Date   : Wed, 26 Mar 2003 15:21:14 +0000
From   : Kate Kneebone <kate.kneebone@...>
Subject: Re: BBC Micros, what else :)

Peter Turnbull wrote:

  > 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).

Actually, I've managed to get all the marker pen off the second lid,
which isn't actually as yellowed as I thought either, or at least the
outside is pretty much the same colour as the inside, so I'm going to do
*both* of them up :)  The 8271y one will stay as it is, and the tape one
will get a 1770 disc interface.

  > 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.

They do have different power supplies, neither of them black ones.  If
the stickers on the back where the power switch is are to believed, one
was made by Wong's Electronics in Hong Kong and the other by a company
in the UK (may well be BSR). So is it a bad idea to plug things into the
power socket underneath the computer then.  I'm pretty sure one of my
disc drives is powered from the computer and one from the mains ('cos it
came off my electron), though they're in Wales and I'm in Cambridge, so
I can't be totally sure.  Would it be better to only use the mains drive
with that computer?


  > 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.

Yep, I'm very wary of stuff like that.  Particularly after an
embarassing incident at work where I picked up the backlight power board
for an LCD display that was turned on and blew a little hole in my
finger :(  LCDs are now Kate-proofed with plastic bags :)

 >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.
 >
On a whim I decided to take a bottle of whiteboard cleaner home last
night, and while it's absolutely hopeless at getting the yuck off our
whiteboard, it's great for cleaning BBC micros with!  Apparently it's
also exactly the right thing to use to remove the ink but not damage the
plastic (I knew being related to an organic chemist would come in handy
at some point!).

 >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).
 >
Yep, that's the plan.  Thanks for confirming it's vaguely sensible!
Addressing both banks of the RAM's not essential, it's just I'd hate to
think half of my five or so pounds worth of memory is going to waste :)

 > 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".
 >
Hmm, I'd not thought of that.  If BASIC's ROM 15 then it looks like I'm
imagining how things work wrong.  I was thinking that the four ROM
sockets counted up from 0 going left to right, but from what you've said
it looks like I've actually got it completely wrong.  I think I've
probably missed some inverters somewhere.  So the ROMS are actually
15-12 counting down right to left then?  If I put my RAM in the socket
to the left of BASIC then that would indeed mean my RAM would appear at
0b1110, 1010, 0110 and 0010.  I think I understand now!

 >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.
 >
 >
Hmm, I'd rather not do any soldering to the computer itself.  The
potential to destroy one of the finite number of BBCs still around is
too much for me!  If clipping the leads on is horrendously un-reliable
then maybe I will though.

Thanks for the help!


Kate