The (mini-) modification consists of adding a switch to toggle between self-oscillation on/off modes.
To do this i had to use an extra 4 HP wide blank panel because there isn't enough space left on the A-124's front-panel to add a switch.
This is not my first 'expansion module',
i made one earlier for my A-127 Triple Resonance Filter

Another piece of wire connects the other side of the switch back to the original position of where the resistor was connected.
I am very happy with the result, i can now manually switch, turn the resonance-knob up to self-oscillation and simply switch back to the original mode again...
Thanks to Tony Steventon from Synovatron for the project review