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Acorn BBC Micro model B+ 64K

My third BBC Micro turned out to be a (slight) roller coaster ride. Again, I bought this one from eBay as a BBC Model B and basically for testing parts for my second one (or vice versa).
When it arrived I found that it'd been poorly packaged having not been put in a box, but simply wrapped in multiple layers of bubble wrap. Also, the plug had been packaged with the metal "prongs" against the computer case so that the machine had quite a few deep scratches (see below for a picture). It also hadn't been cleaned so was full of dust.

(click image to enlarge)

Now, this is one of the many things that annoy me about (some) eBay sellers: they 1) don't package items properly and 2) don't even bother to wipe the item down. Any time I sell anything I make sure the item is as clean and as well packaged as possible. Argh! lol.
Anyhoo, I then decided to have a look inside. For the first second or so I had a feeling of "that doesn't look right..." and then it twigged. Paged ROM sockets were located centre left on the motherboard and the date on the board itself said 1984. So... a much rarer BBC model B+ 64K! Excellent! Not bad for £23.01 (+£14.99 postage & packaging) :).

The model B+ computers are roughly the same as the model B's (main RAM size, ROM size, I/O and physical look).
However, they have a newer CPU (Rockwell 6512A 2MHz) which has a number of bug fixes which affected the 6502A and extra addressing modes. They also have an extra 32KB of RAM split into 20K of 'shadow RAM' for the display and 12KB of 'sideways RAM'. The shadow RAM can be used by the display circuitry instead of main RAM, which then releases a large amount of the main RAM back to the system for user programs.
The sideways RAM mimics a Paged ROM, but allows Paged ROM images to be loaded from disc or tape into it. This can make the computer much more versatile.
Another difference is that the B+ has Basic version 2 which, rather than being in its own Paged ROM, is combined with the OS (also v2.0) in one 32K ROM. The Basic ROM is then seen as a logical 16K paged ROM with the OS part mapped to the &FC00-&FFFF memory range.

Here's a few pictures after I cleaned the machine up a little:

ModelB+_closed ModelB+_opened ModelB+_loading ModelB+_galaforce2
(click an image to enlarge)

One oddity of the B+ is that the keyboard looks like an older style one. Click on the image for a closer look. To me it looks like an old model A style one. [update] It's a type 2 "401KBM-006-90R" and was manufactured circa 1984 - so legit by the looks of things.

(click image to enlarge)

Another oddity is that there is no disc interface Paged ROM installed and upon further examination there is no disc interface chipset either. The model B+ was supposed to have shipped with the then newer 1770 disc interface as standard. It makes me wonder if someone has removed them or (more likely considering the lack of solder around the missing IC holes and complete even fill on others) that this machine was purchased in kit form and home-assembled. Here's a photo of the relevant PCB areas. Notice that ICs 16,17 (used by 1770 controller), ICs 1,2,3,4 + R1,2,3,5,6,7,8,9,10,14 (used by the 8271 controller) and ICs ,8,15 + R1,5,6,7,14 (used by both) are missing.

ModelB+_dfs1770chipset ModelB+_dfs1770chipset ModelB+_dfs1770chipset
(click an image to enlarge)

A final thing to note was that, predictably, the power supply unit on the B+ developed a fault on the third time that I turned it on. Lots of smoke and a nasty sweet-electrical smell. Click HERE to read how I resolved it.

I've now added a WordWise Plus Paged ROM which I had lying around. Despite having it's legs crushed and then straightened out it works fine.
I'm now thinking about moving the MMC card adapter and ROM from my B to my B+. Or I could buy a 1772 disc chipset upgrade kit...

Update 26th April 2014

After having a look at serial numbers thread on the stairway to hell forums, I ended up over at Chris Whytehead's excellent website where upon looking up the serial number of my B+ it turns out it didn't include a disc interface out of the factory. Mystery resolved!

I've moved my MMC Paged ROM and adapter to my B+ and also decided to upgrade the machine to the 1770 DFS (disc filing system). Click HERE for the blow-by-blow non-stop upgrade action :D.

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