Scope Circuit; Power Supply Circuits - Icom IC-756PRO Service Manual

3-5 SCOPE CIRCUITS
3-5-1 SCOPE RECEIVER CIRCUIT (RF UNIT)
A portion of the 64.455 MHz 1st IF signal from the 1st mixer
circuit (Q511–Q514: while receiving) or IF amplifier (Q751:
while transmitting) is amplified at the IF amplifiers (Q811,
Q812), then mixed with the 77.8 MHz scope 2nd LO signal
at the mixer circuit (D831) to produce the 13.345 MHz IF sig-
nal. The mixed IF signal is passed through the ceramic
bandpass filters (FI843, FI841) to suppress unwanted sig-
nals. The filtered IF signal is applied to the FM IF IC (IC841,
pin 16).
The applied 13 MHz IF signal is mixed with the sweep LO
signals from the PLL unit at the FM IF IC (IC841), which
includes the RSSI terminal. The mixed IF signals are filtered
at the ceramic bandpass filter (FI842) then applied to the
limiter amplifier section in the FM IF IC (IC841, 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 amplified at IC871b then applied
to the MAIN unit as the SCPL signal.
Some of the DC voltages from the FM IF IC are amplified at
IC871a to produce AGC voltages for the IF amplifiers (Q811,
Q812), producing wider dynamic range.
By sweeping LO signals (S3LO) are applied to the mixer
section in the FM IF IC, the spectrum scope function is acti-
vated.
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 sig-
nal 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 unit after being amplified at the buffer
amplifier (Q802).
• SCOPE CIRCUIT DIAGRAM
1st LO
signal
RF signals
1st mixer A
Q511–Q514
RF unit
to 2nd mixer circuit
Q811 Q812
IF IF
amp. amp.
D831
to the MAIN 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 unit, L501,
L502).
The same voltage as the PHV line passed
HV
through a fuse (F1).
The same voltage as the HV line passed through
14 V
the switching relay (RL1).
The same voltage as the 14 V line is applied to
14 VA
the AF power amplifier (MAIN unit, IC332).
Common 8 V converted from the 14 V line and
8 V
regulated by the +8 regulator circuit (IC3).
Common 5 V converted from the 14 V line and
5 V
regulated by the +5 regulator circuit (IC2).
Common 5 V converted from the 14 V 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
5V
regulated by the +5 regulator circuit (IC861).
Common –15 V converted from the 14 V line and
converted by the –15 DC-DC converter circuit
–15V
(IC841, Q841, D841). The voltage is applied to
the –7 V, –8 V regulator circuits and etc.
Common –7 V converted from the 14 V line and
–7V
regulated by the –7 regulator circuit (IC501).
Common –8 V converted from the 14 V line and
–8V
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
FI843 FI841
IC871a
AGC
SCPL signal
amp.
IC871b
DESCRIPTION
DESCRIPTION
S3LO signal
(12.79–12.99 MHz*)
IC841
Mixer
16 2
FI842
3 Ceramic
Limiter
BPF
amp.
5
RSSI
12
*depending on sweeping passband width