I've been presented with one of these for repair - a Minx X201. Apparently, it stopped working after a power cut: the owner has tried the switches at the back, including the one that overrides the automatic standby in the absence of a signal. No standby light, nothing.
It appears to have a chunky switch-mode PSU based around a TL494 and I suspect some 'orrible Class D amplifier to produce the claimed 200W output. Although mains power is getting to the PSU, nothing is coming out of it and there's no supply to pins 7 and 12 of the TL494 so I assume it's not starting up: there's a first port of call I suppose.
Before I start trying to work out how the PSU operates from first principles (as I very much doubt any circuits exist), does anyone here have any experience of these? My apologies in advance for tainting the forum with something so modern...
I have a MinxX201 video on YouTube, but it covers a different faulty to what you have.
However, just off the top of my head I would guess your looking in the wrong place, most subs have 2 power supplies a big one for the audio output chip and a small one for the standby rail, without a standby rail the high current supply won't be active, and the driver chip you list suggests a high current mosfets drive.
Many thanks to whoever posted my video, this stuff is a bit technical for me.
I gave up on it in the end and advised its owner to make use of the many eBay sellers offering fixed-price repairs on these things.
