Date   : Mon, 08 Nov 2010 02:51:16 +0100
From   : rick@... (Rick Murray)
Subject: BBC B micro repairs

On 07/11/2010 22:32, Alan Griffin wrote:

> When I turn it on I get the inital short bleep, but not the second
> one, and no "Basic" appeared on the screen. But there is a flashing
> cursor at the top left, but the keyboard doesn't do anything.

Okay - we're past reset, with the video initialised, but we've died 
prior to the BBC 32K message.

Are you sure the keyboard does nothing? While that is probably correct, 
it is perhaps worth pressing Caps Lock to see if the LED goes on and 
off; plus Ctrl-Break? Isn't Break handled specially, or was that RISC OS?

You have swapped the keyboard, so it isn't oxidation on the pins. 
However this fault can be caused by keyboard issues. Take a close look 
at the keyboard connection on both the keyboard and the main board - no 
contamination (tea, etc)?


There is a 6522 just below the 6502 (just left of the ROMs) and there is 
another 6522 to the left of the 6502 next to the floppy controller.
Try swapping them over.
You most likely don't *need* the user VIA (the model A doesn't have it) 
but you very definitely need the system VIA!

If this does not do anything, pull out:
   6522 User VIA (below the 6502).
   ?7002 ADC.
   6850 Serial ACIA.
   6854 Econet chip, if fitted.
   1770 or 8271 FDC, if fitted.
   Every ROM except the MOS (leftmost slot) and BASIC (wherever).

You've pretty much just downgraded to a model A. It will boot like that, 
you ought to get into basic, but obviously stuff like the floppy drive 
won't work!

Does this get you anywhere? If so, put them all back, one by one, until 
it fails.


That said, the flashing-cursor-beeeeeep is usually a faulty keyboard 
(which you have already tested) or a fault with the internal "slow bus" 
on the system VIA. With a little luck, swapping VIAs will coax life into 
the machine.

If not, we're looking next at memory and logic. Try altering link S25 
(if I remember correctly, it is just below the RGB socket) to be in the 
SOUTH position. This will render your Beeb a 16K model. [back to NORTH 
for 32K]

Sadly we can't do a lot if the fault is in the 16K that we can't map 
out, or if it is the memory decode glitching. Your description of the 
lines on the screen make me suspect it is memory/decoding, but this is 
complicated, so let's keep fingers crossed that it'll be something simpler.



By the way:
 > I thought it must be a dry joint, so I took out the circuit board,
 > and resoldered the pins of all the sockets.

You seem to have been lucky - but for others, it is best to diagnose and 
attempt to narrow the problem down before trying drastic actions such as 
this. One tiny solder 'bridge' on an insignificant-looking chip could 
have important repercussions later on.


Best wishes,

Rick.

-- 
Rick Murray, eeePC901 & ADSL WiFI'd into it, all ETLAs!
BBC B: DNFS, 2 x 5.25" floppies, EPROM prog, Acorn TTX
E01S FileStore, A3000/A5000/RiscPC/various PCs/blahblah...