Compaq 800 notebook boot failure

Hi, having an issue with this laptop. Model says series PP2160.
It won't get past POST, and will only display the red Compaq splashscreen
with a F10=Rom Based Setup message. Hitting F10, F2, Fn+F10, F8 all do
nothing.

I've tried:
- booting on battery
- booting on mains
- removing the RAM module and starting
- removing the HDD and starting
- swapped out the RAM module
- tried the RAM module in the other slot
- leaving the battery, HDD and RAM out overnight

I've been able to fix another Compaq using these steps before, but have now
hit a brick wall with this one.
Hoping someone may have had this before, as I hoping I don't have to send my
client to the Compaq dealer.

Thanks,
Xygt