Optima XM 4 zone issue?

Hi all I installed my optima some twenty years ago now and have regulary serviced it up untill the present date.The exterior was looking a bit shabby so I decided to replace it with a more modern sab siren.I realised when fitting the new it sounded for a few seconds then went into engineers hold of mode and would not come out untill I disconnected the sab.On further testing the panel was not putting out the 13v supply for the siren.I have checked the fuses removed the wiring etc but still no output,but have the voltage for the pir's and door contacts.I have now removed the main board for inspection and no electronic components look bad.I also inspected the rear of the board for bad solder joints and for visible burn marks but nothing?.I do realise that this is a old alarm and probably not cost affective to send it to a repair centre,so I would like to have a go myself.I have good soldering skills but I am not familiar with the circuit design nor do I know which components could be at fault.I would really appreciate some guidance in what components are likely to fail and be at fault under these circumstances.thank you to all who respond and all advice from engineers that use this forum welcomed.