Kenwood TH-77A Service Manual page 10

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10
H-77A/E
CIRCUIT DESCRIPTION
3-3 APC Circuit
The automatic phase control (APC) circuit produces attain a
stable transmission output. This circuit also detects the current
of the final-stage amplifier and controls the input power level of
the amplifier.
ee ee
L
Table 4
The APC circuit operation for the UHF band is explained below
(Figure 6). During transmission, 04 is turned on by the 5TV and
5TU signals, and IC1 is activated. The output power of IC101 is
detected as the voltage drop across R4 and R5. The voltage at
both ends of D1, whichis completely stabilized by the constant
current source of 01, is compared with the voltage divided by
VR1, R11, and VR2. The current flowing through D101 is con-
trolled so that no voltage difference occurs. Consequently, the
drive input power of |C101 decreases when the power amplifi-
er module current increases, and the drive power increases
when the module current decreases.
For high-, middle-, and low-level selection, Q2 and Q3 are
turned on then off when the H/L1 and H/L2 ports are con-
trolled. The reference voltage for !C1 is then switched. (See
Table 4.)
O
HA2 H/LI STV
Fig. 6 APC circuit
O
STU
Ic 101
Q2,93
Q7,Q98
Ql
Q4
1cl
R4,R5
UHF Power Amplifier
BAND SW
UHF POWER SW
VHF POWER SW
CONSTANT CURRENT SOURCE
APC AMP SW
OC ERROR AMP
CURRENT DETECT