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WJ-8718-19/FE HF RECEIVER
CIRCUIT DESCRIPTION
The IF output sample of approximately 10 mV from the 455 kHz amplifier
of A4A7 is the input signal for this module.
It is applied to both demodulators
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although only one is actuated at a time.
When FM is selected, the control input at
pin 41 is high (+5 V) and Q2 and Ql are turned on.
This applies approximately +9 V
to limiter Ul.
The input signal is amplified and clipped by cascaded stages within Ul,
so its output is free of any amplitude variations.
The extent to which the amplitude
variations are removed contributes to the AM rejection of the receiver when receiving
FM.
The output of the limiter drives the Foster-Seeley discriminator. Diodes CR1 and
CR2 rectify the composite signals fed to them by C7 and Tl.
When the signal from
the limiter is at exactly 455 kHz, Tl is tuned so that equal and opposite voltages are
produced across load resistors R6 and R7, giving a net output of zero to buffer U3A.
For inputs slightly off 455 kHz, the voltages of R6 and R7 do not cancel causing a
positive output for inputs above 455 kHz and a negative for those below 455 kHz.
(Note that these polarities are reversed by U3C, so the output of the module will go
negative when the signal frequency increases.) Proper adjustment of LI will make the
output voltage vary linearly with input frequency over i8 kHz from 455 kHz.
At the
output of U3A, a low-pass filter, L3 and Cll, reduces higher frequency noise
components which are present in the discriminator output. When the CW mode or any
of the sideband modes is selected, the control input on pin 43 is high (+5 V).
This
turns on Q4 and Q3, applying +9 V to balanced modulator U2. The BFO is also applied
to U2 (approximately a 40 mV level).
This allows U2 to act as the 4th mixer in the
signal path as described in the Synthesizer Relationships section. Its action may be
considered to down-convert IF signals to the audio frequency range.
For sideband
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signals, proper tuning of the receiver places the center of the IF signal at the
frequency corresponding to the carrier frequency of the received signal. This causes
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the audio components out of U2 to reconstruct those of the original signal
transmitted.
For CW signals, the BFO is offset from the signal either by use of the
BFO offset control on the front panel to cause an audible tone at the audio output
when a signal is present.
The output of U2 goes through low-pass filter L2 and C17, which reject
higher frequency noise components, to buffer U3B. OP AMP U3C acts as a summing
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amplifier for the outputs of the FM discriminator or product detector when either is
present.
It gives different amplifications to these two signals to bring them up to
approximately equal levels.
The audio output of this module goes to the Audio
Amplifier.
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3.4.17
AUDIO AMPLIFIER (A4A10)
The Audio Amplifier mounts on the IF Motherboard, A4, (see paragraph
3. 3. 4).Figure 3-23 is a detailed functional block diagram of the Audio Amplifier which
should be referred to in the following circuit description. Figure 6-21, Audio Amplifier
Schematic Diagram, may be referred to for greater component level detail, if desired.
The Audio Amplifier combines the audio outputs of the AM detector and
FM/CW/SSB Detector and feeds them to the LINE AUDIO LEVEL control on the front
panel and the PHONE LEVEL control on the front panel. The signal returned from the
wiper of the LINE AUDIO LEVEL potentiometer drives the line audio amplifier
The
signal returned from the PHONE LEVEL control drives the auxiliary phone amplifier
which feeds the PHONE AUDIO terminals on rear panel J16. A rectifier which samples
lu ?^£Ut ° the line aud,° amPlifiers supplies dc to operate the front panel meter in
the LINE AUDIO setting.
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3-43
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