Thread: 405 NTSC colour
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Old 13th Sep 2004, 4:45 pm   #13
David_Robinson
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Join Date: Feb 2004
Location: Nottingham, UK
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Default Re: 405 NTSC colour

No, that was nothing to do with me. That was a project to build from scratch. I seem to recall it was done by Keith Cummins. My set used a field timebase/line drive board and the IF strip by Bush (RBM), all solid state. The line output (Pye, valves) and the video board (RBM) were hand built to the manufacturer's circuit and the colour decoder was a design from a large size Mullard paperback book that would probably be worth a lot if I had kept it.

I was working as an apprentice at Marconi in Chelmsford at the time, and a lot of people were building colour sets. We clubbed together to buy the major parts. I well remember a big Mullard truck turning up with I think around 15 to 20 26 " picture tubes.

I recall that the RBM IF strip had an unusual design. It had a quasi-parallel system where one channel did the luminance and the other did the chroma and sound. The theory was that you could have a deep sound notch in the luminance path, which eliminated the bane of early 625 IF's, the wiggly sound/chroma beat pattern. This was of course before the widespread use of synchronous detectors.

To use John (the editor) Reddihough's favourite word, HOWEVER, RBM's design had a fatal flaw that I only sussed years later. They had failed to eliminate the sound/luma beat in the chroma IF. Luma at 1.57MHz would beat with sound at 6MHz to give 4.43MHz. I remember noticing at the time that the cross-colour seemed worse than other sets, but I couldn't think of a reason.
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