60/80 metre 10 Watt AM Transmitter
Because of the Covid19 "lockdown" in Britain and having plenty of time on my hands I decided at the end of March 2020 to build an AM transmitter to run about 10 watts output on the 60 and 80 metre bands, this has just been completed at the end of April 2020. The transmitter is made up using mostly parts that were to hand so it is larger than it would normally be. The info on here is purely out of interest with no suggestion that anyone copy it as it can also be described as "utility". Tests so far have show that it is working quite well.
Because of the Covid19 "lockdown" in Britain and having plenty of time on my hands I decided at the end of March 2020 to build an AM transmitter to run about 10 watts output on the 60 and 80 metre bands, this has just been completed at the end of April 2020. The transmitter is made up using mostly parts that were to hand so it is larger than it would normally be. The info on here is purely out of interest with no suggestion that anyone copy it as it can also be described as "utility". Tests so far have show that it is working quite well.
Basic Specification
13.8v Operation.
10 Watts RF Output (40W PEP)
All Spurious better than -45dB.
Frequency Coverage 5.30 to 5.40 and 3.61 to 3.63Mc/s.
VFO Drift better than 100c/s over 3 hours (Ambient)
Illuminated VFO dial.
Digital Frequency Readout.
Bar Graph Modulation Percentage Indicator.
Oscilloscope Monitor Socket.
Switchable Audio Monitor from station Receiver.
Adjustable Audio Monitor detected from Aerial output.
Microphone Input Sensitivity 5mV P to P for 100% Modulation.
Switchable Bandwidth, 9 or 6Kc/s.
Receiver Muting Relays.
2 x 13.8v Auxiliary DC Outputs.
Front Panel
Rear Panel
Close up showing sockets etc
As can be seen the case is covered in square holes as this was originally to contain a large dummy load, this did however make the metal work far easier. The internal view can be seen below prior to fitting the modulator output stage.
Internal View
Probably the most important part of an AM transmitter is the Modulator and its associated filtering. Below is the circuit of the microphone pre-amp which provides about 48dB gain and audio filtering. The bandwidth is switchable between 6 and 9Kc/s (RL1) with a rapid roll off above which can be seen on the graph. The output from this stage proceeds to the Modulator output device which is a TDA2050 IC which provides well in excess of that needed to give 100% modulation.
Circuit Diagram (Mic Pre-Amp/Filter)
Circuit Diagram (Modulator)
Parts List ( Pre-Amp, Filter and Modulator)
R1 2K2 (Electret Bias)
R2 22K
R3 8K2
R4 4K7
R5 1K
R6 1K
R7 22K
R8 680R
R9 22K
R10 2R2
VR1 5K Pot
C1 0.47mfd Poly
C2 0.003mfd Disc Ceramic (RF De-Coupling)
C3 10mfd Electrolytic
C4 0.2mfd Poly
C5 0.001mfd Disc Ceramic
C6 0.02mfd Poly
C7 0.027mfd Poly
C8 0.027mfd Poly
C9 0.02mfd Poly
C10 0.01mfd Disc Ceramic
C11 0.01mfd Disc Ceramic
C12 220mfd Electrolytic (LF De-Coupling)
C13 1mfd Electrolytic
C14 22mfd Electrolytic
C15 0.47mfd Disc Ceramic
C16 1000mfd Electrolytic
C17 0.01mfd Disc Ceramic
TR1 BC239C Transistor or equiv
IC1 TDA2050 IC
IC2 Bar Graph Module ( see below)
L1 50mH Pot Cored Variable Inductor. (Other values can be used but the capacitor values of C6 to C9 must of course be changed accordingly)
T1 Modulation Transformer, open to experimentation here, I am using a Yaesu FT-220 Mains Transformer wired as shown in the circuit.
RL1 Min Double Pole Relay
Mic Hosiden Electret Insert or Similar
Frequency Response Graph showing the 6 and 9Kc/s Bandwidth Positions
Modulation Indicator
This is a very useful modulation percentage indicator which I bought from China (E Bay) which is very cheap and performs a very useful function, the final LED (Red) is adjusted with the small pre-set pot on the module to illuminate when the modulation level exceeds 100% and the yellow LED shows 100%, this of course is set up using a scope monitoring the TX output envelope.
This is a very useful modulation percentage indicator which I bought from China (E Bay) which is very cheap and performs a very useful function, the final LED (Red) is adjusted with the small pre-set pot on the module to illuminate when the modulation level exceeds 100% and the yellow LED shows 100%, this of course is set up using a scope monitoring the TX output envelope.
VFO and Power Amplifier
I will not be including the circuit information for these as they are open to experimentation by the builder. If a VFO is not needed a simple Crystal oscillator stage can be made, the PA section has numerous possibilities, lots of information on the net or a suitable pcb with all components is available from China for about £25.
I used the VFO assembly from Kenwood TS-830 which is silky smooth and it was modified to dramatically reduce its coverage from 500Kc/s to about 20Kc/s to cover the sections of the 60 and 80m bands required meaning that it is very easy to set to the required frequency. The digital frequency display is a very useful addition and again bought from E Bay for about £8.
I will not be including the circuit information for these as they are open to experimentation by the builder. If a VFO is not needed a simple Crystal oscillator stage can be made, the PA section has numerous possibilities, lots of information on the net or a suitable pcb with all components is available from China for about £25.
I used the VFO assembly from Kenwood TS-830 which is silky smooth and it was modified to dramatically reduce its coverage from 500Kc/s to about 20Kc/s to cover the sections of the 60 and 80m bands required meaning that it is very easy to set to the required frequency. The digital frequency display is a very useful addition and again bought from E Bay for about £8.
