The channel switching mod to the old Siemens 6 S Ela 2427c installed. The optocouplers circuit board found a place.
Another view, on the left is the input jack, switch for channels and a remote jack for channels.
I removed the guts from the pilot lamp and installed the channel indicating leds in it. We can see the two new pots installed, a clean volume pot and a mids pot.
Another view where you can see the new wiring to the old pilot lamp now acting as channel indicator.