into same hole of R79 occupied
       by the anode of the 1N914 just installed.  If the resistor lead won't
       fit, solder it to the diode's anode lead on the component side of the
       board.  In addition, solder the anode of the 1N914 mounted on the
       switch in step 10 to the anode lead of the 1N914 mounted as R79.
       Finally, solder the lead(s) on the solder side of the board.  Dress
       the component leads to avoid touching other circuit points.  Stripped
       insulation or spaghetti tubing is handy here.  (This is probably
       the trickiest step.)

 16) Trim the free end of the wire installed in step 11 to a length suitable
       for soldering to the free ends of the two 4.7K resistors.  Dress the
       resistor leads and then solder.  Insulate this junction if possible.

  Operation:

     The AR/ASR switch is now an LFO/AR/ASR switch, with the LFO function
enabled by flipping the switch bat up.  AR is the center position; ASR is
the lower position.  In the LFO mode, the A(S)R Attack and Release controls
become a combination of LFO frequency and symmetry control.  The frequency
range is approx. 0.1Hz (both controls at max.) to 140Hz (both controls at
min.)  As the Attack and Release controls are varied, they will adjust the
cyclic waveform in the sense of a continously re-triggered AR envelope.
The amplitude of the LFO waveform is 5v p-p nominal.

     I will post any changes if I find they are needed.

     Next up: a firmware revision to provide midi note enhancements;
low-key follow, high-key follow,  note-wrapping for lowest and highest 
octaves.

       --Scott

/*    Christopher S. Rider    */
/* http://www.mcs.com/~syzygy */