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Inside The Drake TR-7
by: Ronald Baker / WB4HFN
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The Power Amplifier
|The Power Amplifier Board consists
of two separate circuit boards mounted to a large heat sink.
The Pre-driver Board on the left side of the picture
amplifies the transmit signal to a sufficient level to drive
the power amplifier board. The output drive level is
controlled from the on-board drive level control.
This board operated at 13.8VDC and is switched on during the
transmit mode through the relay mounted on the Low-Pass
amplifier board, right side in picture, has a pair of driver
and final amplifier transistors. Each transistor pair
operates in push-pull to provide a good balance for proper
circuit gain and flatness across all frequencies from 1.8 to
30Mhz. The 13.8VDC supply voltage for the
connected directly to primary power and is not switched.
The bias circuit, transistors across the bottom of the board
in the picture, provide sufficient bias to to hold the
amplifier transistors in cut-off during receive mode.
The Power Amplifier Board takes a very low level signal,
typically between 100 and 200 millivolts, from the
radio and amplifies that to well over 100 watts of output
The ALC circuit, located on the High-Pass Filter Board controls the
overall gain of the Pre-driver Board to limit the output
power to around 100-120 watts output.
Even though this amplifier is capable of producing power
levels greatly exceeding 100 watts, its not recommended.
Running higher power levels over time will significantly
shorten the life of the transistors. Typical
output power levels of the transceiver are 110-125 watts on
all bands below 15 meters, and 80 to 100 watts on all bands
15 meters and above.
High-Pass Filter Boards
|The High-Pass Filter Board consists
of three separated boards. The front and
rear boards form seven different pass-band filter circuits
which are switched into the signal path with the Band Switch.
There is a separate filter for each of the major band
segments except for the two lowest bands which are switched
In the receive mode this filter board connects the antenna
to the Up-Converter Board and controls the pin-diode signal
switching for the 25Khz Calibrator signal insertion.
In the transmit mode the signal from the Up-Converter Board
is pin-diode signal switched to the Power Amplifier Module.
Low-Pass Filter Boards
The Low-Pass Filter is a 4
board configuration, T/R Relay on the left.
The Low-Pass Filter output
side showing the Watt Meter adjustments on the left.
|The Low-Pass Filter Board consists
of four separated boards. The front and rear
boards are switching boards ganged to the Band Switch and
selects one of eight filters and routes the signal through
the transmit/receive (T/R) relay to the watt meter circuit
and antenna connector. The T/R relay provides
antenna switching, linear amplifier key switching and
external receiver muting. The other two
boards form the eight individual low-pass filters which are
switched into the signal path by the front and rear switch
The ALC Board
|The ALC Board (Automatic Level
Control) takes inputs from the forward and reflected
wattmeter and the linear amplifier and provides an output
that controls the variable attenuator on the 2nd IF/Audio
Board. The purpose of this board is to control the
drive level of the Power Amplifier to prevent signal
flat-topping and provide high VSWR protection caused by an
antenna mismatch. This board also lights the green
front panel LED when the ALC circuit is operating.