JUNK STEREO REPAIR UPDATEDon't lose sight of the fact that some meters' continuity buzzer / beeper can still work through resistances etc of around 200 to 300 ohms and also often you can get an initial buzz / beep on circuits that have caps in them as well, so make sure that you can get the signal for a few seconds rather than a quick sounding.
Yep ! Think my Fluke 11 sets the bar at 150 ohms IIRC, but whatever... yes it's taken into account. That meter has an excellent continuity tester. Super fast. You just can't mistake a quick blip with a positive/true continuity. I just love my Fluke 11, still my go to meter 25 years after I bought it !
Anyway, I have spent lots of time buzzing again and I got somewhere.... finally found where that USB power pin was going to. It goes to the drain of a tiny P-channel MOSFET that's miles away, near the edge of the board right by the audio aux connector. Yes, I spent ages checking every single component on the board before eventually checking that "unlikely" area of the board...
MOSFET has 5V on the source and gate, hence not conducting, so not being asked to. Took it off the board anyway, with the hot air station, then put it on the chinese tester, it has SMD pads. still was a challenge trying to hold it steady with one hand with tweezers, while trying to press the push button with the other hand. Took a few attempts... but the FET tested good.
However before soldering back to the board, first I did a quick sanity check : I shorted drain and source with a piece of component leg that I soldered. Wanted to see I could force 5V to the USB connector. Yes I can, I now have a rock solid 5V at the USB connector !
Overjoyed, I rushed and plugged my USB stick.... the 5V supply still holds fine, yes !
..... but still doesn't work, no activity going on on that stick !
tried the SD card, still no go even though the power is good.
I give up. The fact that both USB and SD fail is suspicious to say the least, especially given everything Dragon said about this very problem on these very ALI CPU's....
I think I have run out of options and hope now... time to move on. It's 2H30 AM, I going to bed. Will phone my friend tomorrow to tell him all about it....
No regrets, tried my best.... plus it was a learning experience, I know more than when I started on it 12+ hours ago.
Thanks every one for your help, much appreciated...