It's common for the NEC TurboExpress / PC Engine GT to fail. Common failures include white screen or no screen as well as low to no sound coming out of the speaker. This is caused because the onboard capacitors have failed and need to be replaced.  Some times, other components fail on the system as well (backlight transistor, inductors, power fuse)

A comprehensive repair includes the following:

• remove all the old electrolytic capacitors
  
• clean / restore the capacitor landings
• fix any broken traces / vias 
• electrolytic capacitors will be replaced by high quality Japanese brand (Nichicon, Panasonic, Rubycon, ...)
  
• test the system to make sure 100% operational