Was it plugged into a positive center high current wall-wart? Diode might be toast. ( & maybe more)
I doubt it, but can of course ask him. According to his story the issue started all of a sudden, and not e.g., as a result of connecting or fiddling with cables.
For instance, I'd look for a switching regulator drawing much higher current than normal to start. Maybe due to a leaky cap. Or, maybe it's actually the mains adapter that was able to provide higher current peaks (which would allow said regulator to start) and which is not anymore.
Thanks for the suggestion. I was indeed able to pinpoint an IC that generates a lot of heat while in the faulty power-up cycle. See below for more info.
At 1A there will be something getting hot, so feel for something hot, like a ceramic capacitor, or a linear regulator that is getting toasty. If you have a thermal camera this only takes a few seconds to locate.
Second time around looking for hot components I was able to narrow down the current sink. It is in fact IC4 that gets scorching hot. D5 warms up just as a side effect. IC4 appears to be a SOT-232, but the marking (S40B) doesn't give many hits.
From what I can measure in circuit the pinout looks as follows:
1: short (~0.5ohm) to pin 6
2: ground
3&4: Positive lead on pin 3 and negative on 4 reads like a 1.3V diode
5: (no pin)
6: short to pin 1
When stuck in the faulty power-up cycle, pins measure as follows:
1:
2: ground
3: noise, averaging at 0.02V
4: 2V
5: (no pin)
6: 2.1V
When cranking the current and having the device operate normally same pins measure as follows:
1: 8.1V
2: ground
3: -1.2V (?)
4: 8V
5: (no pin)
6: 8.1V (same as pin 1)
To me this look like pin 1-to-6 isn't a direct short, since pin 1 looks like wanting to go up while in the faulty power-up cycle.
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