820-00840 20v 0a [SOLVED]

[Kappy]

Very small amount of liquid damage on the LCDBKLT caps (C8469, C8411) and around the U3700 Wifi. Stuck it in the ultrasonic cleaner and replaced the 2 caps. Still on 20v 0a.

Started trouble shoots the rails:

PPBUS - 13.10v
PP3V3_S5 - fluctuating
PP5V_S4 - fluctuating
PP5V-S5 -fluctuating

Replaced U7650 along with Q7660 & Q7680 to rule out the possibility that it was coming from a bad MOSFET. still the same

Replaced U7800 still the same. Removed L7690 and the fluctuation is coming from the system side (pin 1)

Check all the above rails and non of them are shorted to ground. Also had a prob on other rails and non of them are shorted either.

Any help of what to try next?

 

[2informaticos]

Which kind of fluctuation?
Do you mean pulsing every 2-3s, or continue fluctuation?

Test the board alone on the desk, nothing connected, apart of USB-C charger.

 

[Kappy]

Continuous fluctuation.

Them readings and the above explanation is on my desk with nothing connected apart from USB-C.

 

[2informaticos]

Post basic voltage for U7650: pins 23, 22, 12, 13, 29, 21, 4.
Mention the ones with fluctuation...

 

[Kappy]

Pin 4 - 0.00
Pin 12 - fluctuating between 3.42 and 0
Pin 13 - fluctuating between 2.0 and 0
Pin 21 - fluctuating between 3.42 and 0
Pin 22- 3.2
Pin 23 - 13.1
Pin 29 - fluctuating between 5.0 and 0

Fluctuations are approximate as I can't get an exact voltage figure.

Just in case you need to know none of them are shorted to ground.

 

[2informaticos]

"PP5V_S4 - fluctuating
------------------
Pin 4 - 0.00"
5V_S4 should stay at 0V all the time.

Remove R7662 and check if fluctuation disappears.
3V3_S5 stable then?

 

[Kappy]

Removed R7662 and the fluctuation is still present on 3V3_S5..

 

[2informaticos]

Keep R7662 out and solder wire on C7668.
3V3_S5 should be 0V; check if SMC_PM_G2_EN is steady high.

 

[Kappy]

When you say solder wire on C7668 do you mean bridge it as the cap is NOSTUFF

 

[Kappy]

3V3_S5 is 0v and SMC_PM_G2_EN is steady high with C7668 bridged and R7662 removed

 

[2informaticos]

3V3_S5 fluctuation may affect SMC_PM_G2_EN too.
Now put back R7662 and remove U7890; tied together its pads 4 & 6, on the board.

 

[Kappy]

I've put R7662 back, left C7668 bridged and I only now get 5.17v 0.27a

With the bridge removed back up to 12v 0a 3V3_S5 is 0v and PP5V_S5 is stable at 5v.

Just in case it helps from U7650 voltages:

Pin 4 - 0.00
Pin 12 - 3.42
Pin 13 - 2.0
Pin 21 - 3.42
Pin 22- 3.2
Pin 23 - 13.1
Pin 29 - Which mentioned above is 5v

 

[2informaticos]

In my previous post, I forgot to mention to remove C7668 bridge too.
Check if 3V3_S5 comes on...

 

[Kappy]

No. With the bridge removed from C7668 I get 0v on 3V3_S5 but PP5V_S5 is stable at 5v.

 

[2informaticos]

Did what I said about U7890?

 

[Kappy]

Yes steps so far. Put R7662 back, removed C7668 bridge and removed U7890 with a bridge from pin4 to 6
I get 0v on 3V3_S5 but PP5V_S5 is stable at 5v.

Just in case it helps from U7650 voltages:

Pin 4 - 0.00
Pin 12 - 3.42
Pin 13 - 2.0
Pin 21 - 3.42
Pin 22- 3.2
Pin 23 - 13.1
Pin 29 - Which mentioned above is 5v

 

[2informaticos]

Then U7650 has all inputs required to generate 3V3_S5.
There must be broken solder/trace, or bad component on its 3V3_S5 section.
Pay atention to R7684/85/86 and C7689.
Did you change Q7680?

 

[Kappy]

U7650 along with Q7680 and Q7660 have all been changed and have come from a working board, so I know that they are all fine.

I don't think this is a problem with U7650 as my first post says 'Replaced U7800 still the same. Removed L7690 and the fluctuation is coming from the system side (pin 1)'

This problem is coming from somewhere else..

I've tried injecting 3.2v 1a into Pin1 of L7690 with it removed and my power supply is tripping on-off as if it is still fluctuating.

 

[2informaticos]

A buck converter can't be tested without its coil; no output voltage in such case.

If your lab PSU fluctuates, there can be a problem indeed.
Supposing you set the amp limit to max; not tripping because amp limit is too low.
1A limit doesn't help to diagnose.

Post actual diode mode to ground at L7690.

 

[Kappy]

I was worried about putting the amp to max as it's going to the PCH and I didn't want to cause any damage to it. I suppose the schematics says that max can be is 7.33A max so I will try this and use the thermal camera.