How good are your electronic skills? :-)
Check on pin 3 of the keyboard connector (KBreset line) which should go low when the reset combination is pressed.
If not, there is possibly a keyboard problem.
Also, if you short this line to ground, it should definately cause a system reset.
The KBreset line goes into U5 (Gary) on pin 5 I think. Check the continuity of the circuit between U5 and the keyboard connector.
When U5 sees a reset state on the KBreset line, it initiates a system reset condition.
I think that's correct?? Though my memory may be permanently damaged after too many beers last night :-P
A500 schematics are here if you need them:
http://amiga.serveftp.netAnthony.