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  • Writer's pictureDan Koellen AI6XG

QRP Tube: T/R Switch and QCX Interface and Modification

Updated: Apr 2

A QCX is used as the companion receiver for the Tube Transmitter, the final amplifier for the QCX transmitter was not populated. The QCX will be interfaced to the Tube Transmitter through a transmit/receive switch (T/R switch). The T/R switch will change the antenna between the receiver and transmitter and also interface the keying to the transmitter. The construction of the T/R switch, QCX modifications and interface are discussed.

This post is part of the series about a backpackable vacuum tube based transmitter teamed with a QCX receiver for portable SOTA activation. Other posts are the overview, concept and results, and detailed descriptions of the tube transmitter and high voltage supply evaluation.

QCX Modification

A QCX CW 40 meter transceiver was built per the assembly instructions except for the transmitter final power amplifier stage and the key shaping circuitry. Figures 1 and 2 are from the assembly manual for the QCX.

Figure 1 QCX Power Amplifier and Key Shaping

Transistors Q1, Q2 and Q3 - all BS710 transistors in the final power amplifier - were not populated during assembly. In addition, L4 - the power amplifier Vcc coil - was not assembled onto the board. This made the transmitter portion of the QCX nonfunctional. The low pass filter was built and used as part of the receiver portion of the QCX.

In the Key Shaping circuit, Q6 and R42 were not installed. The keying signal, from the QCX keyer, is tapped at the drain of Q4.

The T/R switch is built on a separate board and has the LCD display on that board. When assembling the QCX use a high profile female header socket at the LCD position, the T/R switch board is plugged into this socket. The LCD display may be plugged into the female headers during testing of the QCX separate from the TR switch. Instead of the supplied backlit LCD display, a non-backlit LCD display is used for reduced current draw and easy reading in sunlight.

Figure 2 QCX RX and TX Signals

The T/R switch uses the TX and RX logic signal from the QCX to change the state of the antenna relay. The microcontroller in the QCX generates the RX signal, TX is derived from RX through inverter IC3D. TX is tapped from IC3D pin 11 or IC3B pin 5. RX is tapped from IC3D pins 12 or 13.

TX, RX and KEY_OUT are routed to the LCD socket where four of the pins are not used. The remaining unused LCD pin is used to route the QCX antenna connection. A piece of RG316 coax is used for routing to the antenna connection, this routes the receive signal to the QCX. The interface through the LCD socket delivers the signals to the T/R switch.

Another modification is substituting a barrel connector for DC power, instead of the terminal strip. For portable operation a barrel connector is more appropriate than a screw terminal strip. A standard 2.1mm/5.5mm connector was used. To solder in place, wire leads are inserted into the pads for the terminal strip. The barrel connector was soldered to the wire leads for electrical and mechanical connection.

QCX Board Modifications

T/R Switch

Antenna Changeover

The T/R switch connects the antenna to either the transmitter or the receiver, depending on the mode of operation. A latching relay, to reduce current consumption, is used to switch the antenna. In addition, the board makes available the QCX keyer output for use with the transmitter and houses the LCD display. The T/R switch board plugs into the LCD socket on the QCX board and is also secured with m3 nylon screws.

Figure 3 QCX Key_out, TX and RX Timing

The TX and RX signals from the QCX are used to switch the latching relay to connect the antenna to either the receiver or the transmitter. Figure 3 is a diagram of the KEY_OUT, RX and TX signals from the QCX assembly manual. In the QCX there is a key shaping circuit that adds a 5 mSec delay to the rise and fall times to eliminate clicking. The QCX microcontroller adds a 10 mSec delay to eliminate clicks and thumps in the receive audio. We do not use the QCX key shaping but we are still burdened with the 10 mSec delay in the TX and RX signals should full break-in be desired.

A Panasonic ATX229SA TX2SA-L2-5 latching relay is used. This relay has two coils, one to set the relay and one to reset the relay. A 4 mSec pulse is applied to the appropriate coil to set or reset the relay.

Figure 4 Pulse Generator for Relay Switching

The pulse is generated from the TX and RX signals using a classic 1970's era 555 pulse generation circuit adapted from the Third Edition of The Art of Electronics by Horowitz and Hill. The circuit, shown in Figure 4, supplies a 4 mSec pulse to the NPN transistor. When the transistor turns on, it allows current to flow through the relay coil, switching the relay. The transistor may be any general purpose NPN transistor that is able to pass the coil current of about 40 mAmps. Capacitor C1 and resistor R2 form a differentiator so that a negative going pulse edge, applied to C1, produces a low going, short duration pulse to trigger the 555. The 555 will then produce, at its pin 3 output, a positive going pulse of duration determined by C3 and R4. The CMOS version of the 555 is used in this circuit to reduce current loading of the 5 volt supply, the 5 volt regulator will get too hot with the bipolar version. Diode D1 is added to prevent the CMOS ICM555 from latching by the positive going pulse, that exceeds +5 Vdc, that is generated when the signal applied to C1 returns to a high level.

The pulse generating circuit requires a negative going edge to trigger the 555. But the TX and RX signals from the QCX are positive going for initiating transmit and receive respectively. To set the T/R switch to transmit, the RX signal is used as an /TX signal. Likewise, to reset the T/R switch to receive, the TX signal is used as an /RX signal. For example, the circuit in Figure 4 is to reset the relay to the receive state. The TX signal from the QCX is used to trigger the this circuit and is renamed /RX.

The relay switches the antenna between the receiver and transmitter, the normal state for the relay is in the receive mode. The second relay pole grounds the receiver input and enables a LED during transmit.

The timing diagram in Figure 3 is for full break-in operation. The relay will follow the changing mode and switch quite often in full break-in mode. I prefer to operate in semi break-in mode with a second benefit of much less relay switching. This will increase the operating life of the relay and is much less annoying during operation. Full break-in is better implemented with all electronic switching.

Keyer Output

The keyer function of the QCX is used for keying the transmitter. The keyer output from the microcontroller, KEY_OUT, goes to the gate of Q4 as shown in figure 1. Transistor Q4 inverts the microcontroller keyer output, the inverted KEY_OUT is tapped at the drain of Q4 and routed to a jack on the T/R switch board. This signal, /key, is used to key the transmitter. An opto-isolated solid state relay, in the transmitter, cathode keys the transmitter using the /key signal.


The LCD for the QCX is mounted on the T/R switch board. The LCD socket on the QCX is used to connect the T/R switch board to the QCX. A high profile female header socket is used on the QCX, at its LCD socket, to receive the T/R switch board. The LCD itself is mounted on the T/R switch board, the board is layed out so that the display connections may be at the top or bottom of the LCD module.

Figure 5 T/R Switch Board to QCX Connection

Through the connection between the QCX and the T/R switch board, figure 5, are routed the signals used to switch the relay state (/RX_TX and /TX_RX), the keyer output (/key), and the connection to the QCX receiver input (RX_Sig). These connections utilize the unused LCD data pins (DB0, DB1, DB2, DB3). The QCX is modified so that these signals are available at the appropriate socket pins.

Power for the relay and circuit is taken from the QCX 5 Vdc power supply that is available on the QCX LCD socket.

Final Configuration

The schematic of the T/R switch board is shown in figure 6. The board mounted to the QCX is shown in figure 7. The jack in the board's upper left is keyer output to be connected to the keyer input of the transmitter. The SMA jack in the top center of the board is connected to the output of the transmitter. The BNC jack in the top center of the board is the antenna connection.

Gerber and other files available on GitHub

Figure 6 T/R Switch Board Schematic

Figure 7 T/R Switch Board Mounted on QCX


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