The BBC micro went through seven motherboard revisions, although issues 5 and 6 were never produced so exactly what changes they brought in remains a mystery. The issue 1 board is so fantastically rare that even the Service Manual does not deal with it. The most commonly found types are issues 4 and 7, not surprisingly, but over the years I have been very lucky to build up a collection of at least one of each of the main types.
This is a picture of my only issue 2 Beeb. It is my earliest Beeb and works well most of the time but does throw a tantrum now and again but let's face it, it's thirty years old!
More recently, this Beeb has been branded my "Teletext" Beeb as I added a Teletext ROM and used it to receive Teletext pages at the Byte-Back and Bletchley Park shows I attended in 2009 and 2010.
One of the most interesting features about this Beeb is that it has Econet fitted. For issue 4, the Econet circuitry would undergo significant modification so here is a rare example of the early network interface.
The green wire is the write-enable line provided to an Acorn User sideways RAM module so it is not a feature of issue 2 in itself.
I suspect this is an old board in a new case.
Here we can see the early Econet interface a bit more clearly because my issue 3 Beeb is not fitted with the network upgrade. We can also see that the ventilation slot at the back is a single hole going all the way across.
This is my only issue 3 BBC, which came with the UDM DDFS. For the purposes of this picture, I removed the UDM daughter board which explains the lack of a disc controller at IC 78.
The most significant changes at issue 4 were a complete redesign of Econet. Previously, the circuitry was capable of generating the Econet clock and providing termination of the network but by the time issue 4 came out, Clock and Terminator boxes had been introduced to replace this rather curious earlier arrangement. The resistors were also made to lie flat against the board inside of standing on one end.
There were also some changes to the disc interface circuitry to prevent the need for soldering modifications to be made to the board when adding a disc upgrade.
It is not always easy to say which components were and were not socketed in any particular issue because of the proliferation of user upgrades and modifications in circulation.
The final issue made some cosmetic changes to the issue 4 and tweaked some resistor values. The issue 7 appears to be the most common in circulation so it must have been in production for quite a while before the B+ came in.
The printing colour and the colour of the decoupling capacitors seems to vary from machine to machine and is not linked to any particular issue.