Vostro 220 No Powering, Steady Amber Light

This Vostro 220 mini tower has been upgraded to Win10 + 4GB Ram + SSD + Q8400(4C4T) CPU and has been working OK despite its age. Since a few weeks ago, it started having difficulties to power up (needs to push power button a few times). It could not start up yesterday until a full static electric discharge (disconnect & holding power button for over 30 sec).

After this last run, it could not be powered up any more, showing steady amber light, no any fans (PSU, CPU, Chassis), no green light on mobo. However, PSU green light is on.

The following has been tried and it still gave the steady amber light:

1. Plug directly into a different power socket

2. Disconnect & hold power button for over 30 sec

3. No obvious problem with USB on chassis

4. Removal of all peripherals. No SSD, keyboard, DVD, mouse, display, video card. Just mobo, chassis connections, CPU, single Ram.

4. Re-seating of Ram, front panel I/O (did not try direct short circuit of power button)

5. Replacement of good PSU, CPU, Ram

Advice needed:

1. It seems to me the problem is with mobo. Are there any other possibilities? Any diagnosis tests that I can perform to further confirm the problem is with mobo?

2. If the problem has to be with the mobo, are there anything I can do to fix the problem short of a complete mobo replacement?

Many thanks in advance!