Date   : Wed, 23 Jul 2003 19:16:24 +0100 (BST)
From   : Peter Turnbull <pete@...>
Subject: Re: Old BBC B

On Jul 23, 17:20, Jules Richardson wrote:

> One of the things I got though was a very old-looking BBC (with an
extremely
> battered and yellowed case). The serial number on the case says 3619
on a
> sticker underneath, with an identical sticker on the system board. No
other
> serial numbers on the machine. Most of the chip sockets are white,
not the
> normal black plastic. The heatsink on IC6 is bigger than I remember
on other
> machines I've seen. Build date appears to be around December 1981;
the board's
> an issue 3 though.

Sounds like one of the ones ICL built.  Is it an orange sticker?

The ICL machines were the most unreliable, and Acorn only used them for
a short while.  Those white sockets are truly awful, so don't be
surprised if the machine needs all the socketed ICs cleaned and
reseated.

One little curiousity is that ICL decided to fit a trimmer capacitor,
to allow the system clock frequency to be fine-tuned in order to
prevent the time of day drifting.  Look for an extra small trimmer with
a brass screw near the main crystal in the north-east sector of the
board.  Sadly, they got the design wrong, and the trimmer has
absolutely zero effect!

> IC's 81 to 86 are just empty sockets, with pins 1 and 4 shorted on
IC86. My
> schematics aren't to hand to see what those should be.

Most of those are the timing chain for the floppy controller.  IC86-1
is the input to the chain, and IC86-4 is the QB output from that device
(it's a 74LS393) to pin 3 of the FDC.  The other ICs are:

IC81  74LS393
IC82  74LS10
IC83  CD4013B
IC84  CD4013B
IC85  CD4020B
IC86  74LS393

> One of the ROMs is in a dual-height package with a sticker saying
"RAM" on the
> top. A red wire also comes out of this module and is soldered to pin
11 of
> IC76, and there's also a black trailing wire going to an IC pin clip
(which
> currently isn't connected to anything). Any ideas what this is?
System clock
> module, or something more interesting?

IC76 is the ROMSEL "latch" (actually it's a loadable 4-bit counter, but
used as a latch).  Pin 11 is the QD (MSB) output.  I'd guess it's some
sort of NVRAM or clock/NVRAM, like an MK48T02 or similar, and the black
wire is meant to attach to the R/W signal, by being clipped to the leg
of some IC (other things that did this connected to IC33, pins 10, 11,
12, or 13, depending on whether they wanted R/~W (pins 10 or 13) or
~R/W (pins 11, 12)).

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York