Icom IC-M412 Service Manual page 9

RF CIRCUITS (CH70-RX)
RF circuits for CH70-RX consist of RF fi lters and RF amplifi er
(RF AMP), and fi lter and amplify the signal of CH70 (156.525
MHz) only. The circuits are fixed to receive 156.525 MHz
signal only, and are activated even when the transceiver is
receiving other channels (MAIN-RX).
• RF CIRCUITS (CH70-RX)
2-pole BPF
C513 C496
5P 0.75P
1ST IF CIRCUITS (CH70-RX)
The 1st IF circuits consist of 1st mixer, 1st IF fi lter and 1st IF
amplifi er. And it down-convert the RX signals into the 1st IF
signal, then fi lter and amplify.
The filtered RX signal from the 2-pole BPF are applied to
the 1st mixer to be converted into the 1st IF signal, by being
mixed with the 1st Local Oscillator (LO) signals from the TX/
CH70-RX VCO (Q4, 5, D1–3) via buffer (Q7), LO SW (D8)
and LPF (L65, 66, C537).
• 1ST IF CIRCUITS (CH70-RX)
R8V
C533
0.01
Q63
KTC38809
R524
C529
1st IF AMP
0.001 680
RF AMP
C499
C495 C511
0.75P 1.5P 1.5P
C534 C535 L65
0.001 0.001
56N
1st LO
LPF
1st IF filters
FL-413 FL-413
FI5 FI4
R523
680
The filtered RX signal from the trap (BEF; Band Elminate
Filter) are applied to the RF AMP. The amplifi ed RX signal is
passed through the 2-pole BPF.
The fi ltered RX signal is then applied to the 1st IF circuits.
C503
220P
Q61
3SK131
R503
100K
The converted 1st IF signal is passed through two 1st IF
fi lters to remove unwanted signals. The fi ltered 1st IF signal
is amplifi ed by the 1st IF AMP. The amplifi ed 1st IF signal is
then applied to the 2nd IF circuits.
L66
56N
C525
Q62
3SK293
0.001
1st Mixer
4 - 3
C513
5P