Vintage Synth Repair

>>Yes, in the LFO Modulation section, all seven destination LEDs light up:
> OSC1 Freq, OSC2 Freq, OSC3 Freq, PW1, PW2, PW3 and Filter.
> 
> However, turning up MODULATION AMOUNT (knob on left) and/or MOD WHEEL has NO effect EXCEPT on Filter - which works fine so I know the LFO is working.
> 
> Which board should I be checking?<<

the pwm is ok too?
still poring over the service manual myself, but my best suggestion at this point is that you've got one or more dead 4016 ICs on the "common analog board". after that, the control voltages for the oscillators are combined in summing amps, one for each oscillator on each voice card. it can't be these, because you'd have lost all control over the oscillators, & anyway you'd need for 18 op-amps to have failed at once.

anything prior to the 4016s would've wiped out the filter mod too.

thinking about it, & about 4016s generally, it's likely that this is one IC we're talking about. each has four switches internally, so one would be enough for three oscillators-worth of mod switching.

>pores over manual some more< an actual schemo would get me closer, but looking at the parts list I see there are six 4016s on the common analog board, at U3, 4, 13, 14, 24 & 28. if they're socketed, you can swap them around & see if the symptoms change.

d.