Hey, that looks familiar!
You have a scope?
Just follow the ch1 signal through V1 and 2.
And yes, the Vactrol LDR's can go bad.
You can hot down which ones control the channel switching and see if the correct ones are being turned on or off.
If you note which side is passing signal you could jumper across it to pass ch1 signal.
Do not jumper across the control voltage side.