Date   : Sat, 13 Aug 2011 01:34:50 +0100 (WET-DST)
From   : BBCMICRO@... (Peter Coghlan)
Subject: Advice on restoring a dead Beeb?

It's great to get some BBC micro related discussion on the list!

>
>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.)
>

Could you have managed to bend any pins under the ICs when reseating
them or damaged an IC socket? I assume all the ICs went back in the
right way around and in the right sockets!

>
>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.
>

This is a pretty common failure mode. Unfortunately, almost anything
could be responsible. Can you let us know what advice you have followed
so that we know what you've tried or eliminated already?

>
>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.
>

That's a good start. However, when I power up my model B, the caps lock LED
comes on and stays on and the shift lock LED flashes briefly before remaining
off.

>
>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.
>

Except in really odd cases, the MOS lives in the fifth from the left ROM socket.
You need the MOS (in the correct socket) and a language (generally BASIC) in
one of the other ROM sockets. You can leave out the user 6522 (IC69) for the
moment but the system 6522 (IC3) is essential.

>
>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.
>

I'd be surprised if the MOS is corrupted. Mask ROMs are pretty reliable.
If the machine worked properly before being transported and having its ICs
reseated, I would be much more inclined to suspect a connection related
problem somewhere.

I can't recall if the keyboard LEDs come up randomly or not. It may not be
safe to assume that the MOS is running because the lights behave differently
with it plugged in. Do the CAPS LOCK or SHIFT LOCK keys have any effect?

What do you get on the screen? Blank with a mode 7 sized flashing cursor in
the top left?

Check the keyboard cable. Bad contacts here can cause interesting problems.

I would also suggest using the logic probe some more around the CPU. Check
whether there is any activity on each data bus line. Are any of the lines
stuck high or stuck low? Also check on the other side of IC14. Then check
for activity on the address bus lines. Some of them may be permanently
high or low, giving an indication of where in the memory map the CPU is
executing or attempting to execute. Make sure there are clock pulses at
pin 37 and pin 3 and check for activity on the R/W line (pin 34). 

Check that the IRQ and NMI lines (pins 4 and 6) are high at least some of
the time. If either is stuck low, there will be problems. If NMI is stuck
low, ensure the 8271 disk controller and/or 68B54 Econet IC are present
and correct assuming they were originally fitted, as well as IC27 and IC91.
If there is no 8271 (and there is no alternative disk controller present),
jumper S9 should be fitted. If IC91 is not present, jumper S2 should be
fitted.

Regards,
Peter Coghlan.