You must be more informative.
1) What do the User & Service Manuals say about this ?
2) What does CT & OC stand for ?
3) Is this just a status alert or does the scope halt?
4) Any other faults ?
5) Maybe you’re just hoping a 2456B owner will jump-in and say : Go to PCB A4 and change IC #3 ; w/o having to do any work. (Do NOT take the cover off and do wild-probing ! )
6) 95% of us cannot help, with such a vague report. We do want to help ; when we help you, we help all of us learn…………
Jon Batters, Grants Pass, Or.