With a meter on ohms, or a continuity checker. Put the negative lead on the cathode of the SCR ( the large post ), and touch the positive lead to the anode (the base ), the meter should not read ohms or if using a checker, it shouldn't light. if it does it is bad. Now clip a jumper to the positive lead while holding it on the base and momentarily touch it to the gate of the SCR, it should read ohms and stay on, or it should light and stay lit using the checker.
You really cannot troubleshoot this controller by holding in the P switch. you can try briefly jumping out the comparator at terminals 5 and 6 on the 12ea board (comparator Bd.)
That may allow you to see something. Do not leave the comparator jumped out, this is just for testing purposes. It sounds like you have a bad board, maybe the 1ea or 2ea Op Amp board, or something like a six pulse firing board. Assuming you have checked the contacts on the P switch and the brushes on the motor commutator.