Hi Chioccio
It's unclear as to what input on the PIC you are refering. If it's the input from the radio, it might be that the connections needs to be swopped on the cord running from the radio to the interface. i found that sometimes it works better connecting the radio signal in reverse, especially on some JR radio's.
On the serial output of the PIC (pin going from the transistor that is connected to the DB9 plug), the voltages should be reversed. With 5V applied to the base of the transistor, you should get anything from -5 to -15V on the RXD line (pin 2). Applying 0V to the base, should give you around +5V on RXD line.
As a PIC works on a TTL level (either 5V or zero volt), and a serial port works on RS232 levels, an interface is required to connect a TTL device with a RS232 device. The transistor is used to act as a converter between TTL and RS232.
My advise is to measure the voltage on the PIC (across the zener diode). If you do not get at least 5V, you might want to change the resistor between pin 7 on the DB9 plug and the zener diode with a 470ohm resistor. Furthermore, try replacing the 4K7 resistor between pin 2 and pin 4 with a 47K resistor.
Also ensure that you use the correct transistors (NPN or PNP). Have a look at my Joystick/PIC design (just click on "homepage" below). I have tested it on quite a lot of different PC's and radio's, and it work correct on JR and other radio's.
Hope this might help.
Hope this might help you