SIEL EX80 Expander

Hi to all.
I have a SIEL EX80 expander module with a problem. Quick history. I bought it 25 or so yrs ago non functional as someone for some reason, took the processor out. I have replaced the processor, and built a PSU for it. Lo and behold it functions - to a fashion. The main problem is there is a constant note sounding when switched on, and sometimes it will and sometimes it wont respond to note messages.

Is anyone able to help, or point me to a possible solution.

Thanks in advance.