60/80 metre 6 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 6 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.
6 Watts RF Output (24W PEP)
All Spurious better than -50dB.
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.
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                                                                                   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

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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)
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​                                        Circuit Diagram (Modulator)

                                                       Parts List ( Pre-Amp, Filter and Modulator)

R1      2K2   (Electret Bias)
R2      47K
R3      8K2
R4      4K7
R5      1K
​R6      1K
R7      22K
R8      680R
R9      22K
R10    2R2

VR1    10K 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, you may also need to fit a 15K resistor across it to recuce the "Q" and prevent ringing.)

T1         Modulation Transformer, open to experimentation here, I am using a Yaesu FT-220 Mains Transformer wired as shown in the circuit. UPDATE Aug 2020, I am now using choke modulation as this was found to produce a much cleaner sine wave at 100% modulation.

RL1       Min Double Pole Relay

Mic      An Electret Insert can be used directly as the bias is presnt on the mic socket but a good quality Dynamic or Crystal microphone  is preferred with of course the required dc blocking capacitor or in the case of a Crystal microphone a suitable high impedance matching transformer.

                                 
                            Frequency Response Graph showing 4Kc/s Bandwidth 
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                                                                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.
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                                    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. 

UPDATE July 2020

I have just finished building a crystal controlled 80m AM receiver to go with this transmitter which is oversized but fits snugly beneath the transmitter...........

I have now changed to choke modulation which presents a much cleaner waveform over the full audio range, this is the envelope taken from the transmitter output using a 1Kc/s tone showing 100% modulation.

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