Hi,
The opto ensures there is no electrical connetion between
the servos and the rx, it simply passes the pulse on the signal
wire through without changing the width which defines servo
position. It also blocks any electromagnetic interference
picked up by the long servo leads, I have checked this with
a scope and noise generator and it does work. SM Services
version is ok.
PCM consists of equal length pulses each 10 bits assign
a value which is converted by the rx into pulse width for the
servo. Suspect you are using top end channel 9 or 10? Would
be interested if you have the same effect mixing (as an
experiment) with say channel 4.
One thing you could try is a JR matchbox, am using one
on my L39 elevator and the leads on that are 7ft, no problems.
Mike