Date   : Fri, 12 Aug 2011 08:16:53 +1200
From   : car@... (Chris Reece)
Subject: Advice on restoring a dead Beeb?

Hi, chaps.  Long time lurker, first time poster.

I've a BBC B, given to me by a friend and member of this list.  He's currently 
on vacation, or I'd be picking his brains in private.  Before he shipped 
it to me down here in New Zealand it was verified in good working order.
 During transit, or perhaps during my reseating of the major ICs when it
arrived, it stopped working.  (I took reasonable precautions, but you never
know.)

It's failing in what I gather is a fairly common mode -- during reset.  
I get the first "random buffer content" beep, not the second tone.  I believe 
I've run through the advice Pete gave me & that I gathered from elsewhere
when I first looked at this some months ago, but it's only now I've managed
to borrow a logic probe and find some time.

The power supply looks good and measures healthy -5V, 0V and 5V at the connections 
to the board, and 0V and 5V at a number of ICs around the board.  According 
to my borrowed probe, the output of the 555 (pin 3, RST) is starting high 
then holding low.  The input to nearby IC33's pin 3 is the same, as you'd 
expect, and the output of the corresponding not gate (pin 4, !RST) is starting 
low then holding high.  Looks the same at the 6502's pin 40.  Watching the
keyboard lights, about 4 times out of 5 they're doing what I always remember
them doing: starting CAPS on, SHIFT off, then switching CAPS off, SHIFT on.

Removing the MOS prevents the light flash sequence (no surprise.)  This 
is how I identified MOS from BASIC as the only labelled ROM in DFS.  So I'm
guessing that the reset is starting but not completing.  I've tried removing
every socketed IC in turn, also swapping the 6522s, running with just MOS,
MOS & BASIC, MOS & DFS.

I'm going to take a stab in the dark that it's a corrupted MOS or a sick 
6502.  Former seems most likely?  I haven't established that the clock's
good, I've no scope, but I figure that the light sequence is evidence that
we're stepping through the first few instructions of the initialisation.

Any advice or comment appreciated.

Cheers,

Chris.