PC serial ports are pretty dumb.
Any voltage less than about 3 volts will "read" as a "1". Any voltage higher will read as a "0". (for the techos, RS-232 to TTL inverts remember)
If the radio generates PPM at 0 to 9.6V (nominal) then you shouldn't need anything other than a solder connection to plug it in.
Why are the PIC and transistor convertion cables needed?
Is it because not all PPM outputs are 0 to 9.6V (nominal)?
Is there any other problem?