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I suggest it's time Dave pull out his FLIR camera, power up that failed supply board briefly (maybe outside to avoid the stench?) and really see what's getting hot... and perhaps that will lead to more clues as to the real failure cause. There might be something that failed FIRST upstream of that cap? Or maybe the failed cap caused a second failure?? Let's take a look!
It is hard to imagine a failure mode of a buck converter, that can damage the ceramic cap at the output. The more likely sequence is the damaged cap causing secondary faults.
Quite likely but it never hurts to verify the failure cause and not just assume it was the cap failing first. I've seen this happen before with un-expected causes, especially on these apparently very efficient smps converters. Depending on the exact design, sometimes if they are using active mode flybacks that are under-rated, then at higher input voltages the thing starts outputting higher current / high voltage spike AC when an active mode flyback switch fails partially shorted - and it starts taking out even properly de-rated caps downstream. Or other failures where the board assembly house accidentally installs series 1k resistors on the switch gate drive circuits instead of 10 ohm (That gets exciting when the whole board goes up in smoke at power up), etc. Ask me how I know these real world failures can and do happen.
Or it's just the cap, (I agree is likely), but now you still want to know exactly why the second (or more failures) happened. Yes, Dave didn't really clean the charcoal resistor from the output, but the power supply was drawing too much current even with the output switched off (I think). So that's not it.
In any case: since that cap is sitting right on the output, then the exact failure should be investigated to confirm it was the cap that failed first, and then what caused other failure/s?. Dave's got the toys to play with, right? That flaming cap could just as easily have been on the user's circuit side, and you'd try to design the product so that's it's not prone to cascading failure events. In other words you'd like the power supply board not to die every time someone tries to power up some maybe mis-wired prototype circuit. Maybe the user's under-rated cap blows up but you don't want the power supply to die with the cap. That's what we're aiming for, at least in spirit.
Besides, it's fun to see the FLiR in action! This would be a good excuse to drag it out and have some fun, that's all
Every failure is a teaching moment.
Otherwise this power supply seems to be just another example of "you get what you pay for".