tbaker2500
New member
21.5" 2017 iMac, A1418 EMC 3069 with 820-00597 logic board.
Background: computer worked fine in 2020, wasn't used until a month ago. First boot showed these symptoms:
Symptoms:
Power is applied. Diagnostic lights 1,2,3 all light up properly every time.
1/3 of the time LED's 4 and 5 light up and display turns on, and it proceeds to boot. Boot continues for between 5 seconds to 5 minutes, before the display shuts off, LED's 4 and 5 turn off, and the computer hangs.
The other 2/3rd's of the time when powering on LED's 4 and 5 never light, and computer is hung, no startup sound, no display.
When the computer is hung, USB is still powered, fans are active, but computer is unresponsive. On rare occasions display is still showing the OS, but mouse and keyboard activity don't do anything.
Tests done:
Booted off external HDD. Computer is usable for random time up to about 5 minutes, when it hangs.
Removed SSD blade and HDD, no change.
Dropped to one stick of RAM, swapped sticks, no change.
External monitor only works sometimes. Sometimes internal display works, external doesn't.
Disconnected internal display, and behavior is exactly the same, except for the following:
LED 4 lights up dim and 5 does not come on when "working", and 4 and 5 are out when not working. Expected behavior for no connected LCD I believe.
My assumption is something in the GPU area. I've tested ppvcore_s0_gpu and it starts out around 0.807v, sometimes dropping to as low as 0.756v when working, always hanging out around 0.807-0.811v when failing to boot.
12V rail seems solid. All voltage rails I've tested show no voltage dropout when the computer locks up, so I don't think the power supply is flaky.
No visual signs of leaky caps. IR camera shows no hot spots beyond 32c, but its resolution isn't good enough to pick up the small caps.
Any ideas on ares to look at more closely? I'm somewhat new to troubleshooting Mac boards, but very experience in PCB repair otherwise.
Thanks in advance,
Tom
Background: computer worked fine in 2020, wasn't used until a month ago. First boot showed these symptoms:
Symptoms:
Power is applied. Diagnostic lights 1,2,3 all light up properly every time.
1/3 of the time LED's 4 and 5 light up and display turns on, and it proceeds to boot. Boot continues for between 5 seconds to 5 minutes, before the display shuts off, LED's 4 and 5 turn off, and the computer hangs.
The other 2/3rd's of the time when powering on LED's 4 and 5 never light, and computer is hung, no startup sound, no display.
When the computer is hung, USB is still powered, fans are active, but computer is unresponsive. On rare occasions display is still showing the OS, but mouse and keyboard activity don't do anything.
Tests done:
Booted off external HDD. Computer is usable for random time up to about 5 minutes, when it hangs.
Removed SSD blade and HDD, no change.
Dropped to one stick of RAM, swapped sticks, no change.
External monitor only works sometimes. Sometimes internal display works, external doesn't.
Disconnected internal display, and behavior is exactly the same, except for the following:
LED 4 lights up dim and 5 does not come on when "working", and 4 and 5 are out when not working. Expected behavior for no connected LCD I believe.
My assumption is something in the GPU area. I've tested ppvcore_s0_gpu and it starts out around 0.807v, sometimes dropping to as low as 0.756v when working, always hanging out around 0.807-0.811v when failing to boot.
12V rail seems solid. All voltage rails I've tested show no voltage dropout when the computer locks up, so I don't think the power supply is flaky.
No visual signs of leaky caps. IR camera shows no hot spots beyond 32c, but its resolution isn't good enough to pick up the small caps.
Any ideas on ares to look at more closely? I'm somewhat new to troubleshooting Mac boards, but very experience in PCB repair otherwise.
Thanks in advance,
Tom
