I have not built a 25S but I have repaired and calibrated a number of them.
The FM modulation appears to operate more as a square wave on any of the three MO settings. The MO buffer has too much gain so the waveforms clip. Change R245 on PCB3 to 1K so it is the same value as R248. Verify the modulation follows the MO waveshapes. Note the FM modulation is linear, not exponential, so the modulation index does not result in a significantly wide range. Note also that the MO is AC coupled to the linear FM so on low frequencies the MO waveshape is distorted as one would expect.
None of the 3.5mm jacks have a ground wire so the front panel and 3.5mm jacks have to be grounded through the boat. The CCW terminal of either FM control at the bottom is ground so I ran a wire from these to the 3.5mm FM jacks. The grounds should be routed to all 3.5mm jacks so grounding does not rely upon the anodized panel.
Monitor MO output
Set MO Coarse control to CCW, MO fine control to MID
Set Range to Hi
Adjust MO Symmetry trim for best triangle symmetry (adjust first as it affects the frequency)
Adjust MO 27.5 Hz Offset trim to low frequency desired (27.5 Hz)
Select Lo Range and check symmetry; iterate for best symmetry
Set MO Coarse control to CW, leave MO fine control at MID
Set Range to Hi
Adjust MO Coarse Scale to highest frequency desired.
Set MO Coarse to some mid frequency
Adjust MO CV Scale for proper tuning
Monitor CO Sine output
Adjust rear sine trim for best sine shape
Set CO Coarse control to CCW, MO fine control to MID
Set Modulation Type to none and Index to CCW
Adjust CO Symmetry trim for best sine symmetry (adjust first as it affects the frequency)
Adjust CO 27.5 Hz Offset trim to low frequency desired (27.5 Hz)
Recheck rear sine trim for best shape
Set CO Coarse control to CW, leave CO fine control at MID
Adjust CO Coarse Scale to highest frequency desired.
Set CO Coarse to some mid frequency
Adjust CO CV Scale for proper tuning
Monitor Final output
Set Timbre, Symmetry, High to CW
Adjust Timbre trim until waveform just starts to change from sine, then back it off just a bit
Set Mod Type to Pitch
Adjust rear FM Offset trim so there is no change in frequency. You will need to cycle through the Mod Type several times to get this set correctly.
Calibration is complete
I've repaired and calibrated a number of 25S modules. The timbre circuit seems to have a lot of variation which may be related to the tolerances of later vactrols and the LS3958 dual JFET. I decided to put these images of the final waveform of a typical 25S timbre with various settings of the three controls. First is with all three controls set to CCW so we just have a sine wave output.
Turning the Symmetry control CW has no affect but turning the Order control CW increases the amplitude of the sine slightly.
Turning the Timbre control CW with the Symmetry and Order controls CCW produces a lot of folding with square corners.
Leaving Timbre at CW and turning the Symmetry control CW makes a minor change.
Leaving Timbre at CW and Symmetry at CCW and turning the Order control CW creates a ramp-like waveform that is a folded sine with a half inversion.
Leaving Timbre and Order at CW and turning the Symmetry control CW creates a folded sine.
I have not seen a V2 but have corresponded with customers trying to help. V2 appears to incorporate all the changes from the V1 build notes. An additional trimmer TR11 was added on PCB3 for timbre range. It adds a negative offset voltage to the wiper of TR9 so it will offset the control voltage to the timbre circuit. Without documentation, it appears these are the changes.
PCB1: Add R63 2K2
PCB2: Add R134 11K, and R135 10K.
PCB3: You still need to change the value of R245 to 1K so it is the same value as R248. Add TR11, value unknown, but it is a series resistor between -15V and the wiper of TR9. Try 100K but note that if TR11 is set to the minimum, and TR9 is set to the minimum, you ground the -15V supply through both trimmers and burn them out.
I believe TR9 now sets the range and TR11 adjusts the offset, but they will interact with each other. I haven't seen an actual V2 PCB to verify how to set calibrate these trimmers and it isn't obvious from the circuit to determine what procedure to adjust them. I would adjust TR11 to maximum value. (you’ll have to measure between the pins to determine this). Then adjust TR9 to the eliminate folding with timbre off. Then set the timbre to maximum. See if TR11 will back off the maximum. Then check with no timbre and tweak TR9 as necessary.