steady orange led is normal

first try replace cmos battery w a new one and then clear cmos by mobo RSTRCT jumper

if still no go, try a BIST test of stock psu

Now there's just an orange light on the motherboard.

I tried with another PSU and it's the same

Could the motherbosrd be replaced?

If it is very old may not be worth replacement.  but if you really want to do it yes it can be done at low cost probably.  we still do not know your pc model.

try the cmos battery and jumper method.  again orange led is normal