I've never heard of the 'famous C515', but I would VERY, VERY strongly suspect a high ESR capacitor in the power supply - which could very well be C515?. Makes no difference if Yamaha no longer keep parts, it's unlikely they ever kept simple parts like capacitors - there's no point keeping standard parts that are available everywhere.
I got my Sony sound bar for free, as it was dead - same issue, a duff capacitor in the PSU - just one, no other faulty ones. I've repaired a number of others of the same model with exactly the same fault. No idea what the circuit reference is, but I suppose that could be 'famous' as well?.
So replace C515, use a good quality capacitor (unlike the crap they fitted in the factory), I like to use Panasonic or Nichicon - and it will never fail again (it's just a cheap crap capacitor).
It 'might' be blindingly obvious to replace, with the top bulging or even leaking, otherwise you'll have to try and spot the component reference on the board.