No TVT cards for the Sinclair Spectrum, I'm afraid.
I never had a Sinclair, but I assume the coax cable carries a modulated signal on channel 3 or 4? If so, you might look for an electronics retailer who has such a converter.
Seriously, though, if this is the case (and you're in the States) you might want to go to a thrift store like the Salvation Army or Good Will and check out a used analog TV. Since the conversion to digital broadcasting the thrift stores in my area have been flooded with analog TVs in great shape.
Edited by Capn Easy, 21 March 2010 - 05:59 PM.