That was interesting. Maybe the processor checks the output transistors or something and does not start up because it finds a fault?
The yellow wires from the transformer are likely for the filament of the VFD.
Seeing this video made me appreciate my Denon amp even more - in the case that the microcontroller fails I could replace it with a small set of 74HC or 4000 series chips, I would lose the remote control possiblity, but I do not have the remote anyway, so not a big problem (other than the remote, the MCU just switches on the appropriate relays when a button is pushed to select input etc).
tom66 - in a (modern) scope the analog parts are small and can be shielded relatively cheap. The analog part in an audio amp is usually quite big, so you would have to place all those boards in a can. Besides, scopes usually have 8 bit ADCs, so about 48dB of SNR anyway, compared to 110dB (line inputs) of an audio amp. If the amp has a phono input then the requirements are even higher, since a cartridge outputs only 0.5mV (MC) to 5mV (MM) and my amp has 94dB SNR for the MM input.
PA0PBZ - a very similar circuit is used in a couple of my tape decks to control the speed of the AC reel motors (but with a bipolar transistor and in linear mode).