I've literally replaced all but the front ports on my 5,1, swapped out the PS and the backplane. I'm thinking I bought a either a bad PS or BP or both. I have been very careful on all steps and have followed all best practices on this forum to rebuild my machine to top specs, I have a EE background, I'm meticulous. It just won't start consistently, both the original and the new existing. Some where in that mix is a bad PS or BP. Here's the latest symptoms.
Bypassed the button and used the pads to start same symptoms (see steps below)
Did a careful minimum hardware startup as per Apple's tech manual.
LED STBY lamp is on when you plug it in. When it does start, all LED's are as expected.
So I'm starting each time by killing 120 VAC power, (using a power strip switch) and a USB Drive w/LED on any USB port for a visual. This sets a baseline for observation, and a working computer should always be able to start this way.
1. Momentarily push the power button, PS relay clicks, only the USB-LED lights up, nothing else.
2. Seven seconds or so, the PS relay clicks off the USB-LED goes out.
3. Repeat a few times, maybe 4 or 5 times then . .
4. Momentarily push the power button yet again, PS relay clicks, only the USB-LED lights up and STAYS ON, the PS relay does not click off, no other devices power up.
5. Hold in the button this time, LED goes out after six seconds or so, I keep holding another 10 seconds and let go.
6. Momentarily push the button, the computer starts up and runs like BUTTER, smooth and fast.
Damn . . I'd order another PS or BP, but man this is really perplexing. PS I do have a good test bench if needed.
Thank you for considering . .