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 Post subject: Vectronics 1290K Revisited with pushpull 2n7000
PostPosted: Sep Sat 18, 2021 10:23 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 736
Many users had instability issues. It's removed by a 7812 regulator and even the power supply ripples are filtered out. Bit switches S1 and S2 along with trimmer cap set the TX frequency. Squares waves are alternatively driving two 2n7000 mosfets in pushpull configuration. The antenna trimmer capacitor needs to be set for maximum range with a 6 feet antenna. Pushpull output design should keep distortion low, primary of ferrite Rod can be 15T + 15T center tapped of 25 to 27AWG. The secondary winding generates high RF voltage required for the wire antenna, so it will have more turns. Try 200T of thin magnet enamelled wire on top of the primary coil. This design should have great performance and clean and crisp sounding audio. The oscillator inductor should be shielded for frequency stability. Careful antenna matching with the tuning capacitor should give high quality audio, negligible distortion, and a decent range covering your house and yard.

PS: Select value of R so that the DC input to 2n7000/bs170 output pair remains at <100mW with no audio signal present. it's about 15-16ma of DC current at 1/2 the VCC or 6V from LM386 AF modulator.


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 Post subject: Re: Vectronics 1290K Revisited with pushpull 2n7000
PostPosted: Sep Sun 19, 2021 9:45 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 736
Anyone willing to build it? :)


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 Post subject: Re: Vectronics 1290K Revisited with pushpull 2n7000
PostPosted: Sep Sun 19, 2021 8:29 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 736
With 5 crystals and eight DIP switches, we have eight different crystal controlled medium wave band frequencies. CD4069 oscillator driving a CD4040 divider which in turns feeds CD4013 flip flop with 2F signal to produce the required frequencies in 0 and 180 degrees phase shift to drive two 2n7000 or BS170 in pushpull mode and modulated by a LM386. Secondary winding of feritte rod is tuned by a variable capacitor and steps up the voltage to match the high impedance of a 6 foot wire antenna.

Your comments please? No VFO, No PLL but still has 8 different MW frequencies with no drift and easy to build! Four ICs and two mosfets.

Isn't it cool????


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Last edited by Dare4444 on Sep Sun 19, 2021 8:51 pm, edited 4 times in total.
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 Post subject: Re: Vectronics 1290K Revisited with pushpull 2n7000
PostPosted: Sep Sun 19, 2021 8:39 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 736
Will publish complete schematic soon. Comments welcome on the design. Neat little circuit without PLL and LC oscillator!


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 Post subject: Re: Vectronics 1290K Revisited with pushpull 2n7000
PostPosted: Sep Wed 22, 2021 5:13 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 736
Bit switches S1 to S8 covers eight different MW radio frequencies. Number of crystals required is five. A single 2n3904 colpitts oscillator should drive the divider IC CD4040. A ferrite rod with untuned primary and tuned secondary will decide the performance of this circuit.


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