Date   : Sun, 03 Nov 2002 10:03:05 GMT
From   : pete@... (Pete Turnbull)
Subject: Re: BBC keyboard playing up (update)

On Nov 3, 19:07, Michael Foot wrote:
> In message <10210232247.ZM9015@...>
>           pete@... (Pete Turnbull) wrote:
>
> > On Oct 24,  9:57, Michael Foot wrote:
> > > Quoting Pete Turnbull <pete@...>:
> > >
> > > When the keys fail, the speaker emits very faint buzz noise
> > > (interference) whenever one of the bad keys is pressed. It's almost
like
> > the
> > > signal is getting so far but not making it to it's destination.
> >
> > That might be a clue.  The buzzing is quite likely to be due to
something
> > like the CPU diving in and out of the interrupt routine, and if that's
> > true, then it suggests that the counter, decoder, and NAND gate are OK
> > (otherwise there'd be no interrupt) and the fault is with the MUX or
its
> > connections.
>
> An update on this if you're interested.
>
> It's definitely not the keyboard, cable, motherboard connector or any of
the ICs on the motherboard. I have found that if I flex the motherboard a
certain way along where the 6502, 6522 and 8271 are, I can get the all the
keys to respond. I've replaced and resocketed all these chips, but it does
not help the matter so It looks like there is a bad contact on the
motherboard itself.

Hmm... could be a broken pin on one of the sockets, most likely the 6522
socket, since the 8271 probably wouldn't affect things in such a way, and
if it were the 6502 lots of other things would go wrong.  Broken pins are
not terribly unusual; the break is usually on the top side of the pcb just
where the pin enters the plated hole.

The easiest way to check this is to take out the PCB, turn it upside down
(if you're careful, you can do this without disconnecting the BNC video),
and use a soldering iron on each pin in turn.  You'll soon find any broken
pin, a broken one will waggle about when you prod it with the soldering
iron once the solder has melted.  You may want to add a tiny amount of flux
or fresh cored solder to each pin to help the solder melt and flow.

The other possibility is a bent pin on the 6522 itself, so you might want
to give that a very careful visual inspection (or remove it) first.

If it's a broken socket, the only recourse is to replace it.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York