Tandy 102 Service Manual page 36

Reception Filter Circuit
As shown in Figure 4-19, the reception input signal is amplified when passing through coupling
capacitor (C40), and amplified again as it passes through the 3-stage band-pass filter (composed of
an active filter). The signal then passes through the comparator, and after being changed to a
square wave, is input at the RXCAR terminal of the MC14412. Also, to check a carrier signal, this
signal is input to the demultiplexer M33.
The intermediate frequencies of the 3-stage active filter are shown below. The switching of
intermediate frequency for the originate and answer mode is accomplished by switching T2, T3 and
T5 ON or OFF according to the ORG/ANS switch position, thus changing the input resistance of the
filters.
1st stage 2nd stage 3rd stage
Intermediate
(
Originate Mode 980Hz 1150Hz 1330Hz
Frequercy Answer Mode 1940Hz 2120Hz 2290Hz
(d8)
G
f
(d 8)
GG
I
(dB)
0
I
i f(HZ) f (Hz) ; f(HZ)
C109 68P
C41 4700P C43 4700P C46 4700P , R35 10K
>n
470K
0z
y n C I
1110
VDD 1118 1129
Ma f
R30 n = 00
DIR C40 R19 4 732K 590K 332K 28OKF 422K
2 2K ' 0 I
0T7
13
14 1117
9 R15 7 R31 9 8
1 6 7 R41 D29
12 c
M29 M30 r---- M30
ACP
0047y tSK
M29
g
M29 Y
5
M29
n
*
3 10 5
10K
RXM->•0
+ 11
Q
v
+
R „{
e
+
cop
of
+ TL064 + +
1-4 IS2076
Pin-4
VEE
R37
ORIG
ANS
1 VDD
R12
1K
R14 R26 R23
T2 T3
10K 10K 10K
C2712 C2712 C2712
TS
Figure 4-19. Reception Filter Circuit
180K
11160
f00K
93CAR
111311
4-18