There are many, many class AB amps in landfills, probably many millions if you count all the stereo receivers over the years that died and were tossed in the trash. I have some old, old gear for sentimental reasons, though I rarely use it now, but there some impediments to restoring old equipment. Figuring out the circuit and what failed, often without a schematic, finding old parts (often discontinued) or replacements, and having or finding someone with the knowledge, skill, and tools to do the repair can be daunting. I am in awe of what folk like
@restorer-john do; maybe in another 167 years when I can retire I can do something similar. I used to repair and restore gear but my career took a different path, one that led to very little free time or desire to stare at a circuit when I got home after doing it all day long (albeit different circuits).
The component problem has gotten worse IME/IMO with higher integration; fortunately, power supply failures and discrete component failures (e.g. capacitors) seem most common and those are "easy" to fix. I suspect quite a few of us can remember Sanken modules with all the smoke let out, sometimes through the hole in the case... And the smell of a smoked transformer is something I can live without.
All that is a side argument to class D reliability, of course. My limited experience has been capacitors are what tend to die. I just replaced SMPS caps in a 10+ year old TV to get it running again. And there have been a long stream of bad video cards and computer monitors failing due to bad (generally under-spec'd) capacitors. I'd like to think we've learned and modern class D and SMPS designs are past that, especially given the focus on capacitor performance and reliability so there are oodles of good choices these days, but cynical experience says nay...
John's experience with that one amp is concerning. I am tempted to open up my Buckeye NC-252 build and take a look. I'd love to know if the problem is common or an outlier. That said, I have seen quite a few examples wherein it appear to me and my friendly mechanical/thermal engineer that the manufacturer failed to grasp that 80% or 90% efficient is not 100% and quite a few watts can still be wasted as heat even in a class D amp or (especially) SMPS design...
Friday musings - Don