I'm in the process of building a modulator that covers all 13 System A channels. I have posted some pictures on my website
http://homepage.ntlworld.com/david.robinson3 . Sorry the circuit diagrams are a bit scruffy - I haven't got round to Orcad - ing them yet.
The sound modulator of course has the same peripheral components as the vision one, but I couldn't be bothered to draw them. This is very much 'work in progress', but it does work. Top of the to-do list is completing the stereo mixer for sound, and a 3.5MHz notch filter on the vision side to prevent buzz on sound.
If anyone wants to have a go at this, send me a PM and I will send you the software for the PIC.
The modulator chips can be obtained as free samples from Motorola Semiconductors (now inexplicably renamed Freescale). The chip supports System L, hence positive vision modulation. Understandably however it doesn't support the 3.5MHz sound spacing. To get round this I use a second chip with the vision modulator used for the sound. The actual sound modulators in both chips are switched off. The level pot on the sound modulator biasses the input to disable the video clamp on the chip. The chip does have one limitation in that the VHF carrier is derived by dividing the UHF carrier. This inevitably means a squarewave carrier with resultant odd harmonics. This isn't a problem in practice so long as you don't use more than one modulator feeding each TV.
Nothing to do with modulators, there are also some historic pictures of Practical Television mag on the website.
