BBC B Boot problem, LED related. Fixed!

[tnt23]

Hi all,

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 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.

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.

LED solder.jpg (40.67 KiB) Viewed 1456 times

Fixed it with a piece of wire, works like a charm now

[Coeus]

I must admit I couldn't think how the LEDs would affect the boot process but they can certainly be a symptom - like when I first tried powing my old beeb up and had a double row connector for the keyboard (a mod) on the wrong set of holes - no LEDs alight, didn't boot.

The reason for non-booting is, I think, because the OS tries to read the DIP switches which are on the keyboard during the boot process. Any fault on the keyboard PCB that makes the not work can stop booting.

[tnt23]

My guess would be that the track LEDs share, be it Ground or +5V, could get broken and thus spoil the reading of those switches.

[cmj6502]

I have some vague recollection that no keyboard connected is seen as every key pressed and all DIP switches on (which gives mode 0) and as shift and control are seen as pressed and the MOS interprets that as 'don't print the next line'.

Why the cassette motor LED would affect boot i have no idea.I always assumed it was connected to the relay but maybe not. Maybe if it fails short circuit and pulls a line down (or up)?

[raidersan]

I have had the exact same issue with the cassette led but I troubleshooted without knowing the led might be problem so I came from the opposite side of it. I realised the IRQ signal was kept low by the IC3 (6522) and that was the case with 2 other 6522 I had, so the problem was not with it. Looking at what IC3 manages, the keyboard was one strong candidate and the beeb is well known for needing the keyboard to boot. I then saw that all signals on the 74ls251 next to the connector were missing, including VCC. tracing the 5v line led me to the led light and the broken trace around the pad of led. I had 12 M ohms between the two... A bodge wire around the pad solved the issue

[atsampson]

That sounds like a good candidate for the Fault Finding Index - especially as it's come up multiple times.