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Page 309
1
Refer to the "X-Ray View" and "Voltage Chart" (fold-out from Page 308).
CIRCUIT DESCRIPTION
The receiver front end establishes receiver selectivity and
converts the incoming frequency to the 3.395 MHz IF.
Broadband
preselector filters, diode switched for each band,
feed the antenna input to the first receiver mixer where the
input and HFO signals are mixed to produce an
8.3958.895
MHz IF signal. This is then mixed with the VFO input in the
second mixer to obtain the 3.395 MHz IF.
-
In the 80 meter position, the Band switch places +11 VDC
at R702 and
R703, forward biasing diodes D701 and D702.
Diodes D703. D704. D705, D706, D709, D710, D713,
D714, D717, and D718 are back biased. This places the 80
meter filter, which consists of C702 through C712 (less
C704 and
C708) and L701 through L703, in the signal path
and effectively removes all other filters from operation.
Similarly, the filters for the 40 through 10 meter bands are
selected with the Band switch in the appropriate position.
During transmit operation, +11 VDC
is
applied at R701 and
R711, back biasing all preselector filters.
From the preselector filter, the input signal
is
fed to gate 1
of
0701, the first receiver. mixer. The HFO input from
board D, the
HFOlpremix board,
is
amplified by 0703and
injected through the 5.0
-
5.5 MHz band reject filter, C785
through C787 and L731 through L733, to gate 2 of the first
mixer.
An
8.395
-
8.895 MHz bandpass filter; formed by
L725 through L727 and
C763, C764. C766, C767, C769,
C771, C772, C773, and C774; serves
as
a load for the first
mixer and couples the first IF signals to gate 1 of the second
mixer,
0704. The VFO input
is
filtered by C776, C777, and
L728 before being injected at gate
2
of Q704. The mixer
drain
is
tuned to 3.395 MHz and feeds the crystal filter
board from the capacitive divider of the tank circuit.
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