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Inside the SB-104(A) Transceiver

Written By: Ron Baker / WB4HFN

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Receiver Front End - Early Version

Receiver Front End - Late Version

The receiver Front End Board establishes the receiver selectivity and mixes the incoming signal with the internally generated HFO and VFO signals to produce a 3.395Mhz output which is the IF frequency.   Each band RF input tuned circuit is diode switched to the 1st mixer using a switched voltage from the band switch.   The board includes two mixing circuits, the first mixes the incoming signal and the HFO frequency producing a signal in the 8Mhz range.  This signal is then applied to the 2nd mixer and mixed with the VFO signal.  The output of the 2nd mixer produces the IF signal at 3.395Mhz which is sent to the receiver IF/Audio board.

There were two versions of this board.  The early version came unassembled and was wired by the user.   There were several problems with the board design and if not assembled properly produced several problems including poor sensitivity.   The later version of this board came factory assembled and aligned.    The board was totally redesigned using a much simpler circuit while keeping the same functionality and diode circuit switching.   The newer design offered better stability and receiver sensitivity.

The Carrier Generator/ Crystal Filter Board

The Carrier Generator/Crystal Filter Board provides a highly selective and switchable crystal filters for the receiver.   It also generates separate BFO frequencies for LSB, USB and CW, and contains the transmitter balanced modulator circuit.    This board supports two selectable bandwidth crystal filters for the receiver.   The board comes with the SSB filter supporting 2.1Khz bandwidth and has room for the accessory 400Hz CW filter.    When both filters installed the SSB and CW filters are selected from the front panel "CW" mode and 400Hz switches.    The board also contains separate BFO crystal oscillators for the LSB, USB and CW modes.  The BFO signal is used to set the proper frequency off-set for the mode of operation.    The last circuit is the transmitter balanced modulator, where the output of the BFO oscillator is mixed with the transmit audio from the Transmit Audio/Regulator Board.   The output of the balanced modulator is the transmitter SSB signal which feeds the Transmitter IF Board.

The Receiver IF/Amplifier Board

The Receiver IF/Audio Board provides the 3.395Mhz IF signal amplification, gain control, detection and audio amplification.  The receiver IF signal from the Carrier Gen/Xtal Filter Board is applied to the input and amplified.  The amplifier output is applied the AGC circuit which in turn regulated the overall gain of the IF amplifier.    This results in a constant signal level applied to the detector and preamplifier stages.    The audio output from the preamplifier stage is applied to the audio amplifier through the front panel volume control.  The audio amplifier depending on the volume control setting is capable of producing up to 2.5 watts of audio into a 4 ohm speaker.

The Noise Blanker Board

The Noise Blanker was an add-on accessory to the SB-104(A) Transceiver.    The Noise Blanker connects to the receiver IF section just ahead of the Receiver crystal filter.   The noise blanker was designed to eliminate short duration noise spikes about 50us in duration, like from automobile ignition systems.    Other burst of noise such as those generated from atmospheric conditions, lightning storms, or power lines are typically to long in duration for the noise blanker to affect.

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