820-00165 On (I think?) But no display

Hello

Macbook Air - replaced screen and tested backlight works OK
When I power on after disconnecting battery the "Bong" sound goes repeatedly, untill I restart once, then it will "turn on" normally but with no display.
Nothing on the display best I can tell, even with torch in the background

Machine drawing about 600ma with computer connected to power and battery.

Ideas?

Caps Lock light reacts, USB activity, or external video?
You must be sure it doesn't get stuck after chime.

Just in case, check SMC_LID.

Hello
No caps lock or other signs of working.

I did get it to display the apple folder icon once but not seen that again.
Installed a known good test SSD which gets very hot.

smc lid 3.5v

Random symptom may point to dropped machine.
You may have broken trace, solders...