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Tried another RIO flex, same. Bluetooth works. L9501 is ok. Continuity is ok and diode mode to ground reading is 227. C9532/33, no short across.

So the IO board was completely dead with no usb whatsoever, after replacing it, the USB is back but wifi still not working, wifi card tested good on another computer. where should I look?

thank you. problem resolved, turns out wrong CPU from 2010 (i3-550) was installed on this late 2009 board (same socket but not compatible)

Without ram, the iMac will beep. Put the known good ram in, it will turn on with only LED1&2 and Fan. No Chime. No USB. Where should I start looking?

you said your board is "very similar" to 820-2850. But which board do you actually have?

do I put Q7530 back now for testing this ram remove?

same thing if Q7530 is removed. fan spin on and off. already tried known good bios too.

page 72: PP3V3_S3 = 3.5v pulsing PP1V5_S3RS0 = 1.61v pulsing PP1V2_S0 = 1.18v pulsing PP1V8_GPUIFPX = 1.73v pulsing PP3V3_S0 = 3.3v pulsing PP3V3_S0GPU = 3.24v pulsing PP5V_S0 = 5v pusling PP3V3_S3 and PP1V5_S3RS0 seem to be slightly higher.. is that OK?

PP3V3_S0GPU pulses between 3.3v and 0v. PP1V05_S0GPU pulses between 1.05v and 0v. PPVCORE_GPU pulses between 0.4v and 0v.

PP3V3_S5 is stable at 3.3v

Both PM_PCH_PWRGD and PLT_RESET_L pulse between 3.3v and 0v.

SMC bypass mode: high speed fan, but still on and off PPVCORE_S0_CPU is cycling between 1.1v and 0v [diode mode to ground is 063. Known good is 059] PPVCORE_S0_GFX is cycling between 0.86v and 0v [diode mode to ground is 139. Known good is 067]

yes sir. with fuse in place, 12.5v is now stable [ i dont think it was stable before]. Fan starts and stop and cycling... ALL_SYS_PWRGD cycling between 3.3v and 0v. How to do SMC bypass when the board is removed from the case?

sorry case closed. I flashed the gmux with the one from 820-2915 incorrectly.... reflashing gmux with 820-2850 works.

board has garbage distorted screen due to vram issue. Reflowed vram and screen come back OK. Backlight is OK. when close lid, the machine will go to sleep, but screen wont come back when lid is open. Actually there are stuff displayed on the screen and can be seen when shine light from apple...

Under normal situation, when the fuse is removed. pin 1 should have stable 12.5v right? [and it is not] May I ask what is the rationale behind this to put the fuse back instead of looking troubleshooting pin 1? [really trying to learn]

you mean F7040 instead of F7140 right? If so, both pin 1 and pin 2 diode mode reading is 377 when the fuse is removed.

Removed F7040. when plug real charger, pin 1 slowly drop from 12.5v and then cycling. When plug in fake charger, pin 1 dropped from 12.5v to 0.1v and stay there.. .

I think it turns out that D4610 is bad as I was getting intermittent result... removing D4610 then the USB port is working.... Let me replace it and see what happen

Verified that it booted up fine with linux on usb via the good USB port [with a usb hub and mouse/keyboard attached] and also connected LCD panel and display is OK. Diode mode reading to ground: L4600 (good port): Pin1&2: 440 Pin3&4: 435 L4610 (bad port): Pin1&2: 459 Pin3&4: 458