I repaired an A4000D for the same fault last week, I've seen it a few times in other machines too.
Can be caused by a number of things, but most of the cases I've seen are the SMD electrolytic capacitors leaking electrolyte onto the PCB which corrodes tracks, causes bias upsets on op-amps, etc.
The 22µF ones seem the worst, check around the base of the capacitors (the small round silver ones) for signs of discolouration or corrosion. Sometimes the shiny soldered legs on the capacitor also turn black, indicating there is corrosion underneath.
I would not recommend trying to remove and replace these yourself unless you have good SMD soldering skills. It's easy to lift and damage PCB tracks.
I have schematics here if you want to do some more faultfinding yourself.
Drop me an email if you need a copy.
Anthony.
