Optiplex 790, no post, blinking yellow light

From the manual- Power off computer, leaving the computer plugged in. Press and hold the power supply test button on the rear of the power supply unit. If the LED next to the switch illuminates, the problem may be with your system board. If the LED next to the switch does not illuminate, disconnect all internal and external peripherals, and press and hold the power supply test button.If it illuminates, there could be a problem with a peripheral. If the LED still does not illuminate, remove the PSU connections from the system board, then press and hold the power supply button. If it illuminates, there could be a problem with the system board. If the LED still does not illuminate, the problem is probably with the power supply. what do the diagnostics lights show? https://www.dell.com/support/article/us/en/04/sln284978/a-reference-guide-to-the-dell-optiplex-diagnostic-indicators?lang=en#Beep_codes

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how to fix

No sign of life from original MB, only fans would run while holding PSU test button in.  Changed to identical good MB and having same issue here, but without the beeps. 

Plug unit in, power button blinks steady amber, push power button in for ten seconds, light goes out.  Push power button again, starts steady amber blinking.  Computer will not boot with power button.

Computer will boot to BIOS if holding PSU test button in.  Green PSU test light stays on when pushed in.  All cooling fans working.  Changing relevant BIOS settings have no effect.  As soon as PSU test button is released, power goes off.

Please help.  Thanks.

No relevant answers to this issue in link to reference guides.

Just for followers, for me this "did" mean bad psu, I blew out the dust on its intake grill and blew through the thing itself and it started working again FWIW :)