Spender DHS3600, Service Manual, Page: 4 / 32

Service manual SPENDER DHS3600
4
CIRCUIT DESCRIPTION
1. FREQUENCY CONFIGURATION
The receiver utilizes double conversion. The first IF is 45 MHz and the second IF is 455 kHz. The first
local oscillator signal is supplied from the X2 (44.545MHz).
2. RECEIVER SYSTEM
2-1 Front End (RF AMP)
The signal coming from the antenna passes through the transmit/receive switching diode circuit, is
amplified by the RF amplifier (Q10), and passes through a BPF. Then passes through the low-pass
filters and goes to the mixer.
2-2 First Mixer
The signal from the front end is mixed with the first local oscillator signal generated in the PLL circuit
by Q11 to produce a first IF frequency of 45 MHz. The resulting signal passes through the F1,F2
MCF to cut the adjacent spurious and provide the optimum characteristics, such as adjacent
frequency selectivity.
2-3 IF Amplifier
The signal then passes through the first IF (Q35), and is amplified and goes to the IF IC (U8). U8
incorporates the functions of the second OSC, second mixer, second IF amplifier, detector, noise
amplifier, and noise detector. The signal input to the IC is mixed with the RF signal of the second
OSC to produce a 455kHz second IF signal. The signal is amplified by the IF amplifier. The signal
passes through the ceramic filter (F3) to provide the necessary selectivity. Finally, the signal is
detected by the IC (U8) and output as an AF signal.
2-4 AF Amplifier
The AF signal from the IF IC is amplified and decoded by IC (U8), and the signal passes through AF
VOL circuit and enters the audio power amplifier IC (U1) to drive the speaker.
2-5 Squelch
Part of the AF signal from the IF IC enters the FM IC again, and the noise component is amplified
and rectified by a filter and an amplifier to produce a DC voltage corresponding to the noise level.
The DC signal from the FM IC goes to the analog port of the microprocessor (U4). U4 determines
whether to output sounds from the speaker by checking whether the input voltage is higher or lower
than the preset value. To output sounds from the speaker, U4 sends a high signal to the MUTE and
turns U1 on though Q4, Q14, Q17.