Circuit Description - Kenwood TR7400A Service Manual

CIRCUIT DESCRIPTIONS
input pulse and i t is reset. The operating frequency has
been improved because resetting and presetting are done
in one cycle of input pulse but not in half a cycle, and the
delay time, t2, of the high speed D-flip-flop in l C l O is
much smaller than the delay time, t l , from IC5,
6
and
7 and logic circuit t o point A.
Next, operation is explained in relation t o the T X off-
set switch setting.
1 +600
During reception, this is the same as i n ( 2 ) . During
transmission, SR 1 is turned on and becomes U in Table 2.
Gate U therefore opens and gates N o and D are closed.
A t this setting, Ns
=
I N
+
197 (137
+
6 0 ) , and i t operates
as an extender when IC5, IC6 and IC7 take code 8, 0 and
5 respectively, t o perform division of N
+
197.
2
No (SIMP)
( F ) and ( G ) make up N o in Table 2. Gates N o and U
open and gate D
I S
closed. A t this setting, the relation,
Ns
=
N
+
137, holds between preset value N and actual divi-
sion ratio Ns. I t is enough t o decrement the counter after
division of N (decrernenting) has completed and perform
resetting and presetting just when the count has become
137. For this purpose, IC5, IC6 and IC7 do n o t take code
8, 6 and 3 respectively (as already described), but it
operates as an extender at code 5 and performs division
of N
+
137. Since the gate is of code 197 (137
+
60), the
extender operates before this code triggers the circuitry.
3 -600
During reception, SR2 is turned on as in ( 2 ) . During
transmission, gates No,
U and D open as D in Table 2.
A t this setting, Ns
=
N
+
77 (137
-
60), i t operates as an
extender t o perform division of N
+
77 when IC5, IC6 and
IC7 carry code 9, 2 and 5 respectively. A t this time, the
extender operates at code 77 even when all gates are open
Table 1 shows the case of N
=
400 (144.00 MHz).
-
T S B l
A c t i v e f i l t e r
n
TONE SQUELCH CIRCUIT
*d
Fig. 5 shows the circuit. The tone squelch circuit
employed in this equipment is the so-called CTCSS (con-
tinc~oc~s tone controlled squelch system). Tone signal of a
certain frequency is superimposed with audio signal at the
trans~rlission side, which is separated at the reception side
t o drive the squelch circuit. When set t o SOU (tone squelch)
as shown in Fig. 5, a voltage is applied t o T S B I and TSB2.
When no signal is received or signal received does not have
tone component, 0 2 0 and 21 remain off and no sound is
reproduced since the voltage of TSB2 is applied t o the
base of Q13 through D l 4 and the A F circuit is turned off.
When signal including tone component is received, the tone
signal separated from discriminator output with 0 1 9 ,
LPF
and amplifier, is applied t o an active filter. The active filter
which serves t o the tone frequency and 0 1 1 give steep
characteristics at the frequency. I t selects tone output
equal t o the active filter and its output passes through D l 1
(on during reception) and is detected in D l 2 and 13. I t
turns on 0 2 0 and then 0 2 1 and turns off Q13 and the A F
circuit ( 0 1 4 ) operates t o reproduce sound from speaker.
In the A F circuit, an active type high-pass filter of 0 2 4
and 2 5 cuts off tone signal output t o arnplify audio signal
alone. During transmission, 0 2 2 is turned on, and the
active filter and 0 1 1 forin an oscillating circuit t o deliver
output with the same frequency as of the active filter.
This o c ~ t p c ~ t is passed through V R 3 and modulated in T X
unit together with audiosignal. The maximum frequency
deviation for audio signal is f 5 kHz and that for tone com-
ponent for tone squelch is k0.5 kHz, which results in a
ratio of about -20 dB. This would result in buzzing sound
when unmodulated signal is received, but a high-pass filter
of 300 Hz in cutoff frequency corporated in the equipment
reduces the tone level t o prevent buzz. Operation is the
same even in the SUB (sub-audible) since a voltage is applied
t o T S B I , and sub-audible control is performed.
I
I
2
1 3
H8D5022
I
I
Dlsc
L ---A
O u t p u t
e
L.P.F.
AMP
T o base
o f
013
Fig. 5
T O N E SQUELCH Circuit