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Inside The Drake TR-7 Transceiver

by: Ronald Baker / WB4HFN

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"The Road Map"

One of the best ways to understand what is inside the TR-7 Transceiver is to first understand how the signals flow.   The transceiver operations is divided into four major functions, receiving, transmitting, frequency range synthesization, and fixed frequency generation.   Each of those major functions are described in detail just below the Signal Flow Chart     The Signal Flow Chart shows each function with separate colored lines and arrows showing the signal path.    The one board common to every board and function in the transceiver is the Power Supply Board.   This board provides all the proper voltages to the entire radio. 

Drake TR-7 Transceiver Signal Flow Chart

The most important board in the entire radio is the Pass-Band Tuning Board.   This board is considered the "heart-beat" of the radio because it generates several fixed frequencies and reference signal needed to operate the radio.   Those signals are indicated with the black arrow lines going from the Pass-Band Tuning Board.    This board generates the 40.0Mhz master oscillator frequency from which the entire operations of the radio depend on.   From that master oscillator frequency this board derives several other fixed frequencies which are supplied to several other boards.   

The receive function signal flow is shown with the blue arrow lines.   Starting at the antenna the receive signal flows through a high pass and low pass filter to limit the broad range of incoming frequencies to a narrower tuning range set by the band switch.   The signal then travels to the UP-Converter board where its mixed with the VCO frequency to produce the 1st IF frequency.  From there it travels to the 2nd Mixer Board where the signal is mixed with the injection signal from the Pass Band Tuning Board to produce the 2nd IF signal at 5.645Mhz.   From there the signal is sent to the IF Switch Board where in the receive mode is sent to the IF Filter board.     The Filter Board limits the bandwidth of the receive signal with selectable filters according to the selected mode of operation.    The output of the Filter Board goes the IF/Audio Board where the signal is AGC controlled, demodulated with the audio then amplified to drive the speaker.

The transmit function signal flow is shown with the red arrow lines.    Starting at the microphone or key, these devices are attached to the Transmit Exciter Board.    This board takes the audio from the microphone amplifies it and mixes that with the BFO frequency to create the 1st transmit IF frequency.   This board also controls the VOX functions of the transceiver.  Next the signal goes to the IF Switch Board where in the transmit mode that board switches it to the Filter Board where the signal only passes through the SSB filter to limit the signal bandwidth of the signal to 2.3Khz.    From there the signal passes to the IF/Audio board which contains a variable attenuator used to control the output power level of the transmitter.   The transmit signal then is sent through the 2nd Mixer board and the UP-converter Board to produce the operating transmit frequency.   Through each of these boards the input signal is mixed with a fixed frequency to create the operating frequency at the output of the Up-Converter board.    From there the low level transmit signal goes through the high pass filter board to eliminate all harmonics of the actual frequency.  The filtered signal then goes to the power amplifier where up to 150 watts of RF power output can be created depending on the drive level set by the variable attenuator.    The high power signal then goes through the low pass filter to further eliminate all harmonic frequencies generated by the power amplifier to produce a clean signal which is sent to the antenna connector.

The frequency synthesizer function produces the correct injection signal frequency to the Up-Converter Board, refer to the violet arrow lines for signal flow.  This injection signal is mixed with the incoming signal to the Up-Converter to produce a correctly tuned output frequency.    In the receive mode this frequency would be the 1st IF signal going to the 2nd Mixed Board, and in the transmit mode the output is the actual transmit frequency which goes to the High-Pass Filter Board and PA Amplifier.    The Synthesizer has three major interdependent boards.   The VCO Board produces the actual injection frequency from the data supplied from the other boards.    The Translator Board takes signal inputs from the PTO, Pass-Band Tuning Board, and VCO Board, mixed those signals together, then divides that signal in frequency to produce an varying output reference signal proportionate to the VCO frequency.   That varying reference signal is sent back to the VCO Board to the phase detector circuit.  The Phase Detector compares that varying reference signal to the 500Khz reference signal from the Pass-Band Tuning Board.   When the Phase Detector determines both frequencies exactly match the detector locks the VCO frequency.   When the VCO frequency is locked, that VCO frequency is set to the correct injection frequency to operate the transceiver.     In this process of producing a locked frequency the Digital Board determines the actual VCO frequency tuning range from data supplied from the band switch.    The Digital Display Board determines the actual operating frequency and displays that frequency at the front panel.      

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