Okay, the default programming for op3 is for a speaker and you have a sounder connected, so that will not work and is why you don't see 12v on that connection - the speaker output is not driven with a DC voltage.
The op1 trigger voltage is being held high, so I would suspect faulty wiring or siren. The transistor is trying to pull the op1 output low, but the wiring or faulty siren is holding it high and the transistor cannot drive enough current to pull it low (it would blow a fuse if it could). Remove the op1,op2 and op3 connections for now. Take the positive (red) of the internal siren and connect to +12v. Take the 0v (black) of the internal siren and connect to op1. Now test the bell. If it works, then the panel is fine.
You said that you tested the siren by closing the lid tamper switch, applying 12v and 0v, then connecting 0v to the trigger and it did not sound. If that's the case, then the siren is faulty. Double check this on the bench and if faulty, get it changed out. The AG6 are really poor volume wise anyway. The latest texecom sirens are much better.
The op1 trigger voltage is being held high, so I would suspect faulty wiring or siren. The transistor is trying to pull the op1 output low, but the wiring or faulty siren is holding it high and the transistor cannot drive enough current to pull it low (it would blow a fuse if it could). Remove the op1,op2 and op3 connections for now. Take the positive (red) of the internal siren and connect to +12v. Take the 0v (black) of the internal siren and connect to op1. Now test the bell. If it works, then the panel is fine.
You said that you tested the siren by closing the lid tamper switch, applying 12v and 0v, then connecting 0v to the trigger and it did not sound. If that's the case, then the siren is faulty. Double check this on the bench and if faulty, get it changed out. The AG6 are really poor volume wise anyway. The latest texecom sirens are much better.