Much older crystal controlled radios usually had separate Rx and Tx oscillators. This made it possible to convert a single mobile radio into a repeater. All of these radios, Syntor , Syntor X , Syntor X 9000, Syntor X 9000E and Spectra , use a single synthesizer for both receive and transmit. This means repeater conversions of these radios require two separate radios. It is not practical to make any one of these radios both receive and transmit at the same time. Even if you add a custom offset oscillator/mixer/filter for the receive and transmit frequency offset, keep in mind that the transmitter audio directly modulates the VCO frequency (it it true FM, not phase modulation) which means the receiver would also be attempting to use a modulated VCO (. it would be very detrimental to the receiver). You would have to make a custom oscillator/mixer/filter that you could modulate, disconnect the existing VCO from the Tx buffer, then connect the new Tx modulation circuit to the Tx buffer. Needless to say it would be a very messy and very complex modification. To get sufficient modulation deviation you would probably have to use a doubled, tippled crystal or a phase locked loop for your custom oscillator, which would make your modification almost as complex as the second radio you are trying to avoid using. The other equally complex solution would be to make a custom Rx tuning oscillator. Just using two radio drawers is so much easer.