Kenwood TKR-750 Service Manual page 18

TKR-750
2-4. Audio Amplifier
The audio amplifier circuit is located in control section of
TX-RX unit (X57-626 B/2). The recovered audio signal ob-
tained from IC9 is amplified by IC608, input to the AINR ter-
minal of CODEC IC (IC614), and audio processed by DSP
(IC618).
The processed audio signal from AOUTR terminal of
IC614 is amplified by IC619 (B/2) to a sufficient level, anti-
aliasing filtered by IC619 (A/2). The audio signal goes to an
electronic volume (IC615) V3/V4, to the input of multiplexer
IC (IC622), and is amplified to drive a loudspeaker by an au-
dio power amplifier (IC629). The 4W audio output can be
provided to external 4 ohms speaker through the 15-pin test
connector "SPO, SPG" on the rear panel.
2-5. Squelch Circuit
The output signal from IC9 enters FM IC again, then
passed through a band-pass filter.
The noise component output from IC9 is amplified by Q7
and rectified by D5 to produce a DC Voltage corresponding
to the noise level. The DC voltage is sent to the analog port
of the CPU (IC604).
IC9 outputs a DC voltage (RSSI) corresponding to the in-
put of the IF amplifier.
Display unit
(X54 B/2)
MIC JACK
J601
TXRX unit (X57 B/2): CONTROL
IC605 (A/2)
MIC
MIS
TA
TAS
18
CIRCUIT DESCRIPTION
Display unit (X54 A/2)
IC501(A/2) Q501,502
VR501
AMP
DET
D501,502
IC610
SW AMP
AINL
TXI TXO
SW
Fig. 2 Microphone circuit
3. Transmitter Circuit
The transmitter circuit consists of the following circuits :
3-1 microphone circuit, 3-2 modulation level adjustment cir-
cuit, 3-3 driver and final power amplifier circuit, and 3-4 auto-
matic power control circuit.
3-1. Microphone Circuit
The signal from the microphone is passed through AGC
circuit located in display unit (X54-333), so that it does not
saturate. This circuit consists of IC501, D501, D502, Q501,
and Q502. The AGC is operated by controlling the + and –
side levels of amplitude using the current obtained by posi-
tive and negative detection of the amplified audio signal.
The audio signal goes to control section of TX-RX unit (X57-
626 B/2) from display unit (X54-333).
The transmit audio signal goes to the input of the multi-
plexer IC (IC605) for microphone muting. The audio signal is
amplified by IC610, input to the AINL terminal of CODEC IC
(IC614), and audio processed by DSP (IC618). The pro-
cessed audio signal from the AOUTL terminal of IC614 is
amplified by IC616 (B/2) to a sufficient level, anti-aliasing fil-
tered by IC616 (A/2), and amplified by the summing ampli-
fier IC611 (A/2).
3-2. Modulation Level Adjustment Circuit
The output of the summing amplifier IC611 (A/2) is
passed to an electronic volume (IC615) for maximum devia-
tion adjustment before being applied to a varactor diode in
the voltage controlled oscillator (VCO) A2 located in TX-RX
unit (X57-626 A/2).
MI
ATT
IC616 IC616
IC614
(B/2) (A/2)
AMP LPF
CODEC IC
AOUTL
IC605 (B/2)
D-SUB
SW
TD
TDS
IC611
IC615
(A/2)
AMP
MO
DAC
MB