This is the first of a my Paia Fatman mods. It will provide an LFO function to the filter. This mod requires no trace cutting, just some component swap-outs plus a few extra parts.
The are two ways of doing the mod. The first method does not need an updated firmware ROM; the second method requires a small change to the Fatman firmware, but uses less parts. Read through them both then take your pick.
The above schematic shows the A(S)R circuit as modified for the added LFO function. Notice that the SPST switch S3, which controlled AR/ASR has been replaced with a SPDT Center Off switch. This switch's positions are LFO/AR/ASR when read left to right. Also note the four added resistors Ra, Rb, Rc and Rd. Two other resistors, R85 and R103, will be changed to new values. Resistor R79 will be replaced by a 1N914 diode, and a second 1N914 will be placed between the base of Q7 and the "LFO" lug on switch S3. Finally, Q7 is changed to an NPN Darlington type and Vcc is wired to Ra and Rb.
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,
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.
Here's how it works: Positive feedback has been added to the comparator stage IC8:c to change it into a Schmitt Trigger. The hysteresis is the peak-to-peak voltage of the LFO and is set by the ratio Rc/Rb. When the switch is in the AR and ASR positions there is little change in the operation of the circuit. The output of the Schmitt (comparator) signals the computer that the peak output has been reached and the computer responds by turning on Q7 to initiate the Release. When the switch is in the ASR position this signal is grounded and since the computer never sees that the peak has been reached, it never turns Q7 on. Except of course when an NOff or 0V NOn is received from MIDI.
In the LFO position of the switch the "peak reached" signal from IC8c is routed back so that it turns Q7 on. When the peak is reached the Schmitt output goes high, turns on Q7 and the Release starts. Release continues until the voltage on C22 falls below the lower Schmitt trigger point at which time the output of IC8c goes low, which turns on Q7 - and so on.
It is also worth noting that the computer's AR control bit "O15" does not affect the LFO. This bit is activated (dropped low) only once at the beginning of a note, and goes high whenever the AR "peak reached" signal is sensed by the computer on its "I2" line. If the LFO already has the I2 line high when O15 goes low, O15 goes high within a several microseconds (The amount of time it takes for the firmware to see I2 high and react by setting O15 high.) This is far too short a time to affect the time constant of the 2.2uF storage capacitor and attack/release pots, even at their minimum settings. If the LFO has the I2 line low at the time O15 goes low, the AR/LFO is already in the attack/upslope portion of its cycle: the imminent peak of the cycle will, by setting I2 high, cause O15 to go high.
Vref, which is produced by the voltage divider R85 and R86, is lowered because otherwise the higher threshold of the Schmitt will be so high that the voltage on C22 never gets there. Vref is set at 3.2 volts, and provides the maximum possible peak-peak voltage from the LFO. Vref, however, also sets the peak value reached by the ADSR. In order to compensate for this new lower peak voltage, the value of R103 (which scales the VCA V/I converter and is not shown here) should be cut. The ADSRs peak value has been reduced to a little less than half it's previous value so 22k or 27k replaces the present 39k.
A note about Rc, Rd: These values will determine the peak-to-peak output of the LFO. Since a single comparator is being used to provide the triggering (instead of the more common dual "window" comparator), the ratio of Rd/Rc is unusually small in order to provide the widest possible hysteresis. This in mind, observe that as Rc is decreased, Vp-p will decrease. It is OK to use a 3.9K resistor for Rc; the Vp-p will be about 4.25 volts. Beyond 4.5K, the Schmitt comparator tends to latch up, making 4.53K (a common 1% value) the upper limit. As for Rd, loading effects of the VCF's V-I converter as the A(S)R level control is rotated toward maximum limit this resistor to 3.9K.
In this method, the only changed components are the ROM (IC3), switch (S3) to a SPDT on-off-on (center off) type and resistors R85 & R103, which are replaced by 15K & 27K. The only added parts are resistors Rc and Rd (4.53K and 3.9K), as well as adding a wire from the open lug of switch S3 to pin 17 of the 8031 chip, IC1.
Resistors: 3.9K, 4.53K, 15K, 27K Semiconductors: 27C64 ROM w/LFO software update Hardware: SPDT or DPDT on-off-on (center off) miniature switch Misc: 10" #22 AWG insulated hook-up wire
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. Note: steps 7 to 13 can be omitted if you don't mind using the DIP switch labeled "spare" to switch the LFO in and out. The AR/ASR switch must be in the AR position for the LFO to work, however. 7) Unsolder connections to AR/ASR switch, remove switch. 8) Mount SPDT (or DPDT) on-off-on (center off) switch. 9) Reconnect wire from point "M" on board to *top* lug of switch. (Note: this wire previously went to center lug) 10) Reconnect remaining wire to *center* lug of switch. (Note: this wire previously went to top lug) 11) Connect 10" length of insulated hook-up wire to bottom lug of switch. 12) Route wire down through hole used by "Gate" RCA jack mounting tabs. (There should be enough room) 13) Trim wire to appropriate length and solder to pin 17 of IC1. 14) Remove ROM IC3, replace with update ROM (file listing below)