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WJ-8718-19/FE HF RECEIVER
CIRCUIT DESCRIPTION
When the AM detection mode is selected, the control input to pin 47 is
high (+5 V). The output of U1A is +14 V, which reverse biases CR1.
The gate of FET
Ql will then assume the same potential as its source and Ql will be on, acting as a
closed switch for AM audio.
Both demodulators of the FM/CW/SSB Detector will be
off so the output of UID will be AM audio only.
When any other detection mode is
selected, the control input to pin 47 will be low (0 V) and the output of U1A will be
approximately -14 V. This will tend to forward bias CR1 and will cause gate of Ql to
be similarly negative, cutting off all signal flow through Ql.
The audio signal from
the FM/CW/SSB Detector will appear at the output of UID.
The signal into line audio amplifier U2 is the output of UID attenuated by
the LINE AUDIO LEVEL control, Rl. The two sections of U2 act as a push-pull bridge
amplifier, driving output transformer T2 located on the inside of the rear panel. The
amplifier U2 uses a supply voltage of +24 Vdc from A1-J35.
Transistor Q2 is biased as
a current limiter to protect U2 in case of short circuit on any of U2's outputs.
A
circuit within U2 provides a bias voltage at pin 1 which is equal to one-half the supply
voltage.
This is connected to the non-inverting inputs of both amplifier sections of
U2.
Both amplifiers use unity feedback at dc, that is, the only dc path to the
inverting inputs is from the outputs, so there is very little dc difference between their
outputs at pins 2 and 13.
The input signal is applied to the non-inverting input of U2B, pin 9.
Although pins 6 and 9 are at the same dc potential, pin 6 is bypassed so no ac signal
appears there. The operation of amplifier U2B will be clear if pin 7, the inverting
input of U2A, is considered to be at ac ground.
With this assumption, U2B simply
appears as a non-inverting amplifier with a closed-loop ac gain of 50. Its ac gain is
determined by the ratio of feedback resistors R20 and R19.
On the other hand, U2A
may then be viewed as an inverting amplifier with an ac gain of nearly one. Its 'input
is the full output of U2B and its gain is determined by R20 and R19 acting as input
resistors and R21 as feedback resistor.
As with inverting OP AMPs, extremely little
signal voltage appears at the amplifier inverting input terminal, thus satisfying the
assumption made to explain the behavior of U2B.
The net gain of the combined
amplifier is 100 and its outputs are balanced with respect to ground. Due to the high
current U2 can pass, it is grounded separately from the other circuits on the Audio
Amplifier module to prevent ground current coupling which might lead to instability
and parasitic oscillations.
The output signal of U2A is rectified and filtered to indicate LINE AUDIO
level on the panel meter. The rectifier is a voltage doubler consisting of CR2, CR3,
C12, and C13. It responds to peak-to-peak input voltage and is calibrated by resistors
R22, R23, and R24 to indicate the RMS value of a sine-wave at the LINE AUDIO
terminals of J16 on the rear panel. Its calibration is therefore most accurate for
sine-wave voltages.
The auxiliary phone amplifier U1B and U1C is a low power bridge amplifier
and is therefore similar to U2. It operates from both +15 V and -15 V supplies and
has its inputs biased at ground.
Comparing its circuit with that of U2 it should be
apparent that it also uses unity dc feedback and has a closed loop gain of 100 for ac
signals. The phone amplifier output is transformer coupled to the rear panel terminal
J16 . Output level is 100 mW nominal across 600 ohms.
3-45
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