Date   : Sun, 03 Nov 2002 19:07:39 +1300
From   : Michael Foot <mjfoot@...>
Subject: Re: BBC keyboard playing up (update)

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.

Now I just need to figure out where the connection problem is. Hopefully
it's not underneath a socket or somewhere extremely hard to get to.

Cheers,

Mike.

-- 
Michael Foot <mjfoot(at)paradise(dot)net(dot)nz> *
http://homepages.paradise.net.nz/mjfoot/
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