Icom IC-551D Instruction Manual page 36

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TRANSMITTER CIRCUITS
MICROPHONE AMPLIFIER AND LIMITER CIRCUIT |
Audio signals from the microphone are fed to Pin 6 of the audio amplifier 1C5/2. Amplified
signals appear at Pin 7 and are sent through C172 and J4 to the MIC GAIN control variable resistor
on the front panel. Adjusted signals are fed to Pin 2 of 1С5/1 through C171, 045 and C170 in the
FM mode, as D45 is turned ON by the voltage supplied through R188; and through C171, D44,
C169 and R184 in the SSB and AM modes, as D44 is turned ON by the voltage supplied through
.R187. 1C5/1 functions as a limiter amplifier in the FM mode. In the SSB and AM modes, since
the input level is approximately 10dB lower than in the FM mode, because of R184, 1С5/1
functions as a normal amplifier.
BALANCED MODULATOR CIRCUIT
іп the SSB and AM modes, the amplified audio signals from Pin 1 of the audio amplifier ІС5/1 are
fed to Pin 5 of the balanced modulator ІСЗ (same as the receiver detector), through C164 and
R182 level adjust trimmer. The BFO signal is fed to Pin 7, resulting in a 9.0115MHz suppressed
carrier double side band signal brought out from Pin 3.
In the CW mode, a voltage is applied to D17 and 016 through 051, then 017 and 016 are turned
ON, therefore the BFO signal from Pin 7 of the ІСЗ is fed to the transmit mixer circuit.
The keying control is done by stopping the oscillation of the BFO. During key up, a voltage is
applied to the source of Q20, BFO oscillator, through R128, R127 and D30. Thus the BFO
oscillation is stopped, and therefore no RF output. During key down, the applied voltage is
shunted to ground through 029, thus the' source voltage of 020 becomes normal and the BFO
starts oscillating.
TRANSMIT IF AMPLIFIER CIRCUIT
When the RF POWER CONTROL is in COMP OFF position, DSB signals which appear at Pin 3
of 1C3, аге fed to the gate of 012 through D17, 027, 019 and 020 on the Pass Band Tuning unit.
The DSB signals are amplified by 012, and then fed to Р11 through the switching diodes D9 and
D11. DSB signals are converted to SSB signals by the filter and sent to the transmit mixer circuit
through D13, R61 and D8. In the AM mode, BFO signals from Pin 7 of the balanced modulator
ІСЗ аге fed through C69, R66, 016, 017 and 08 to the output of ҒІ1 and mixed with 558 signals
to generate AM (A3H) signals.
When the RF POWER CONTROL is turned clockwise to turn ON the speech processor circuit,
те DSB signals are fed to the 1st gate of O1 on the Pass Band Tuning unit through 017, D27 and
D1. This DSB signals are amplified by Q1 and fed to the balanced mixer consisting of D2 —D5,
where it is mixed with the VXO output frequency of 19.7615MHz. It passes through the crystal
filter ҒІ1 and becomes an SSB signal of 10.750MHz. This signal is mixed with the V XO output at
another mixer consisting of D6 — D9 again to become an SSB signal of 9.0115MHz, amplified by
Q2, Q3 and Q15, and passed to D25 and D26. D25 and D26 are the limiting circuit, and it clips
| signals above а fixed level.
Q14 is the clip level control circuit. The clipping current of D25 is applied to Q14's base, and
amplified the voltage of Q2's 2nd gate is controlled by Q14's output, and controls 02% gain and -
fixes the clip level. The clip level is adjustable with R63 and the output level is adjustable with
R65. Some splatter is included in the output signal which has passed through the limiter circuit,