Re: Vectronics 1290K AM TX modification with 4 transistors

by Densil Sun Oct 31, 2021 1:48 pm

Hi dare.
You have done a lot with transmitters and i c u have lots of posts. the am transmitters in this power range often used a 1:2 autotransformer at the output of the audio amplifier. this made the audio modulation twice as high and ran the pa at the full supply volts (12v). it also made the mudulation depth of almost 100% possible. another + was that the audio 1:1 autotransformer blocked a lot of rf from the pa to the af stages.

just a suggestion. mebe can help farther improve the tx?

/D

by dare4444 Tue Oct 12, 2021 10:28 pm

Vectronics 1290K AM TX modification with 4 transistors Vectro11

The output network

by dare4444 Tue Oct 12, 2021 10:26 pm

Trying to improve vectronics 1290 AM transmitter. It was a good transmitter with 85% modulation but had some design flaws. A 100n cap was directly connected to its pin 5 of LM386 Therefore heating it up. A simple 100uH RFC solves this problem. The oscillator here is colpitts. The 270p NPO cap sets the center frequency which can be fine tuned by 50p trimmer. The two feedback caps are 0.0033u Mylar. NP0 and Mylar together they minimize frequency drift by compensation. Set VR1 so that DC input to T4 is <100mW with no audio signal. A binocular ferrite transformer would have been a good option to drive low impedance base of the PA transistor. From old notes, I found a buffer circuit consisting of BC548/BC557. It can drive loads as low as 10ohm. It's nothing but a type of Darlington pain to boost the current. We already have enough volts from the oscillator. Voltage gain of this buffer is about 1. If you want to replace the pair with 2n3904/2n3906 then go ahead it can be done. So this Darlington transistor buffer eliminates the need of a ferrite matching transformer and winding coils! The base of the PA stage is grounded by a 100uH RFC to ensure class C operation and a resistor with a value 47 to 100ohm puts up a constant load for the buffer. Its value could vary anywhere between 22ohm to 1K. Experiment for best stability/range/100mW DC in and maximum clear and crisp audio modulation. A 7805 regulator provides 5V to osc and buffer for stable operation and it removes all power supply ripples as well. The collector of T4 goes to the Vectronics 1290 output matching network. I recommend 1.4 to 1.6 MHz operation for obtaining the maximum range. The oscillator stage should be shielded from air currents. Mount the 7805 at a sufficient distance to keep its heat away from the oscillator. An ugly dead bug construction would be nice Vectronics 1290K AM TX modification with 4 transistors Icon_smile

Putting out another circuit out there, this time a modified Vectronics 1290K AM transmitter. The original design had many issues but it actually tried to match its output stage to a high impedance wire antenna and this was the reason for its good audio quality. I have not built this circuit but years of experience says that it will work. Sorry for the unclear diagram and writing as my hands were shaking after taking back pain medication.

Do not remove the 7805 voltage regulator. Regulated 5V supply to osc and buffer stage is necessary for frequency stability. It also isolates the PA stage from the rest of the RF section. Stability should be excellent.

All the coils can be readymade axial Inductors. So there are no coils to wind! First ugly bug testing, and then a proper PCB. A neat modified version of the famous Vectronics 1290k AM transmitter with a bit of thought.

Vectronics 1290K AM TX modification with 4 transistors Img_2149

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