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Inside the SB-104(A) Transceiver

Written By: Ron Baker / WB4HFN

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The HFO/Premix Board

The HFO/Premixer Board (High Frequency Oscillator) contains a high frequency crystal oscillator for each band of operation.   The high frequency signal is mixed with the VFO signal producing the premix signal.   The premix signal then passes through another band selectable pass band filter to produce a clean signal which is then amplified and sent to the receiver and transmit IF boards.    Each band crystal oscillator and the selectable pass band filter are diode switched with a selectable control voltage from the band switch.

Transmit Audio/Regulator Board

The Transmit/Regulator Board provides the transmitter audio and two voltage regulator circuits to power the radio.    The transmit audio section takes microphone level audio and amplifies and processes the signal which then feeds the high level audio to the transmit balanced modulator.    This board also contains the transmit VOX, Anti-VOX and transmit delay circuits.

There are two voltage regulators on this board, +5VDC and +11VDC.   Each regulator has a separate IC regulator that feeds the chassis mounted pass transistor.    The +5 volt regulator is supply voltage for all the Integrated Circuits (IC's), and the +11 volt circuit feeds other circuits requiring high stability voltage regulation such as the oscillator circuits.

The Transmit IF Board

The Transmit IF Board amplifies the 3.395Mhz IF signal, then mixes the IF signal with the premix signal from the HFO board to produce the desired output frequency.   This output signal then passed through two stages of amplification with the output going to the Transmit Driver board.    The last stages of amplification has band selectable pass band filters on the input and output of the amplifier.    This ensures a clean transmit signal and suppresses distortions and harmonics produced in the circuits.   Each of the pass band filters is diode switched with a switched voltage from the front panel band switch.

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