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A2779 (820-02841-A) Gets 20V, 5V->20V handshake loops and does not boot. Can enter DFU. Liquid Damaged.
- Thread starter PaulPCGuy
- Start date
Update 2: I can't find the short. It's a 0 ohm short, and no amount of amperage (6.1A at 1.2V) produces heat on a thermal camera. I've pulled off just about every IC and MOSFET that's either directly connected to PP3V8_AON, or indirectly past a csense or an inductor. This includes the WiFi IC. The only IC's I haven't pulled off are the 6 SPKRAMP IC's. This is because there was never any corrosion or signs of heat near them, and I've never seen them fail before.
"(6.1A at 1.2V)"
This means 7W aprox, something should go really hot.
Be sure to inject voltage in the correct points.
Use powerful cables; you want to heat something on the board, not the cables.
BTW, set PSU amp limit to max first.
This means 7W aprox, something should go really hot.
Be sure to inject voltage in the correct points.
Use powerful cables; you want to heat something on the board, not the cables.
BTW, set PSU amp limit to max first.
Use a good thermal camera...
If WiFi chip is dead, can't be replaced; it is married with CPU.
In such case, think if is worth to continue with this board.
Possibly problems appeared reballing U7700, or heat applied on the board affected something else.
In such case, think if is worth to continue with this board.
Possibly problems appeared reballing U7700, or heat applied on the board affected something else.
Which power rail was shorted to ground?
It was PP3V8_AON. Specifically, the subrails for WiFi and for HDMI. It's unclear if WiFi was caused by my, but HDMI was definitely from the prior damage.
I will note that U7700 had a sticky circle on top of it near one of the edges when I first got the board. Looking at the boardview, this edge holds PP1V8_S2 and PP1V_AWAKE.
I removed U7700 again and tested the behavior on PP1V8_S2, PP1V8_AWAKE, and PP1V8_S1_CLVR_VDDH. Since S2 and AWAKE are generated by it, they were expectedly low. PP1V8_S1_CLVR_VDDH was also low though. Previously, if I tried to inject voltage into any of these 3 rails (specifically while attempting to boot), my PSU would show the same jittering behavior. Now that U7700 is not present, none of these 3 rails exhibit the jitter. Though I will also note that they act similarly to if I tried to power them without USB power, so this isn't really a conclusive test.
I'm thinking about replacing U7700, is that a reasonable next step?
I will note that U7700 had a sticky circle on top of it near one of the edges when I first got the board. Looking at the boardview, this edge holds PP1V8_S2 and PP1V_AWAKE.
I removed U7700 again and tested the behavior on PP1V8_S2, PP1V8_AWAKE, and PP1V8_S1_CLVR_VDDH. Since S2 and AWAKE are generated by it, they were expectedly low. PP1V8_S1_CLVR_VDDH was also low though. Previously, if I tried to inject voltage into any of these 3 rails (specifically while attempting to boot), my PSU would show the same jittering behavior. Now that U7700 is not present, none of these 3 rails exhibit the jitter. Though I will also note that they act similarly to if I tried to power them without USB power, so this isn't really a conclusive test.
I'm thinking about replacing U7700, is that a reasonable next step?
Possibly overvoltage on 3V8_AON rail, caused by lekage from backlight area.
There may be more chips damaged...
There may be more chips damaged...
3V8 stays at a stable 3.8V. So if there was an overvoltage spike, it quickly burned out whatever made the connection. I'll check what IC's and FET's have both 3.8V and 1.8V connections. I think I'm also going to replace U7700 anyways just to be safe. It'll be a little bit before I work on this again, as I have cleared my bench for other MacBooks to work on.
Think if is worth to spend more time on it.
Will customer accept the machine without embedded WiFi?
Needs to connect external adapter...
Will customer accept the machine without embedded WiFi?
Needs to connect external adapter...
I believe I have found the issue. Or at least, the current one. The original SPMU (U7700) did not have a hotspot, but would not generate 1V8S2/1V8AWAKE properly. Presumably other rails were also affected, but this is the only one I actually looked at.
Much earlier in the diagnostic, I tried swapping it with a donor from a liquid damaged A2442. When that one came back with a very hot hotspot, I assumed it was just dead from liquid damage. Now my replacement from Sentrix has come in and it has the same behavior. I pulled it off and started probing. I found that what I assume is the main power input for the SPMU is shorted. Tomorrow I will figure out what on VSS_ANA_SPMU is shorted, and hopefully with some luck this MacBook will be booting tomorrow.
Much earlier in the diagnostic, I tried swapping it with a donor from a liquid damaged A2442. When that one came back with a very hot hotspot, I assumed it was just dead from liquid damage. Now my replacement from Sentrix has come in and it has the same behavior. I pulled it off and started probing. I found that what I assume is the main power input for the SPMU is shorted. Tomorrow I will figure out what on VSS_ANA_SPMU is shorted, and hopefully with some luck this MacBook will be booting tomorrow.
Ooops, VSS is a ground point!
Looking at page 39 you can see a lot of VSS pins; where they go???
In general, in electronics, VDD and VCC are labels for positive power supply designation.
VSS and VEE are used for ground, or negative power supply.
Looking at page 39 you can see a lot of VSS pins; where they go???
In general, in electronics, VDD and VCC are labels for positive power supply designation.
VSS and VEE are used for ground, or negative power supply.
Thanks for that. I found that (at least) PPVDD2H0_S2SW and PPVDDQ0_S1 are shorted. I pulled off 6 caps which were suspect, but it didn't go away. I pulled off the CPU, and it still hasn't gone away. At least now I can safely drive more voltage into it but still this is getting insane lol
Is U7700 soldered on the board?
If yes, remove R7810/40 and check where the shorts remain.
You may also need to lift L7810/11 and L7840-43.
If yes, remove R7810/40 and check where the shorts remain.
You may also need to lift L7810/11 and L7840-43.