I added an extra SID chip to my 128D and ran it thru the composite a/v rca jack. I was using a nice Magnavox TTL RGB+Composite monitor so I had no use for the RF modulator jack. It became a second audio channel jack.
Ofcourse the sound output was much lower...maybe I didn't have it grounded right at the jack...
Other than the stereo SID player, nothing else supported it. Oh well...
Anyone want a 128D for $45 shipped in the US?