Hi Tom,

HP's and Compaq's have this switching circuit on the board that deliver signal to the inverter.  Please check the adjacent IC or transistor close to where you connect the data cable or the cable that goes to the inverter board.   If you are sure that your data cable is good then the culprit is the IC.  Im not so sure about availability of IC part locally, as some ICs are in-house and not retailed. If you can find similar motherboad that people sell in auction then that could be a good start to salvage parts.