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Inside the SB-104(A) Transceiver

Written By: Ron Baker / WB4HFN

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The Counter Board

The function of the Counter Board is to measure the premix and VFO signals and sends that measured frequency to the Display Board, which is the operating frequency of the radio.    The Counter Board uses a 1Mhz time base which is derived from an on-board 10Mhz oscillator.     One of the inherent problem with the SB-104(A) are the strong birdie signals you hear as you tune across the any of the bands with the antenna disconnected.    The birdies are actually internal frequencies being generated within the radio.   The Counter Board and HFO Pre-Mix Boards are the primary source of those signals.

There is a very common problem found with the Counter Board when the display shows 6.606Mhz and doesn't change when rotating the tuning knob.    In most cases the radio tunes and operates properly, the display just doesn't change.    There are two possible causes.  The Counter Board has lost the input signal at connector J101, typically due to a loose or bad connection to the board.   Also the Counter Board may not be making a solid connection to the mother board and the board needs to be reseated.   

The Display Board

The Frequency Display Board uses three display tubes to show the transceiver operating frequency.    Each display tube consists of a dual 8 segment number display to provide a 6 digit frequency display.    Each display tube requires 180VDC on the anode to operate.   Each segment in the display is switched on and off by the driver IC's mounted on the back of the board.   The high voltage for the display board is generated from the Converter Board.     This is the only board in the transceiver that has high voltage on it. 

The Converter Board

The Converter Board consists of a 25Khz oscillator which switches +5VDC at the 25Khz rate through a small transformer that steps-up the voltage on the output.   Once the output is rectified and filtered it produces 180 volts DC used for the operating voltage for the frequency display board.    The circuit board is enclosed in a shielded box to prevent the switching noise from interfering with the operation of the radio.

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