I suggest starting a new thread elsewhere with photos of the handset internals.It is likely to have the NE5044 seven channel encoder chip.You may be lucky because that may allow substitution of the switch and no more.If there is an NE5044 and switch centre connection is removed and the wire voltage goes to 2.5 volts,that is the case.The channel 5 servo would centre with the centre cable disconnected.

If not,you could trace the switch cables to the circuit board,note the coloured bands on the connecting resistors or measure their resistance.

There may be up to five resistors connecting the switch to 0,5 volts and the decoder.
http://images.google.com/images?hl=e...tor+Color+Code
Note: surface mount components were introduced at about the time of that set.They were used in the following Futaba models