Date   : Mon, 08 Sep 2003 08:15:30 +0100
From   : Mike Tomlinson <mike@...>
Subject: Re: newbie question

In article <001101c375a2$59f228e0$0100a8c0@...>, Bob Devries
<bdevries@...> writes

>Mmmm, I can see that it draws *something* but triangles it aint. It is
>broken up by bars of solid colour, and sometimes black.

Quick history: The BBC B was supplied in 2 models: 16k and 32k.  The
memory is divided into two 16k banks controlled by CAS 0 and CAS 1.  The
16k model has the CAS0 bank of memory fitted only, the 32k model has
both.

Try changing link S25 to South.  S25 is located to the right of the
6845, and behind the RGB connector.  This turns the machine into a 16k
model, the machine will report "BBC Computer 16K" when you turn it on,
and you will lose the ability to choose modes 0 to 3.  Change the test
program to select mode 5 instead of 2.  Does it now work?  You should
get a series of random black, yellow, red and white triangles being
drawn on the screen, with the words "On test" scrolling up the left.

If this works, it suggests a problem with the upper 16k of memory or the
6845's ability to address it.  Carefully feel the back of each of the
RAM chips with the back of a finger.  Do any feel hotter or colder than
the others?  Since you mentioned in an earlier post that line MA7 is not
"wiggling" properly, I'd suspect either ic2, the 6845 (rare for this to
fail) or ic9, 81LS95.  Also check with a scope the address and data
lines of ic53 to 60 inclusive, but again it is rare for the actual RAM
chips themselves to fail.

There's a BBC B circuit diagram at:
http://www.8bs.com/boards/BBCBschematicJTribbeck.png

You *may* be able to "piggyback" the 81LS95 with a known working one to
test whether this cures the fault, but it's never worked for me, only
removal and substitution has.

Also try Mike's suggestion of ic39, 74LS283.

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