1. When the TAM is started up, the green LED should light up and away it goes - have you tried the usual CUDA reset, PRAM reset. ensured RAM is good? Does it boot to a desktop - or what you think is the desktop - giving it a few minutes to start up, press the power key on the keyboard and see if you get any sounds/press 'R' to restart
Yep - did all the resets, and it appears to boot up to the desktop (I can restart with keyboard, play music with the CD player)
2. The backlighting/inverter could have gone on the LCD - try shining a torch down on the LCD while on and see if anything is displayed.
I tried this and can't see any kind of image. completely dark
3. The TAM display is the exact same part as from a PowerBook 3400 (and in turn easily obtained), but there exists an analogue to digital converter board between the LCD and TAM motherboard. I have come across a failed A-D board but it displayed lines and static on the screen after a minute of use.
So it sounds like what I describe might be worth trying to replace the display (I found a TAM display for $75 euro so might do this instead of tearing apart a 3400c). Also do you know if this A-D board is part of a new logic board? I haven't been able to find the A-D board, but did find a logic board for sale.