I have an Issue 7 with the standard no boot, single beep! The story so far...……...

PSU has been recapped and voltages are good.
Moved S25 SOUTH and hey presto! I have a 16K Beeb, so CAS1, socketed IC53 - IC60, is OK.
Memory test confirms no stuck bits high or low...
Used the 74LS04 trick so that I could boot to CAS0. I get a corrupted screen, Mode 0 allowed me to
run a memory test that returned 4. soldered IC63 was duly replaced!
Rebooted and the screen was still corrupt, weird, but it now passed the memory test, both at 0 and 255.
Current state of play is that it will still only boot with S25 at south. In S25 North mode I get a continuous
beep and the CAPS light comes on, followed by the SHIFT LOCK.
IC14 and IC15 were commented about recently so I have replaced them..
Any further pointers appreciated.
BP