Alinco DX-70, Service Manual

Page: 5 / 116

Passing through BPF, the signal turns ON/OFF in the switching diode, D29 and
D30. This preamplifier is the parallel grounded gate operation of Q9 and Q10
(2SK2171), so the unit can obtain a good performance at a high level input signal
with low NF.
The wide range frequency from about 1 MHz to 60MHz is amplified about 10dB.
This lOdB preamplifier and 20dB attenuator in the Filter 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 L52, L53, L54, C103, C104, C105, and C106, 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 Q10 and Q11 is the balanced mixer, in which
the local oscillating signal is fed to the gate of 2SK2171.
7 he 3rd intercept point is about 20dBm, and local oscillator of about 2V P-P is fed
to the gate. The receiving signal is converted into the first IF of 71.75MHz.
As the ratio of the spurious interference is decreased in 50MHz band mode, the
trap of 71.75MHz consisting of L72 and C107 keeps the ratio of spurious interfer­
ence 70dB or more in all band.
b. The First IF Am plifier Circuit
FL1: A and FL1: B 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. H ere the image ratio is
determined 70dB or more (approx. 80dB}. The first IF amplifier circuit of Q12 is
located between the crystal filters to prevent the loss in the front-end and mutual
interference.
The first IF amplifier circuit Q12 decides the sensitivity after passing the mixer.
AGC voltage is applied to the second gate.
c. The Second Mixer Circuit, The S econ d Amplifier Circuit
DBM (Double Balanced Mixer) consists of L14, D7 and L16. The signal is passed
in the opposite direction while receiving or transmitting in this DBM. Approximately
OdBm is as the second \ocat oscitaVing \evel, and the third IP is approximately
lOdBm.
The receiving signal (71.75MHz) and the second local oscillating frequency
(71,295MHz) is mixed, and unwanted signal is eliminated in LPF consisting of L17,
L73 and C36, then the signal of 455kHz is generated. After passing through the
switching diode D8, the signal is amplified in Q22. The source of Q22 is controlled
by the output of the noise blanker circuit.
d. IF Filter
After passing through the transmission/reception switching diode D9, the signal is
led to one oi 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(CFJ455K8) 1.0kHz/-6dB 3.0kHz/-60dB
FM, AM FL.4(CFW455G) 9kHz/-6dB 20kHz/-50dB
Each filter has 4 switching diodes (D3-D48) in front and rear to isolate the filter.