I have many times tried to repair the Huawei 12V 2A power adapter HW-120200E6W. It was used with some Huawei 4G router.
It had failed randomly. Sometimes it worked, sometimes it didn't. I did stress tests with 1.8A of current for ~1 hour. Sometimes it failed, sometimes it survived.
I re-applied solder to many parts, it seemed to have small effect, but not much.
Lately the output was completely lost, so it was again an opportunity to fix it. I opened it up, applied 230V and checked the circuit with thermal camera. Only some resistors got warm. I measured zener ZD2 (24V is written on it) pins that are on the switching mode power supply controller - only 2.5V. I removed the 230V, drained high voltage capacitor with 47K resistor, then soldered wires to ZD2 to give it external supply. I gave 10V and ZD1 got warm in thermal camera. ZD1 had marking 5J which means 18V. I measured current to supply. It was a few milliamps. This is not normal, and the main chip cannot start when ZD1 is consuming that much current. I removed ZD1, replaced it with a 16V 0.3W zener (probably will add another one in parallel). The leakage current disappeared and the circuit started to work.
So, faulty ZD1 (ideally replace with 18V zener). On the image, it is between the yellow dots. It was probably this zener (look for 5J). Here you can see also top and bottom images of PCB.
I will need to do some more testing, but now I've got 12V out of it, again!
Texts on PCB: HKA02412020-3Q, rev:B, sch: A, 2012/8/17