Q:Regarding the source of the fault, I have seen many posts on various forums (e.g. http://www.volvoforums.org.uk/showthread.php?t=152688) with many owners experiencing this problem. Note in the link there is a post from an audio engineer (Marcel) who has met the problem before and notes that it is most likely a main board connection fault or the DSP micro.
Also, my vehicle has a central dash speaker. It seems that this is powered directly by the HU850 head unit, not via the amplifier. As all channels failed at the same time, this points towards the head unit not the amp being the problem, as if it was the amp, there would still be sound from the dash speaker..
A: You are correct I have had few of these repaired in the past and the audio chip failed, I have used a chip of different radio hu-650 to repair hu-850 and it worked fine,