Chosen Solution
Around 2017 iMac was upgraded with an Angelfire SSD drive in primary position using OWC NewerTech AdaptaDrive bracket and OWC In-line Digital Thermal Sensor. I used the opportunity to disassemble more to access the SPARE SATA connector and connected a SATA to eSATAcable, pulling it out through the memory door. The iMac was working well for 2 years. It stopped working when wife was using it, abruptly black screen but able to restart once and again several days later but since then, upon restart, it sounds the start-up chime looping in 3 sec distance. Tried resetting RAM and moving it around, Command-Option-P-R keys, Shift+Control+Option keys with Power, no help. The screen stays black. Ultimately, I have to hold the Power button to force it shut down. I was looking on ebay, realized sale prices are around $400, nonworking $150. Today finally found time to go back to it and explanted the SSD drive which is readable so it not the problem, but rather hardware. Having the data, shall I just sell it as-is with no security updates for High Sierra after Nov 2020 :-( or take it to a local Apple certified repair shop for diagnostics ? But then I am looking for the dig fee plus repair work plus parts cost. Is there any more troubleshooting I can do on my side ? Update (08/22/2020) EDIT: I corrected the title and some text to be more precise about the sound.
Dan, thank you for the hint. With screen off and starting this time from the original Apple drive I kept on shelf (to rule out the role of the SSD), the problem is still present. The LEDs are up, initially the first 2, with the first sound also the 3rd. I can’t see the 4th through the holes on the bottom but it should not be UP anyway if the screen is off. Can the GPU still be faulty despite the 3rd LED is ON ? Reading the web, GPU seems to be an Achilles heel of this iMac. People even upgrade with non-original video cards, flash them and are able to run Catalina, pushing past the High Sierra limit.
The video card was faulty (despite the third LED on MOBO was ON) and a replacement made the iMac working again.
For starters Apple won’t service this system as its too old for them ;-{ Vintage and obsolete products Your system offers some onboard diagnostic LED’s
LED 1 - Indicates that the trickle voltage from the power supply is detected by the main logic board. This LED will remain ON while the iMac is connected to the AC power. The LED will remain on even when the computer has been shut down or put to sleep. The LED will turn off only if the AC power is disconnected or the power supply is faulty. LED 2 - Indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly. LED 3 - Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement. LED 4 - Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement. Let us know what you discover!
