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I removed the corroded U6420 and the buzzing is gone. Since this audio amplifier seems extremely hard to find, I decided to leave it as it is. Since there are 2 speakers on each side, I can barely hear the difference with one of the four nonfunctional.
Yup, you're right. Upon closer inspection, the liquid damage extended to left speaker amplifiers. I see some potentially corroded capacitors and inductances. Any tips on troubleshooting an audio circuit?
This 820-00840 board had liquid damage, mostly in the area of the USB-C circuitry. I replaced some corroded resistors and the machine starts up and functions normally. Except that any time an audio signal is produced, the left speaker buzzes audibly for a few seconds, then goes quiet. Startup...
This 2015 Air board generates all power rails, Vcore, ALL_SYS_PWRGD, and PM_SLP_S4_L. Fan spins but does not POST, chime, or start up. The only abnormal signals I noticed are SMC_PROCHOT = 3.4 and CPU_PROCHOT_L = 0. Any ideas?