2017 iMac 27" LED 1 2 3 on, LED 4 dim and black screen

KMCC

New member
LED 1 2 and 3 on, chime good, LED 4 seems dim, black screen. Using ext. Cinema via TB2 and boot the cinema seems to get bright but nothing. Tried SMC, PRAM resets, and removed coin battery, still no change. no evidence of liquid damage or physical damage on the board. PS visual looks good to. Once booted to chime and LED 1 2 3 on w/ 4 dim (pic attached) the fan starts spinning higher then reboots. this repeats. Using a simple IR to scan for anything abnormal, shows cold board or nothing higher than 95F. This is on the bottom of the board by the little IR flat cable.
Is it possible to be a PS issue even though 3 of LED lights on? BIOS chip and boot ROM is damaged? Logic board? Recommendations to test and narrow down? Don't like using the "swap-tronics" method of troubleshooting plus don't have a spare PS or board available.
Original issue was customer kept filling up drive via emails dumping server and capacity errors so after removing files or running a clean up then reboot it never came back on. New 3.5" tried, new SSD (orig. 32GB SSD), new memory modules (these done by a different repair shop) but only resulted in $700 of charges for those "test parts".
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I hope I posted this correctly..
Mike4 led.jpg
 

KMCC

New member
After installing the screen and powering up the following: LED 1 2 3 and 4 all came on. After 7-8 seconds from power button pressed the 4th LED GPU goes dim to full lit. After 55-60 seconds it reboots. This is repetitive. Any ideas? Please..
 

KMCC

New member
I plugged in a USB tester (all 4 USB show 5v). I may sound dumb here but in case, USB C/TB3 (both ports don't show any voltage). I do see a backlight come up on the external via TB3 to HDMI (flat scree) and TB3 to TB2 Cinema Display. After chime both ext. screen setups brighten up like a backlight. Does this help? Not sure if the TB3 should read anything on back using the same Multi-USB Tester.
 

2informaticos

Administrator
Staff member
Read about how to properly check USB activity.

Seems that boards sends something on video output, if external monitor reacts somehow.
But we already know something goes wrong with video.
Need to confirm if board is still alive on that moment (USB activity), or gets stuck.
 

KMCC

New member
Wow.. thank you for the info!! Results.
Did the SMC reset and the fan did spin up to high so it was successful correct?
I connected only a apple wired keyboard and TB3 to HDMI TB3 to TB2 and tried booting into Safe Mode (did not) Diag Mode (did not), tried both video outs by going through the SMC reset and PRAM reset, to try and boot into safe mode or diag mode. All I get is 3 LED 4th dim (if panel on it's ON) and the ext. screen lights up. The keyboard does get detected since I can perform SMC but when trying to boot Safe or other the keyboard doesn't show on or Caps Lock light Num lock light doesn't show or work.
Is it possible the BIOS or EFI or Boot ROM chip is "out of commission)?
Just seems like this pristine system that's in mint condition should work with no evidence of anything discolored, broken, etc.
Should I try using compressed air to see if a component gets hotter than rest at boot up? I've tried IR gun but the entire board (bottom or facing out when installed) is room temp except around the Coin battery Pads. Just below is a 470 cap and this gets to 98-102 F while everywhere else is room temp. Also, after boot up or normal boot attempt, once the 3.5 or 4 LED's on and ext. screen lights up that after 60 seconds it reboots. This will continue at least 5 times then I powered down. But like clock work it reboots at 60 seconds.
All info all help all ideas -- I am very thankful!!!!
 

KMCC

New member
820-00609-A

I know from older macs like 2011 ish, that the GPU's failed often and some would try to reflow/heat to get them to work temporarily. Or use the Tiresias which I have to bypass the GPU onboard so is this "possible" GPU issue caused by overheating or high temp? There's not much on GPU resolutions for this model yet.
So if GPU then is it possible to have some protection cap or fuse to blow to save the GPU? LED 3 and 4 on with also something coming out of the video like backlight (i describe it as it brightens up the screen right after the chime).
 

KMCC

New member
820-00291
is the boardview/schematic using
Pic shows the area I'm describing
Below the Coin pads is a 470uF cap which reads find. The 470uF caps to the right 12 of them in a row, all read positive side to ground as short (continuity test). Is this normal? I only looked by the BIOS ic because it's the only place on the board bottom side that is hotter than the rest.
Possible that an issue here could cause no video out but backlight perhaps to work?

I'm sure I'm way off the mark here, but it's observations
 

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KMCC

New member
IMG_90811.jpg

Trying to show actual board and describe previous post. I don't know if I'm going in a direction right or wrong. circle seems more like it.
 

2informaticos

Administrator
Staff member
These caps are for GPU core power I suppose; read very low value as normal.

New BIOS is worth to try.
Sorry, I don't have a known good one for this board.
 

KMCC

New member
Just wanted to update from previous post regarding this 2017 iMac 27" 820-00609
I have not replaced the BIOS chip yet and wanted to get a better understanding of how a BIOS chip could cause this issue. New info: Cable replaced for rule out. If BIOS then what are symptoms of a problem BIOS chip? I also have now all 5 LED's on and bright. The screen shows backlight or brightness but no video. Once it chimes, all 5 LED's are on it will reboot and it repeats this every 59 seconds. (i stop watched it over 10 times with almost on the 59 second mark). I do get a white vertical line in the middle. This screen is one from another system that was in excellent condition but had a broken panel. Broken panel would work enough to boot, install an OS, or anything actually. So i know the screen works. What would cause an iMac 2017 to chime, turn all 5 LED's on, show screen brightness and a vertical but reboot every 59 seconds? No hands have compromised any component on the board. I did notice on the LVDS connector to be bent. Which was I'm guessing the original shop during panel removal and lost control while still connected and it bent. I tried seeing if anything under a scope looks bad but not enough exp. to say one way or another.
Attached pics are of LED's all lit and a zoom in on LVDS connector on the board.
If any ideas come to mind or anything I'm all ears (eyes).
Note: did the do SMC bypass and it to has all 5 LEDS's on, screen brightness like described but it doesn't reboot after 59 seconds!!
LVDS connector pic below shows the corner that was bent over.
Many thanks
 

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2informaticos

Administrator
Staff member
If video cable was disconnected (by error) when the board was working, GPU could be damaged now.
BTW, stop saying LVDS; this board uses DP (display port) LCD.

Turning off as an exact same time, could be caused by ME region corruption (BIOS).
But if board stays on in SMC bypass, is sign of SMC sensor problem.

Try to confirm consistent USB activity after chime; as in above linked thread.
 

KMCC

New member
Thanks for the response!!! I don't have a clue why I kept using LVDS?? Brain poof!!
I have a generic USB tester and ran the following tests.
Plugged into USB C TB3 port and voltage showed 5v and it came up on the tester screen just after pressing power and stayed on past the chime and didn't go out or drop off until the 59 second mark and it reboots.
Plugged into USB ports and same results as the TB3 port(s). I tested both TB3 ports and all 4 USB ports and results were the same for all 6 tests. I attached a pic of the tester plugged in both the TB3 and USB ports. Pic's taken after chime and I clocked it so that I take the pic around 55 seconds after power up. This process is very repeatable and all ports.
So does this mean it's possible the BIOS chip was disrupted/interrupted or messed up when screen pulled with power and LCD cable attached?
I have 2 preprogrammed BIOS chips I got from an eBay seller that programs per order I guess for this exact model. I have not tried replacing the chip as you mentioned earlier in this thread as try it because my smd solder skills aren't 100% trusted yet.
Looking forward to anymore advice or ideas. Thank you again ahead of time and I appreciated the help!!! Pics show which port it's plugged into.
 

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KMCC

New member
Thank you again. I hope this is what you are asking for.
Using a USB to SATA adapter that has a blue activity light.
I tested both USB c and USB 3.0 ports and here's the findings.

USB 3.0 port
Power on..
LED 1 on already
LED 2 on and as LED 3 turns on the Blue activity LED on the adapter comes on.
LED 1 2 3 on and then Chime.
after about 5-6 seconds the blue activity LED blinks a second then on then blinks a second then stays on. (like blinking blue LED twice then on) and no further change.

USB c / TB3 port
Power on
LED 1 on already
LED 2 on and Blue LED on and LED 3 come on.
Chime just about to finish and Blue LED activity blinks 2 times fast then goes out and stays out until it reboots.

This is repeatable.
Picture attached is the adapter and LED blue activity I'm using.

I hope this gives some insight into this issue.

Also, it turns on or try's to boots up as soon as it's plugged. If I force power off then it stays off and pushing the power button works as normal.
Just more info.
 

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2informaticos

Administrator
Staff member
May not be BIOS issue, but is worth to try; the only way to discard it.
Just be sure to make a backup of original BIOS first.
 
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