Hmm, well, I did some searching, maybe this can help?
There is a DIY for atari of such a board utilizing the MC1377.
Can be found at:
http://www.preromanbritain.com/gwem/martbean/ataridiy/conv.htm
Ironically, cascade surplus used to sell the MC1377 chip for 0.50 USD a piece just a few months ago, but it is no longer in their stock.
I found a similar chip called NTE879 that is going for 4.39 USD here:
http://www.galco.com/scripts/cgiip.exe/WService=Galco-Web/WA/WCat/Catalog.htm?listtype=Catalog&search-part=NTE879-NEWT
Other similar chip is likely to be found here:
http://www.xs4all.nl/~ganswijk/chipdir/f/tv.htm
and here:
http://www.chipcatalog.com/Cat/105.htm
