Kenwood TH-77A Service Manual page 8

In-//A/CO
CIRCUIT DESCRIPTION
2-4-3 External speaker detection
The microcomputer detects that the REM pin of the speaker
jack is zero or the remote switch voltage when the speaker
plug is inserted. The audio input is then switched from IC6 to
IC5 by IC3.
2-4-4 Audio amplifier
Amplifier IC6 (NJM386BM) is used for the internal speaker,
and amplifier C5, for the external speaker. The power for IC6
and IC5 is produced by 016 (2SB1182) and Q17 (2SC4617). D4
(MA110) and O18 (DTA144WE) are a power on/off control cir-
cuit.
2-4-5 Squelch circuit
Since the squelch circuit for the UHF band is configured identi-
cally to that for the VHF band, only the squelch circuit operation
for the VHF band is described below. The detection output sig-
nal of C2 is filtered by an internal amplifier in IC2. The noise
components of the signal are amplified by 013. The resulting
signal is rectified by D16 to produce a squelch signal. The volt-
age at the input pin of Q11 is turned on or off by the squeich var-
iable resistor. The SCV signal and hysteresis switches are acti-
vated by Q12.
2-4-6 Signal-strength meter circuit
The TH-77 employs signal-strength meter circuits |C2 and
1C202 (MC3377D) for the transmitting-receiving unit. For the
VHF band, the microcomputer input voltage is adjusted by VR1
of the transmitting-receiving unit. For the UHF band, it is ad-
justed by VR6 of the control unit. The liquid-crystal display for
the VHF and UHF bands appears at the same time. Each signal-
strength meter signal is thus added to the input pins of differ-
ent analog-to-digital converters. The liquid-crystal display con-
sists of five steps in a pair for the VHF and UHF bands.