Date   : Sat, 19 Apr 2008 08:49:52 +0100
From   : ken.lowe@... (Ken Lowe)
Subject: Reviving a dead BBC+

Mike Tomlinson wrote:

> I'm pretty sure I have a spare 6512 somewhere.  I used it in a BBC B so
> that I could run the Master BASIC4 ROM.  You're welcome to it if you
> cannot find a replacement.

Sorry for the delay in responding to everyone, and thanks Mike for the offer 
of a replacement 6512.

I made a major breakthrough last night. The story so far...

I managed to get hold of the following replacement ICs: 1 x 6845, 1 x 6512 & 
2 x 6522 (thanks Sprow for these). I swapped these out, but they never made 
any difference.
Next in line for replacement was the RAM. The B+ uses 8 off 120ns 4864s 
(HM4864AP-12). I managed to find some 150ns 4864s (HM4864P-2) but I wasn't 
sure if these would work.
I replaced one bank of 4 memory chips (IC55,56, 60 & 61) with the slower 
4864s, and the B+ sprung into life with the Acorn OS 64K, Acorn 1770 DFS, 
BASIC, > power up display
I carried out a number of tests, and everything seems to be in order :D.

Whilst digging around for spare parts, I came across an old Brainsoft 32K 
sideways RAM module lying in a box, and decided to try getting this working 
with the B+. The Acorn 1770 DFS (v 2.2) expects to find RAM in pages C/D 
and/or 0/1 (assuming Basic is in pages 14/15). If the DFS finds RAM in any 
of these pages, it will update the power up message with Acorn OS xxxK, 
depending upon how much memory it finds.

Unfortunately, these four pages are not mapped into any of the ROM sockets 
and it was necessary to find the page select signals elsewhere on the PCB. 
Fortunately, these were relatively easy to find and I now have a B+ which 
reports Acorn OS 96K on power up.

For those who are interested, I have the sideways RAM plugged into the 
socket that normally controls pages 4&5 (IC57) with the exeception of a few 
pins. Pin 1 of the RAM chip (A14) has been wired out to pin 14 of IC45 
(signal QA - which is used to select either the upper or lower 16K bank of 
the 32K RAM). Pin 20 of the RAM chip (notCE) has been wired out to pin 9 of 
IC46 (page C/D select). Finally, pin 27 of the RAM chip (R/W) has been wired 
out to pin 24 of IC84 (write).

I intend to get hold of another 32K RAM chip and build a small circuit borad 
that will hold both 32K RAM chips, and a single 32K ROM. This circuit board 
will then plug into one of the existing ROM sockets, with flying leads to 
the three external points highlighted above, and a fourth to pick up page 
0/1 select. This should provide me with a fully expanded 128K B+ with all 
ROM sockets still available.

Ken.