In May of this year, my business partner gave me a not-dimming E550 machine after well repaired. Today I came across the same errors with the same machine. So I can say it’s like a common error issue. Thus I would like to share this with you guys.
Title: E550

Model of Motherboard: NM-A221

Error: no dimming, without stand by voltage 3V and 5V, total power supply is normal. When switch it on, voltage is normal. Sometimes it can be triggered by electricity, but power off after a while.
Repair: firstly I found it’s not stand by, normal main power supply, normal linear power supply, stand by 35V, no short circuit.
It was triggered by electricity. I switched it off. After a while, I switched it on, but it failed. Repeated but still failed. Check EC conditions, ensure power supply, reset, ACIN, LID, on/off, EC_ON are all normal.



Switched on EC_ON to supply stand-by chip, which is controlled by MAINPWON_EC.



After cutting off control,MAINPWON_EC has still no voltage. Check MAINPWON_EC further.









Followed one instruction mentioned on a repairing forum, tearing down PU2, but it’s still not working. Then tried to tear down PD2 and cut off temperature control, then it worked.



No need to change circuit, tear PD2 down directly.

Summary:
MAINPWON_EC is controlled by temperature control resistance. There will not be any trigger without MAINPWON_EC, even there’s stand by.