Date   : Tue, 16 May 2006 23:10:17 +0100 (BST)
From   : Pete Turnbull <pete@...>
Subject: Re: Spotting an upgraded model A

On May 16 2006, 10:18, Kris Adcock wrote:

> I've been given a BBC B and a Master - both boxed, with manuals. The
original
> owner is of the impression that the Beeb is actually a model A that
has been
> upgraded to a B. So I thought I'd check here and ask if anyone knows
the
> telltale signs. Briefly:
>
> 1. The manual that comes with it is a "provisional copy", not the
standard
> spiral-bound user guide.
> 2. The motherboard is marked as "Issue 3".

The only place I've seen those is with very early machines that were
part of the first batch issued to schools.  That doesn't really jibe
with an Issue 3 board, though.

> 3. Half the RAM ICs are socketed - not soldered directly on.

This suggests a Model A.  If you were to take out the PCB, I suspect
you'd detect signs of the five IDC connectors at the front having been
soldered on later, along with the 6-pin DIN RGB socket and 5-pin domino
serial socket.  I expect you'll find some other things socketed that
would *usually* be soldered directly to the PCB on a B:  the serial
chips near the serial socket, the 74LS163 that selects ROM banks, a
couple of 74LS244s and a 74LS245 that drive the 1MHz bus, and a 74LS00
near the back left.  Those are most of the things not fitted to a Model
A, which also lacks the uPD7002 ADC chip and the User VIA, but they're
socketed on a B as well as on an upgraded A.

Incidentally, if the upgrade is done properly, there is no difference
between an upgraded A and a B, except that some devices are socketed
rather than directly soldered.

> 4. There are one or two board modifications done (a short wire or two
going
> from a point on the board directly to a leg on an IC, for example).

That just means someone did the proper fixes for things like the
printer port mistake on an Issue 3 -- the one you're referring to is
probably the leg of a 7438 near the left edge of the board.

> 5. The plastic keystrip just says "BBC Microcomputer" rather than
specifying
> a RAM size.

Original Beebs were all like that.  It's the B+ that has the memory
size on the ruler.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York