[Shadowfire]

The fact that he's already tried 3 different keyboards that work in other motherboards, rules out any problems with the keyboard itself pretty definitively.

Since he apparently has no problems with power-on reset (i.e. the machine comes up every time), we can assume that the U14 (555) timer circuitry and Gayle are operating correctly.

This leaves R624 and U13 (6570 keyboard controller) as the remaining possible culprits (U13 far more likely), unless you have the U36/R623/Q622 devices installed on the machine, of which the most likely to fail is U36 due to potential ESD from the keyboard.

Pin 43 on the keyboard IC is the system reset line, and is almost certainly an input on the keyboard IC. The keyboard controller resets the machine by asserting pin 34 low, which triggers the 555 to pulse the KB_RESET line, which feeds into Gayle (which should assert the system _RST line). Based on the description of the symptoms, I'm willing to be that you don't have R623/Q622 installed on your machine, and the problem is that U13 is toast.

Edit: Wait... you already replaced U13. Please let us know if your machine has R624 or R623 installed.

If R624 is installed, it should be 0 ohms, so check for continuity.

If R623 is installed, check that when you press Ctrl-Amiga-Amiga, that as long as the keys are pressed:
U36.1,U36.2, and U36.13 are LOW (<.6V) and U36.12 is HIGH (>.6V) (validates U36 is working)
If the above passes, check Q622. Q622.3 should under normal operating circumstances be pulled to 5V by R611 (which is 1Meg). Which giving the machine the 3-fingered salute, Q622.1 (base) is pulled high by U36.12, which activates Q622, which discharges C611 and triggers the 555. When you let go of CTRL-A-A, the usual power on reset timings generated by the 555 timer and R612/C612 take over, and these are known good (unless you are having cold boot problems that you aren't telling us about).