The sidetone spurs in FT-736R

Fig.1 Carrier spectrum at reduced power (-6dB) from a FT-736R. The serial number is 3J740239.

The origin of the many spurs is the CW sidetone oscillator. The oscillator is built from CMOS gates, Q26 and Q27. On each transition low to high or high to low these gates draw a short current pulse from the 9V supply voltage. The current pulses modulate the transmitter with pulses at twice sidetone frequency. The problem can be cured by a filter that prevents these current pulses to cause voltage drops on the +9V line.

Fig.2 Carrier spectrum as in figure 1 when the FT-736R is modified. The two VCC pins of Q26 qnd Q27 are cut and bent a little upwards, connected to each other with a wire and to the +9V line through a 47 ohm resistor. A 47 æF capacitor to ground from the wire decouples the common VCC for Q26 and Q27 and serves as the reservoar from which the current spikes that the CMOS gates need can be taken without affecting the +9V supply. (Without the resistor, a much larger capacitor is needed. 1000 æF reduces the spurs by less than 6 dB)