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Inside the SB-104(A) Transceiver

Written By: Ron Baker / WB4HFN

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Wiring the SB-104(A) Power Connector

This diagram details the power connector wiring.   The connector is a female 11 pin octal socket.     Pins 1, 2, 3, 4, and 11 are wired to the positive polarity, and pins 7, 8, 9, and 10 are wired to the negative polarity.  AC power is switched using pins 5 and 6.    The transceiver requires +13.8 Volts DC at 20 amps.

The SB-104(A) Rear Panel Connections

The diagram above shows how the SB-104(A) transceiver is wired to the accessories.    Don't forget to add the jumper in the "Accessory" plug between pins 2 and 5.  This jumper in necessary for the antenna relay to key in the high power mode.

When troubleshooting the RF Power Amplifier the first thing to check is the bias voltage on the base of the output transistors.   With the transceiver in the LSB or USB mode, High Power Mode, key the transmitter with the microphone switch.    If the power output transistors shows a +.65 volts the transistors are probably good.   If you measure more than .65 volts like 1.2 volts or a near zero voltage the transistors are bad and need to be replaced.   Its best to replace all 4 output transistors at the same time to avoid mismatch transistors.

Common Problems Found With The SB-104(A) Transceiver

Birdies:  One of the common problems and complaints with the SB104(A) are the birdies you find as you tune across the band with the antenna disconnected.  These birdies are signals generated from within the radio.    The most significant contributor of birdies comes from either the HFO/Premixer board or the Counter board.    The birdies are an inherent characteristic on the radio.    To minimize the level of the birdies make sure the boards and seated properly, well grounded and the shields are properly installed.    Another contributor of birdies comes from the VFO.   The VFO has an internal output level adjustment.   With the output level set to high, birdies normally fairly weak are strong.   There is a fine line when adjusting the VFO output level,  too much level results in strong birdies,  the level turned to low results in poor receiver sensitivity.   So properly adjusting the VFO output level is a critical setting.   

Loose Circuit Boards:     One very common problem is loose or misaligned circuit boards.    This can cause a wide variety of problems from a dead radio, to no receive or transmit, and all sorts of intermittent problems.   If you suspect this to be the problem, pull each board completely from the socket and firmly reseat the board by pushing down firmly on the board.    Do this to every board in the radio, don't forget to turn off the power before removing the boards.  If the contacts are dirty, use a small amount of contact cleaner and degreaser to clean those contacts,  just use sparingly and let dry before applying power.

VFO Output Level Set:  The VFO has an internal output level adjustment.   With the output level set to high birdies normally fairly weak are much stronger.   There is a fine line when adjusting the VFO output level,  too much level results in strong birdies,  the level turned to low results in poor receiver sensitivity.   So properly adjusting the VFO output level is a critical setting.   

Front End Boards:   There were two versions of the receiver Front End board.  The early version came unassembled.   There were several problems with the board design which produced several problems including poor sensitivity.   Typical receiver sensitivity was around 1uV for a detectable signal.   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 later version on the board typically had .25uV sensitivity for detectable signal.

RF Power Output:    Typically the transceiver output power is around 100 watts.   However, any power level between 80 and 120 watts on any band was considered acceptable by Heathkit.  There were several factors that determined the actual power output.  There were a few design changes that affected the power but mostly how well the kit was built, tightness of the connections and grounding, were the main issues related to power output.

 

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