Thought I'd share own experience of fixing booting problem with my BBC Model B+.
After having it open and fitted the S26 to add chroma to composite video out, the computer would no longer boot. I have checked everything I could find on boot faults, from PSU to poor IC sockets to bad keyboard connection (wut?). Voltages OK, connectors and sockets are fine, but still only a blinking cursor and single sound on power on.
Long story short, I came across a post on Vintage-Radio.Net forum (http://vintage-radio.net/forum/showpost ... ostcount=4):
I gave my LEDs a slight wiggle with my finger and whoa! The computer booted just fine. There was indeed something related to those LEDs. Checked the keyboard PCB copper and found Shift Lock LED had loose connection and some sloppy solder work over what appeared as snapped track.I also had a similar problem on my old BBC B, which was caused by a duff connection on one of the LEDs (Cassette Motor, Caps Lock and Shift Lock) - I was never quite sure why this caused the fault, but it occured twice and both times, resoldering the broken joints on one of the LEDs solved the problem.
Neil.
Fixed it with a piece of wire, works like a charm now