I have a question about automatic firing mode in S-125:
in salvo mode it launch the two missiles from channel 1 & 2.
but in non salvo mode, it don't use missiles in channel 2, even if the two channel 1 is empty !
we can launch channel 2 missiles manually, even if automatic firing mode is ON !
but if we change to other launcher (and automatic firing mode still ON).... the missile in channel 1, Go!
some explanation?

You nearly described all the inbuilt logic of the system

1, It will automatic launch if the "Razreshenie" lamp illuminates, and no missiles were launched to the same target earlier.
(You can reset this by selecting new launcher)
(Automatic launch is for the first salvo, at the furthest possible range)

2, Automatic launch will launch No1 missile all the time first.
(if there is no missile at No1 launcher, it will not work)
(If a salvo set, No2 missile will be launched)

3, At manual mode, if salvo is set, you can still decide if you want to launch 1 or two missile.
If you launch No1, than No2 will be launched automatically.
If you launch No2, than there will be no salvo.
(as salvo is only capable of launching No2)