Date   : Tue, 25 Mar 2003 18:54:23 GMT
From   : Peter Turnbull <pete@...>
Subject: Re: BBC Micros, what else :)

On Mar 25, 10:19, Kate Kneebone wrote:

>  They both claim to have issue 7 boards, but other than that, they're
> quite different.  The one which is currently in the best cosmetic
> condition has a power supply made in the UK and yellow printing on
the
> PCB, but no disc interface and looks like I remember Model Bs to look
> from my schooldays (I had an electron and a Master 128 at home).  The
> other one has a power supply made in Hong Kong, white printing on the
> board and an 8271 disc interface with Watford Electronics' DFS, and
all
> the bits of disc interface other than the 8271 are soldered to the
board
> (scuppering my attempt to move it to the other machine!).
 Intriguingly
> this computer has a slightly different case from the other one, it's
a
> different texture and has what looks like a space for an extra
sticker
> on the bottom, on the other side from all the connectors to the
normal
> sticker.

As others have suggested, I'd take the best case and the best PCB, and
put them together.  It was quite normal for factory-fitted DFS to be
mostly soldered (except for the 8271 and the ROM).  However, if you buy
a 1770 DFS, you need to remove part of the 8271 interface (IIRC).

I'd check the power supplies, too.  The old linear power supplies are
all black, and have no external auxiliary socket; I doubt you have one
of those as they were only fitted to very early machines.  However,
Acorn had at least two versions of the switch-mode power supply, the
original made by Astec, and another almost identical made by BSR
(apparently under license).  The BSR ones are not nearly so reliable,
and have slightly lower capacity.  If you remove the PSU, *make sure*
it's unplugged from the mains first, as there are some extremely high
voltages (DC higher than mains) in there, and it's easy to get a very
serious shock.  Keep your fingers out of the PSU as sometimes the
high-voltage capacitors don't discharge very quickly and there are
still lethal voltages in unexpected places after it's switched off.

Wouter is right about the case texture.  What he calls "spiritus" is
what we call "methylated spirit"; you could also try iso-propyl alcohol
(aka IPA).  Either should remove the ink, unless it has soaked right in
to the plastic over the years and stained it (in which case nothing
will ever remove the last traces).  His comment about the resistor for
the sound fix is good, too.

Having had a quick look at the diagram I think your idea with IC76 and
A14 might work.  I assume you mean to address the 32K RAM as two
sideways banks of 16K, with the RAM chip selected by one of the normal
ROM selects from IC20, and A14 held high for one bank, and low for the
other, according to the state of QC (for example) from IC76.  So it
would appear as banks 14 and 10, let's say (it would also be seen as 6
and 2, but the OS discards multiple images).  BTW, I wouldn't make it
bank 15 because that's the most significant bank, and if you ever get a
corrupt image in it, you need to switch the Beeb off to clear it,
whereas if it's in a lower priority than BASIC you can always get
control back with BREAK and write zeros into the RAM.  Mine has a
little 2-way with centre-off toggle switch fitted, wired so that up is
"write protect", down is "disabled", and centre is "normal".

Your idea about the clips is OK, but I'd warn you that such connections
tend to get unreliable after a while.  If you want to try it, you can
buy miniature clips from Farnell Electronics in Leeds, or CPC in
Bolton.  I think you'd be better to find a decent soldering iron (NOT a
minature 15W thing, but at least 25W, and a 50W thermostatically
controlled one would be better) and practice on an old PC expansion
card or something.

-- 
Pete						Peter Turnbull
						Network Manager
						University of York