JUNK STEREO REPAIR UPDATEOK I think I might be getting somewhere. I touched up the 4 pins of the USB socket, no luck.
Then I started with the basics.... power supply. So for the past 20 minutes I have been playing in this area and found interesting things....
I measured USB Vdd then, expecting to find 5Volts isn't it, and instead found.... 800mV ! A second later... 600mV... a second later...400.... then 300mV... stayed there. I thought hey strange... so maybe that means USB power is not permanently supplied by the 5V rail, for some reason ? Maybe the CPU switches power to USB only when the input selector on the front panel is set to USB ? OK why not, even if I don't see the point of doing that but well...
Still, if the CPU switched the power off, I would expect to get Zero volt, not something as high as 800mV, and mysteriously dropping as I am watching it on the meter....
So, to test this hypothesis, I switch to different in puts : SD card, CD, Radio.... and... strange things again. For a split second, during the change of input, USB power pin goes up a bit, 1.2V say, then immediately drops back to 800mV and starts again dropping slowly.
So.... nah. not sure what happens with USB power but it doesn't look like the CPU is controlling / switching it.
So kept experimenting : I unplugged the USB stick, unplugged the stereo, plugged it back in while watching at the same time USB power. Oh !!! Now we have something new ! USB power is now 3.28 Volt ! Much better.. but still a far cry from 5V.....
Then I plugged the USB stick, again while watching at the same time USB power : instantly the voltage drops to 800mV ! Same scenario as before !
Oh, so USB power is too weak, it can't stand ANY load whatsoever ?! So I unplugged the stick, to see if the voltage would climb back to 3.28V but it doesn't, it stays at 800mV.
SD card power pins : measured that. They get the same voltage that USB power does, it exhibits exactly the same symptoms, in unison. So that's good. That would explain why both USB and SD don't work.
So I have a power supply issue in the USB/SD area.
Noticed something else too : found a local 3.3V 3 pin linear regulator near the CPU, tiny one, you can see it on the picture I posted earlier.
OK so the CPU does run at 3.3V not 5V. So the 5V must be only for USB and SD. but these get only 3.3V, why....
USB and SD do seem somehow to get the voltage from that 3.3V regulator : output of the regulator measured precisely 3.28V just like what I measured on the power pins of the USB and SD card connectors. Strange. Sure enough USB is only 5V right ? Or is there some standard somewhere that says it should also be compatible/work with 3.3V as well ?!
Still, there IS a 5V rail on the board, so why not use it ?...
Anyway. Regardless, it looks like USB/SD are having power supply issues, be them with 3.3V or 5V doesn't matter. They don't get proper power and I need to investigate that don't I... The CPU works fine and the output of the 3.3V regulator is solid, no issues there. So even if USB was indeed using 3.3V rather 5V, then the problem is not with the 3.3V regulator. There must be some discrete components in between the regulator and USB/SD connectors. That's plausible because when I do a continuity test, I don't get continuity between, the power pins of Sd and USB, nor do I get continuity between USB and 5V rail or SD and 5V rail.
tried looking at traces around the USB power pin. did some buzzing. I see an MLCC cap between that power pin and ground. But it's not shorted nor low impedance (could try removing it anyway just to see...). then there is an inductor in series with the pin, then it buggers off to I don't know where because the other side of the inductor goes to a burried via.. does not come out the other side of the board
.....
OK back to the bench, oh dear....