Power Supply Circuits - Icom IC-756PRO III Service Manual

3-5 SCOPE CIRCUITS
3-5-1 SCOPE RECEIVER CIRCUIT (RF-B UNIT)
A portion of the 64.455 MHz 1st IF signal from the 1st
mixer circuit (Q1203–Q1206: while receiving) or IF ampli-
fier (Q1551: while transmitting) is passed through the
PIN attenuator (Q2203) and amplified at the IF ampli-
fiers (Q2202, Q2201), and then mixed with the 77.8 MHz
scope 2nd LO (S2LO) signal at the mixer circuit (D2101)
to produce the 13.345 MHz IF signal. The mixed IF signal
is passed through the ceramic bandpass filters (FI2003,
FI2002) to suppress unwanted signals. The filtered IF sig-
nal is applied to the FM IF IC (IC2001, pin 16).
The applied 13.345 MHz IF signal is mixed with the sweep
LO (S3LO) signals from the PLL unit at the FM IF IC
(IC2001). The mixed IF signals are filtered at the ceramic
bandpass filter (FI2001) then applied to the limiter ampli-
fier section in the FM IF IC (IC2001, pin 5). The applied IF
signals are converted into DC voltages according to the
applied IF signal strength at the RSSI section in the IC.
The converted voltages are output from pin12 (IC2001) and
amplified at IC2002, then applied to the MAIN-A unit as the
SCPL signal.
Some of the DC voltages from the FM IF IC (IC2001) are
amplified at IC2002 to produce AGC voltages for the IF
amplifiers (Q2201, Q2202), producing wider dynamic range.
By sweeping LO signals (S3LO) are applied to the mixer
section in the FM IF IC (IC2001), the spectrum scope func-
tion is activated.
3-5-2 SWEEP LO CIRCUIT (PLL UNIT)
The sweep LO signals (S3LO) are generated by the DDS
IC (IC801) using the 32 MHz system clock. A 10-bit digital
signal is converted into analog wave signals at the D/A con-
verter (R801–R820). The converted analog wave is passed
through the bandpass filter (L802, L803, C809–C813) then
applied to the RF-B unit via (J801) after being amplified at
the buffer amplifier (Q802).
• SCOPE CIRCUIT DIAGRAM
1st LO
signal
RF signals
1st mixer A
Q1203–Q1206
RF-B unit
to 2nd mixer circuit
Q2202 Q2201
IF IF
amp. amp.
D2101
to the MAIN-A unit
3 - 9

3-6 POWER SUPPLY CIRCUITS

3-6-1 PA UNIT
LINE
The voltage from an external power supply via
PHV the common filter circuit (FILTER-A unit; L501,
L502).
The same voltage as the PHV line passed
HV
through a fuse (F1).
The same voltage as the HV line passed
14V
through the switching relay (RL1).
The same voltage as the 14 V line is applied to
14VA
the AF power amplifier (MAIN-A unit; IC332).
Common 8 V converted from the 14 V line and
a8V
regulated by the +8 regulator circuit (IC3).
Common 5 V converted from the 14 V line and
5V
regulated by the +5 regulator circuit (IC2).
Common 5 V converted from the HV line and
H5V
regulated by the H5V regulator circuit (IC1).
3-6-2 FRONT UNIT
LINE
Common 5 V converted from the 14 V line and
5VF
regulated by the +5 regulator circuit (IC861).
Common –15 V converted from the 14 V line
and converted by the –15 DC-DC converter cir-
–15V
cuit (IC841, Q841, D841). The voltage is applied
to the –7 V, –8 V regulator circuits and etc.
Common –7 V converted from the –15 V line
–7V
and regulated by the –7 regulator circuit (IC501).
Common –8 V converted from the –15 V line
–8V and regulated by the –8V regulator circuit
(IC881).
Common 18 V converted from the 14 V line and
+18V converted by the 18 V DC-DC converter circuit
(IC821, Q821, D822).
S2LO signal
(77.80 MHz)
Ceramic Ceramic
BPF BPF
FI2003 FI2002
IC2002b
AGC
SCPL signal
amp.
IC2002a
DESCRIPTION
DESCRIPTION
S3LO signal
(12.79–12.99 MHz*)
IC2001
Mixer
16 2
FI2001
3 Ceramic
Limiter BPF
amp.
5
RSSI
12
*depending on sweeping passband width