Alinco DX-SR9 E, Service Manual

Page: 3 / 66

3
CIRCUIT DESCRIPTION
SA901 and R903 are installed in the input part of antenna terminal as the
countermeasure against the thunder. The electric charge of antenna is
discharged at R903, and when the voltage becomes over about 300V, the gap
of SA901 is discharged so that the receiving input circuit is protected.
The input signal from antenna is passed through the Tx/Rx selecting relay
(RL903) and passes thru the attenuator of about 20dB (RL906 ON or OFF).
The followings are prevented in LPF consisting of L904, L905, C913, C914,
and C915: 2m band image receiving, passing through the First IF (71.75MHz)
and leaking of the first local oscillating frequency (71.88654~106.75153) to the
antenna terminal.
The receiving signal output from PA Unit is fed to Main unit through CN108.
HPF, consisting of L122, L123, C154, C156, C158, C160, C167, and C168,
eliminates the strong radio signal of MW band of 1.6MHz or below. In case of
receiving the signal of 1.6MHz or below, the received signal is passed through
the low pass Filter (L118, L119, C155, C162, and C163). BPF consists of 8
filters. Each filter covers the following frequency range. The frequency of
2.5MHz or more consists of Chebyshev BPF, and under 2.5MHz frequency
band is LPF.
Range For amateur band
-1.6MHz BPF1
1.6 -2.5MHz BPF2 1.8MHz
2.5 -4.5MHz BPF3 3.5MHz
4.5 -7.5MHz BPF4 7MHz
7.5 -10.5MHz BPF1 10MHz
10.5 -14.5MHz BPF2 14MHz
14.5 -21.5MHz BPF3 18,21MHz
21.5 -30MHz BPF4 24,28MHz
50 -54MHz BPF5 50MHz
Passing through BPF, the signal turns ON/OFF in the switching diode, D120
and D121. This preamplifier is the parallel grounded gate operation of Q128
and Q130 ( 2SK2539 ), so the unit can obtain a good performance at a high
level input signal with low NF.
The wide range frequency from about 1MHz to 60MHz is amplified about 10dB.
This 10dB preamplifier and 20dB attenuator in the PA unit are combined, then
by pressing RF gain switch on the front panel, one of four steps, -20, -10, 0, or
+ 10dB is selected.
The LPF consisting of L146, L147, C235, C236, C252 and C253, prevents the
following first receiving mixer from the local oscillation leaking, and also
prevents the first IF and image of the spurious receiving.
The first receiving mixer consisting of Q128 and Q130 is the balanced mixer, in
which the local oscillating signal is led to the gate of 2SK2539. The 3rd
intercept point is about 20dBm, and local oscillator of about 2V P-P is led to the
gate. The receiving signal is converted into the first IF of 71.75MHz.
1) Receiver System
1. PA Unit
2. Main Unit
a. Front End
XF102 and XF103 are the crystal filters of 71.75MHz. By the combination of
two filters, the unit has the characteristics of the band width of 15kHz or more
3dB and the value of guaranteed attenuation of 70dB or more. Here the image
ratio is determined 70dB or more (approx. 80dB). The first IF amplifier circuit of
Q124 located between the crystal filters to prevent the loss in the front-end and
mutual interference.
The first IF amplifier circuit Q124 decides the sensitivity after passing the
mixer. AGC voltage is applied to the second gate.
DBM (Double Balanced Mixer) consists of L114, D111 and L115. The signal is
passed in the opposite direction while receiving or transmitting in this DBM.
Approximately 0dBm is fed as the second local oscillating level, and the third
IP is approximately 10dBm.
The receiving signal (71.75MHz) and the second local oscillating frequency
(71.295MHz) is mixed, and unwanted signal is eliminated in LPF consisting of
L101, L102 and C119, then the signal of 455kHz is generated. After passing
through the switching diode D108, the signal is amplified in Q110. The source
of Q110 is controlled by the output of the noise blanker circuit.
After passing through the transmission/reception switching diode D110, the
signal is led to one of three ceramic filters of 455kHz. The selectivity is decided
here except CW narrow.
SSB, AM-NARROW FL3(CFJ455K5) 2.4kHz/-6dB 4.5kHz/-60dB
SSB-NARROW, CW FL2(CFJ455KB) 1.0kHz/-6dB 3.0kHz/-60dB
FM, AM FL4(CFW455G) 9kHz/-6dB 20kHz/-50dB
There are two switching diodes for input and output of each filter (D129 to
D150), securing isolation. The isolation required is more than the guaranteed
attenuation for each filter (about 70dB). The filters not used are shorted by
diodes parallel to the filters and cut by the diodes in series, therefore the
combination achieves high level of isolation from the signal. The filter switching
is done by the Q141, Q142, Q143, Q145, D128, D145, D146 and D151, and
the switching configuration depends on the mode, Tx/Rx, and Wide/Narrow
status.
After the filter, passing thru a Tx/Rx switching diode (D128), the signal is
amplified by the Q138 and Q139, and buffered by the Q137. The second gates
of the Q110, Q138 and Q139 are controlled by the AGC circuit. The level of the
received signal for which AGC is applied is of high amplitude and constant at
the output of the Q137.
This output is used for demodulation of SSB, AM, and CW modes besides
used for AGC detection. In the FM mode, the signal having amplified by the
Q138 is partly input to the IC110 (MC3357) thru the C353 and is amplified and
demodulated. The demodulated signal is amplified by an op-amp inside the
IC110. A feedback resistor (R351) has a parallel capacitor (C365) for
de-emphasis. The Q110, Q138 and Q139 are also operational during the FM
mode and the AGC is effected.
b. The First IF Amplifier Circuit
c. The Second Mixer Circuit, The Second Amplifier Circuit
d. IF Filter
e. Second I.F. Amp