Icom IC-502A Instruction Manual page 17

When transmitting, R8 is grounded by the microphone PTT switch, or mode change-over switch
(in the case of CW-T), to make 0175 base voltage zero and output voltage zero also. Likewise, for
the transmit section regulated voltage, the reference voltage of D3 cathode is applied to Q3's
(2SD355) base through D4(1SS53), and a regulated voltage of about 9.5V is obtained at its emitter.
During reception, since the PTT switch is not grounded, positive voltage is applied on the transmit/
receive change-over control Q2(2SC945) base through R8 to turn on Q2, while the Q3 base is
grounded through R13 and 02, thus making the transmit voltage zero.
When transmitting, the PTT switch is grounded and the Q2 base is also grounded through D3
(15553) to turn off 02 and apply the reference voltage to the Q3 base, and so a proper voltage is
obtained. Also, the rise time for transmit/receive change-over is delayed by C13 and C14, respec-
tively, to prevent transmission signals from entering the receiving section during the change-over
operation. |
RIT CONTROL AND FINE TUNING
When the RIT switch is turned OFF in the receive mode and during transmit mode, the positive
voltage at the base of Q5 goes to zero and as is turned off, and Q6 is turned on. So, a voltage
divided by R3, and R4, R32 and R33, is applied to D1, and the VFO frequency is adjusted by the
FINE TUNING only, thus the receiving frequency and the transmitting frequency are the same.
VFO UNIT
The oscillator circuit is a parallel tuned Colpitts circuit. Q1(2SK33) enables the oscillator. fre-
quency to be varied by changing the variable capacitor C2. In this oscillator, a range of 36MHz to
37MHz is produced and output is fed through to the buffer amplifiers Q2(2SK33), Q3(2SC710)
and Q4(2SC710). The low-pass filter composed of L5 and L6, and C26, and C29 to C33, mini-
mizes spurious radiation. |
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