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C.RCU.T DESCRIPTION
WJ-8718-19/FE HF RECEIVER
3.3.4.3.
455 kHz Filter Switch (A4A3) (791595)
The 3rd IF output from A4A2-19 drives the high-impedance input of the
455 kHz Filter Switch, A4A3-13. When AM, FM or CW Mode is selected, the signal is
routed through one of three voltage-selectable circuit paths within A4A3.
The
wideband path, selected by +5 Vdc of pin 15, passes the full bandwidth, determined by
A3 and A4A1, through A4A3.
The other two circuit paths, selected by +5 Vdc at
pin 17 or 19, narrow the bandpass from that determined by A3 and A4A1 down to
1.0 kHz or 0.3 kHz respectively (see Table 3-1 for summary of 2nd and 3rd IF
bandwidth selection).
The amplified, band-limited output from A4A3-57 drives the
high-impedance input of the 455 kHz IF Amplifier/Demodulator, A4A7-57.
3.3.4.4
AGC (A4A6) (796175)
The AM Audio output from A4A7-51 drives the input of the AGC,
A4A6-51.
In AGC mode, selected by 0 Vdc at pin 13, the detected DC level from
A4A7-51 is amplified to become the IF GC Voltage at A4A6-47.
This voltage adjusts
the gain of the IF Amplifier, A4A7. Under strong input signal conditions, a gate
circuit transfers the IF GC voltage to pin 19, RF GC, reducing Input Converter gain.
A4A6-41, AGC Signal Strength, is a DC sample of the IF GC voltage that drives the
front panel meter in Signal Strength Mode.
In MAN mode, selected by +5 Vdc at pin
13, the IF and RF GC outputs respond to the MAN GAIN, A4A6-17, signal from the
front panel RF GAIN control.
A4A6-12, MAN Signal Strength, is a DC sample of the
IF GC voltage that drives the front panel meter in the Signal Strength Mode.
3.3.4.5
455 kHz Amplifier/AM Detector (A4A7) (726002-2)
The amplified 3rd IF output from A4A3-57, A4A4-57 or A4A5-57 drives
the high impedance input of the 455 kHz IF Ampl/Detector, A4A7-57.
The signal is
amplified by a two- stage gain-controlled (for AGC purposes) amplifier. The amplifier
is untuned to give the required 50 kHz bandpass characteristic.
Following this, the IF
signal is split to provide three outputs:
the input to the AM Detector, the third IF
output at A4A7-13 which drives the input to the FM/CW/SSB Detector and the 3rd IF
output at A4A7-17 which drives the rear panel IF Output jack J2.
The AM Audio
output from the AM Detector at A4A7-51 contains a DC level proportional to RF Input
signal strength and audio resulting from modulation detection by the AM Detector.
This AM Audio output drives the parallel-connected inputs of the AGC, A4A6-51, and
the Audio Amplifier, A4A10-51.
3.3.4.6
FM/CW/SSB Detector (A4A9) (791599-4)
The amplified 3rd IF output from A4A7-13 drives the high-impedance
input of the FM/CW/SSB Detector, A4A9-13.
In the FM Mode, selected by +5 Vdc at
pin 41 (see Table 3-2 for a summary of detection mode selection), the signal is
amplified, limited and drives an FM discriminator.
The audio from the discriminator as
FM Audio at A4A9-57.
In the CW or any of the sideband modes, selected by +5 Vdc at
pin 43 (see Table 3-2), the signal is applied to a product detector.
The BFO
Synthesizer is also enabled and supplies, a fixed (SSB mode) or variable (CW mode)
55 kHz BFO signal to A4A9-17. This BFO signal mixes with the 3rd IF signal in the
product detector.
The audio from the product detector appears as CW/SSB Audio at
A4A9-57.
Audio from A4A9-57 drives the input of the Audio Amplifier, A4A10-57.
3-16
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