AN actor was yesterday put on probation for 18 months for having sex with a 14-year-old girl. Yeung Chi-wai, 17, was found guilty of having unlawful sexual intercourse with a girl under 16. Seven other charges against him, including living on the earnings of prostitution and claiming to be a triad member, had been dismissed by the judge after the key witness' testimony did not stand up in court. Prosecutor Roderick Wu called evidence to show that the girl met Yeung in Sai Kung on December 23, 1992, and joined Yeung and friends for mahjong and cards at a nearby shop. Yeung later suggested the two of them go to another floor of the shop to have sex. The girl agreed. Charges that Yeung had unlawful sex with the girl on four other occasions were dismissed as were charges that he had forced her into prostitution - the girl was unable to recall clearly the alleged incidents. Defence counsel Selwyn Yu said Yeung was ''taking care of his family'' and urged the judge to put Yeung on probation. Judge Jackson made reference to Yeung's difficult family life - his father, mother and step-mother have suffered from ''various mental or physical disabilities''. Yeung was arrested at a checkpoint while trying to leave for Macau.