Kenwood TH-235A Service Manual page 7

TH-235A/E/234
CIRCUIT DESCRIPTION
@ APC and transmission output selector circuit
The automatic power control (APC) circuit is used to obtain a
stable transmission current. This circuit detects the output
power of the power transistor (Q14) and controls the APC
voltage which then controls the transmission power.
The output amplitude of Q14 is rectified by D13 and D19,
then compared with the reference voltage. Q16 forms a dif-
ferential DC amplifier which controls the APC voltage that is
generated by Q17 and Q18. This APC voltage controls the
output of the drive amplifiers Q12 and Q13 to stabilize the
transmission output power.
Either Hi power or the Low power of the transmission output
is selected by the switch Q15 which changes the reference
© Temperature protection circuit
When the thermistor detects about 100°C, the temperature
protection circuit turns Q23 on, reduces the APC voltage to
prevent the drive amplifier and the power transistor from ther-
mal breakdown.
voltage.
"7 on bebo + Transmission
oi TT! bm " sure
From drive stage I | y
i 6 = g
Ei
7? . I rr
AVA
Ahh Ah
ANy SIS —4
= re |
Qi7
B 4 + —
Qs
T
Qi6
D9
ryt +
hhh he
TX
POWER
Fig.5 APC circuit, transmission output selector circuit and temperature protection circuit