I don't know about TVB series, but this is either the themesong or subtheme song of a very old and obscure ATV series called Blue Moon [蓝月亮]. I definitely remember hearing the song in that series because after I heard that song in the series, I went around look for it. It's Joey Meng's first series, released in 1989. I really like this series. But it's very very hard to find now.