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Originally Posted by kbmuri
The oscillator and audio/video amps look unchanged. I guess I don't follow how the audio oscillator at 4.5 Mhz adds in, but I guess that's the signal that affects the shape of the sine wave at Q5's output, so I guess that's ok and the harmonics are expected. I don't think they're filtered well though, as like I said, tuning channel 3 also pretty clearly tunes channel 8 (the 3X harmonic).
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Seeing the entire diagram I can make better sense of it now. Q3 looks to be an oscillator at 4.5Mhz, and its internal PN junctions' capacitence is being varied by the audio signal voltage. That creates an FM signal. General Electric used a scheme like this to do Automatic Frequency Control in their solid state AM/FM radios they made around 1969. Using voltage to shift the frequency. I'd adjust L1 after I succeed in getting a video signal seen on the TV set. Adjust L1 to get the sound carrier demodulated properly by the TV sound circuits. Be sure the audio input level isn't too high.
The RF power amp is being "plate modulated" to modulate the video and FM subcarrier onto the TV channel carrier. Some older AM radio station transmitters used this method. Video is amplitude modulated onto the TV channel. Real TV stations also partly filter out most of the lower sideband, but VCRs and other such devices don't bother as they are usually in closed circuit environments (not radiated into the "air").
You could connect a TV coax cable (the F connector type) to where the 75 ohm resistor R12 is, and remove that resistor. The 75 ohm termination at the far end would be a 75 to 300 ohm balun in turn connected to a TV set.
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However there's one other flaw, I can't trace down nor explain, in that there's a slight, sudden increase in amplitude at the antenna, then a slight decrease, at a specific period (60 Hz?) that results in a series of lighter vs. darker blocks of picture, about half the picture height each, which slowly "walk" up the picture as you're watching your program. It's noticeable-to-annoying, and no amount of tuning removes it completely. Like I said, the regulated 9V is rock solid, but I don't know, maybe the regulator is radiating something?
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What is likely happening is that some of the output RF signal is finding a path thru the power supply rectifier diodes. As the rectifier diodes turn on and off at the powerline frequency, this RF path also is switched at the powerline rate. One solution is to insert choke coils in both power supply wires from the power supply. A few milliHenrys should do. You can usually find a suitable coil used in the power feed of a dead computer monitor or PC power supply. Those usually have a pair of windings on the same core, one for each power supply wire (the + source and - return aka ground).
This kit's schematic looks to be adequate for the job. Not great, but it should work reasonably well. Things like the power supply chokes should have been added to make success more likely.