Repairs_2019/Beaglebone_Blacks/docs/notes

took a look at this

may be power regulator

or may be something downstream

it turns off for a second, then turns off. 

hard to tell what is the problem.

need to check all voltage rails when this is powered on.

note that I found you can hit the power button to activate the vreg. but 
this might be dangerous to hit repeatedly.

let's try replacing the vreg, as I want an excuse to use my hot air station

it's possible someone just gave it too much power.

https://www.eevblog.com/forum/repair/could-i-repair-a-beaglebone-black-that-had-was-fried-by-the-wrong-adapter/

hehe, they say don't bother fixing. Wow...


Steps for this:
Attempt replacement of vreg
if that doesn't solve it, likely something downstream.
connect to power rails, with PSU, and watch what heats up with iso alcohol.

I have 4 or 5 of these beagles, so I only need to find it once. In fact, maybe 
I should just skip to the inserting the Power supply in. there are a few 0805
caps that are relatively easy to tap into...
The board files are allegro, and I don't run win but 
the schematic shows cap labels, so this is probably the best plan of attack.
If I don't find anything heating up, then it would point to the power supply...

Let's do that. 


EDIT: testing all 4 beagles

1 works, it appears. 
1 turns on all 4 status leds, but no blinks. 

2 have a brief led light on power reg, but then off. protection circuitry.

I found 1 of them with a blown resistor on the ethernet shield to digital ground.
it was a 0.1 ohm resistor. 

I found 2 of the 2 with brief led lights to have a cap (3.3v rail) at 12 ohms. The others
are 400 ohms. This was mentioned on forum. Probably someone put 12v on the input, and blew the 10v
caps... flawed design, i think.

I'm going to remove those caps first. off to the hot air station

i should buy some 10uf 0805. i only have through hole and 1uf 0805 (though in a project
bag there might be one somewhere...)

After replacing cap, there's still 12 and 5 ohms on those respectively.
So it wasn't the caps, but something on the rail. Looks like the 3v3 rail is the only 
rail that has the short, per the other caps, didn't give me any unusual resistance, and
also 3v3 is the one you would expect to be shorted (main v rail).

can't find c16, which is on rtc. I need allegro to do layout searching

3v3 goes into 
main CPU (arctic)

usb host tps2051

SPI board id
24lc32

lan adapter (3v3b)

HDMI (3v3b)

and some 74 logic.


I think the cpu is the issue. Let's disconnect R14, and see if power rail boots.
there's two inputs. one is where r14 is, another is on USB. Hopefully I don't
have to track down usb input. don't have layout.

removed R52, and R14, but it's still shorted. 
I believe the USB input to CPU is causing this. There aren't many other things it could be

EDIT: looks like the 74 logic here is max 6.5 volts or so. could also be that.

lets try to track down the short with the 6 digit dmm

NOTE: there are two distinct 3v3 rails a and b

3v3B comes from TL5209

that is enabled after 3v3A is turned on.  (see notes in schematic updates)

C20 on 3v3b looks to be 0.4K ohms, so OK.

Must be 3v3a, which doesn't have much. 

Nice idea, to separate 3v3 rails.

based on my tracking with the 6 ohm dmm

i found the lowest resistance somewhere around 5.5ohms in between the cpu and the ddr

the ddr is 3v3b, so it must be the cpu.
 let's remove hte cpu, and see if power rails start.

removed CPU. first bga removal for me.

And sucess. The low resistance on C16 is now 0.4K ohms. So the CPU is the weak point.
I guess this means by design, of the bbb, I will be doing some hot air bga soldering.
it must be gods will. (ti's will)




I removed the second bga, but this time, I pulled some pads. Of course you can
pull pads on a bga... May not be repairable, but lesson learned.