Let me just relate this to you...I have been working on a FAV6800 for 2 weeks now that I ordered a control and panel for. The first 2 controls I got from WP appeared to be used and did not function. The control panel they sent also had been tried before, as the cover film on the touch pads was missing. For whatever reason, WP is giving out some really bad parts the past 6 months or so. Understandably, the Neptune TL has not been made since 2004, so the parts are old from sitting in the warehouse.
The control board supplies the power to light up the screen. You can actually take the console off and place a 120 volt test cord on the board at the L1 and Neutral terminals on the right rear area of the board to make the display work on the bench. If nothing lights up the board has pobably got a bad transformer. Be sure the ribbon connector from the touchpad is securely clamped in the board receptacle also.