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Stage B: Receiver Audio Amplifier
The secret to the fine performance of the LM380 audio amplifier lies in
careful selection and physical positioning of the several external components
required to complete its circuit. The LM380 is a self contained general
purpose audio amplifier capable of over 2 watts audio output with a voltage
gain of 50. Audio from the FM discriminator (U1) is fed through C7 through
the 10K volume control (R7) to pin 2, the amplifier input. The amplified
output at pin 8 is available through C34 to both the speaker jack and pin 4 of
the Packet I/O Jack. Capacitor C41 in series with R108 across this amplified
output are recommended good practice to prevent self-oscillation of the
amplifier IC.
Pin 1 is bypassed to ground through C48 in normal operation. If pin 1 is
grounded directly, the internal bias of the LM380 is upset, and the amplifier
is silenced. We are able to put this characteristic to practical use, using Q6
as a simple switch. When 8 volts is applied through R107 and D22 to the
base of Q6, the transistor collector grounds pin 1 of U2, thus silencing the
receiver during transmit. The COR output of U1 (pin 16) also mutes the
amplifier, a feature which we'll discuss in the context of Stage CR.
LM380 Audio Amplifier IC
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STAGE A: DC POWER INPUT REGULATION & DISTRIBUTION and
PACKET RADIO I/O CONNECTOR (J1)
The "power supply" for your FX- transceive r is, basically, any "12-volt"
battery or well designed power supply operating from 120VAC or other
source. In theory, all "12V" sources should provide pure DC voltage to your
FX- transceiver DC input. In fact, there are many variations and
imperfections in common "12VDC" sources, ranging from weak batteries,
poorly filtered AC powered supplies, vehicle ignition noise, or just badly
made power supplies.
Stage "A" of your FX- Transceiver is designed to help your radio survive
quite a wide variety of imperfections. It's not a "power supply" in itself, but it
comes close, because it performs all the essential functions expected from a
good solid-state DC source. Just add 12-15VDC from a battery, vehicle or
bench supply.
Much of the circuitry operates on the regulated 8 volts supplied by voltage
regulator VR1. If you have already looked around the schematic diagram
and also seen "+8R" or "+8T", these are points where the regulated 8V
output is switched for Receive or Transmit by the PTT (push to talk) circuitry
(Q12, U4c, U4d, Q13, Q14) which we'll discuss in more detail when it's time
to build it in Stage M.
The Receiver IC (U1) and the digital frequency synthesis circuit is powered
by +5 volts regulated by VR2. The op amps used in the circuit (U4 and U5)
operate from this single supply through the use of voltage divider networks
at the respective IC's. The full 12-15 volt input is supplied to the transmitter
RF output section and to the receiver audio amplifier (U2).
The large 1500 uf capacitor (C42) and inductor L20 are installed at the DC
input to filter out ignition noise, etc. Fuse F1 is contained in the power cord
and the DC power switch is integral to the volume control. The DC negative
(black) wire is soldered directly to the PC board ground plane.
The Packet Connector
The Packet I/O Jack (J1) is not a "stage" or section of your transceiver in
itself. It is simply a convenient "port," to use computer terminology, which
gives a packet TNC (terminal node controller) convenient access to the
microphone and receiver audio circuits. We'll install J1 and its associated
bypass capacitors at the beginning -- because now is as good a time as any,
and it gets some parts on your PC board quickly and easily! Also, having J1
in place will give a little extra protection to other parts when you are working
on the solder-side of the board.