Osc2 and osc3's pots. On the left are the ones with the 1µF cap and the 100k bleed resistor (and the non-switched LED). The blue cable is the osc1 sync.

"As standard, Oscillators 1 and 2 were meant to be tracking audio generators and Oscillator 3 an LFO. As the circuitry is almost identical Oscillator 3 may be converted to be the same as Oscillator 2 or all three may be made range switchable." (from Graham Hinton's page)

Just do the mod as described on Graham Hinton's page but use the push-pull switches.






 Pots to replace with push-pull ones :
- osc1 "sine level"
- osc2 and osc3 "square level"

Wiring with dual shielded cable for osc 2 : 2 caps and LED on the switch in the AKS. 1µF cap and bleed resistor in the A MKI.

My experience implementing this mod into several Synthis revealed that some LM394 supermatched pairs (depending on the batch) can cause a constant repetitive problem : switching between hi and lo range a few times (or even only once) results in a slightly evolving high whistle, the frequency pot not acting anymore.
If the Synthi is turned off and restarted, it works again... untill the next switching (I didn't know Synthi issues could be fixed the same way as Windows ones).

According to Digitana who kindly helped me to diagnose this issue, the LM394's common maximum collector current of 20mA could be too low for use with the 1µF cap. 2C746's and SL301B's is 50 mA.

If this issue occurs, the best solution is of course to use another supermatched pair as 2C746, 2N2916, SL301B (as EMS used)... but sourcing these can be difficult.
Modern MAT02EH will work too.

Another solution is to add a bleed resistor, placed on the push-pull switch, to empty the cap when not in use. It works fine but is not perfect (sometimes issue still occurs but is very very rare).
But doing this you won't be able to use a LED, unless you're ok for it to be lighting all the time (partially hidden by the knob when pushed though), because both ways of the standard push-pull switch switch are used. 
Is a pity 3PDT push-pull pots are not available. 
If you ever know where I could find these, please let me know, thanks !

The lugs wiring is :
A D : twin cable to the 15nF cap on the board
B E : 1µF cap
C F : 100k or more resistor

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