Kenwood TH-22A Service Manual page 8

TH-22A/AT/E
CIRCUIT DESCRIPTION
e APC and transmission output selector circuits
The automatic power control (APC) circuit is used to ob-
tain a stable transmission current. This circuit detects the
drain current in the final stage of the power module and
controls the transmission output.
To differential DC amplifier |C204, two voltages are ap-
plied: the reference voltage produced by dividing the voltage
of constant-current zener diode D202 by variable resistors
VR201, R210, R211, and R212 for transmission output ad-
justment, and the detection voltage generated across
R61,R62 and R63 in proportion to the drain voltage in the
final stage.
The voltage, proportional to the difference between the
reference voltage and the detection voltage, is obtained at
the output pin (pin 6) of IC204. This voltage is reversed by
Q203 to provide the APC voitage.
This APC voltage controls the power contro! pin of the
power module and stabilizes the transmission output.
When transmission output is changed, each switch is
changed as shown in the table below. The reference voltage
is changed, and the transmission output is fixed at about 5 W
(high), 0.5 W (low), or 50 mW (EL).
Wr 1C3 121
Red 0.99 POWER MODULE LPF
R63__ 0.39 45 |
Ly
z & Qo 0 ANT
es oa > > A
he
DRIVE
AMP
CONT
=
SB ~«- t zs
Q202 N
fol
EL >——
Q205 Q201
3
a
is
g
=
Hi>—~
x R209
a
a
ec
be RX
D
ee,
$———<3T
11
=
F
a.
Fig. 6 APC and transmission output selector circuits
e Temperature protection circuit
When the thermistor detects about 100°C, the tempera-
ture protection circuit turns Q15 on, reduces the APC voltage
to prevent thermal damage to the power module.
H/L and EL are control signals from the microprocessor unit. The
logical relationship is shown in the table below.
=
2 eg Transmission Reception
Daw °
& a5
E -—
52
5 83/ WL EL HIL EL
-
Hi L L L H
L H L L H
EL L H L H
Table 5 Transmission output selecting condition