... Could you lead me a bit more? i don't know where to find what I'm looking for on that site. Sorry, RF has never been my stronghold.
...
This page under the heading "ATV RECEIVERS/TRANSMITTERS/DEMODULATORS" has many circuits, but unfortunately most are just schematics with few details on actual construction. They all seem to be based on a simple RF oscillator that is amplitude modulated by the composite video signal and simultaneously phase modulated by the audio signal. The ones that will interest you are among the first nine that are listed. Most of the remaining circuits pertain to amateur radio television transmission on amateur radio frequency allocations. None of these circuits produce a broadcast-quality RF signal with NTSC modulation, but they may be good enough for your purposes.
Perhaps the simplest is
this one which uses a single 741 op-amp for audio amplification to modulate the base (and hence the phase) of the single-transistor RF oscillator. The video signal is injected into the emitter circuit to provide amplitude modulation of the RF carrier produced by the RF oscillator:
It is not necessary to use an op-amp for audio amplification. A common-emitter amplifier with a gain of 100 or so, followed by an emitter-follower to provide a low-impedance output, should work. Everything is AC coupled with capacitors between stages.
Note the very small 10 pF capacitors used to couple audio into the RF oscillator. This is the tricky part. You want enough audio to phase-modulate the oscillator, but not so much that it prevents RF oscillation on CH 3 or CH 4. Just about any high-gain, high-frequency, NPN transistor should work for the oscillator. The 2N3904 transistors that Dave's circuit uses are inexpensive, but I don't know what the Soviet equivalent part number is.
The adjustable inductor in the RF oscillator is about 10 μH (NOT mH!) at the 50% point of its adjustment range. I think this will be perhaps five or six turns of stiff enameled copper wire on a 10 mm diameter coil form with an adjustable ferrite core. Coil inductance depends on diameter, number of turns, and the ferrite core, so I may be wrong about the number of turns required. It may be difficult to salvage a coil-form with a suitable adjustable ferrite core. I would try using the inter-stage IF coupling transformers from a junk TV, removing all the windings and adding a few turns of wire to see if it will oscillate in the circuit on CH 3 or CH 4.
A grid-dip oscillator would be of immense help in roughly tuning the RF coil, using whatever small-value capacitors you can find to make it resonate on CH 3 or CH 4. Note the schematic shows a variable capacitor AND a variable inductor. You only need one or the other, not both. The variable inductor is usually the easiest way to go. Also note that the 20 pF variable capacitor (also usually hard to find) is the main resonating capacitor used with the coil, but the 4.7 pF capacitor from collector to emitter is also important because it provides feedback to make the transistor an RF oscillator. Its value will also have some effect on the oscillator frequency.
It can be frustrating working with RF circuits without appropriate test equipment, such a 100 MHz bandwidth oscilloscope and a 250 MHz grid-dip meter/oscillator. I am guessing your "test equipment" will be the TV itself. Tune the TV to CH 3 or CH 4 without connecting an antenna, except for a few meters of "twin lead" you will use to couple a signal from your oscillator circuit into the TV tuner. Power up the oscillator, then adjust (tune) the coil to see if you can blank out the "snow" on the TV using just the RF oscillator signal. Use just enough coupling from the oscillator to the TV antenna twin-lead to produce an observable effect. Once you are sure the RF oscillator is producing a signal on CH 3 or CH 4, apply video and audio from your digital converter and then
farkle around with gain and tuning adjustments until you get acceptable results. [Note:
farkle is American slang term that means to do whatever is required to make it work, without letting anyone know you don't have a clue about what is going on. You must
farkle with a very serious face, and perhaps a frown once in awhile, until the circuit bows to your command. This definition has nothing to do with motorcycle accessories or a game involving six dice since I just now made it up.]
All the above seems like a lot of trouble to go through, just to get audio and video into a working TV that must already have circuits to process audio and video. If you are allowed to "modify" the TV, it should be fairly easy to find the audio and video circuits that are already there and feed those circuits with the output audio and video from your digital converter.
Best of luck, Señor Lopez.
73
de AC8NS
Hop
