Date : Thu, 24 Oct 2002 09:57:17 +1300 (NZDT)
From : Michael Foot <mjfoot@...>
Subject: Re: BBC keyboard playing up
Quoting Pete Turnbull <pete@...>:
> > In article <38cd6894b%mjfoot@...>,
> > Michael Foot <mjfoot@...> wrote:
>
> > > Has anyone encounted this type of behavour before? What ICs control
> the
> > > signals to/from the keyboard?
> >
> > The system via (you could try swapping this with the less useful user
> via)
> > and the shift register on the keyboard itself,talking to eachother
> over
> the
> > not-so-reliable grey molex cable,
So far I've tried replacing the keyboard (and molex cable) and replacing the
6522 but this has not helped.
The motherboard had some slight damage around the 74LS163 (IC76 below the
keyboard pins) where a flying lead had been soldered, but I think this IC is
only used for the ROM paging and appears to be working ok as the ROMs are
fine. 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.
> It works like this:
[snip]
Many thanks for the explanation. It gies me an excellent idea what ICs and
tracks to check.
> The most likely fault is a cracked track near the counter or MUX; I've
> seen
> similar situations where a crack right beside an IC becomes heat
> sensitive
> (when it warms up, the metal expands and -- counterintuitively --
> widens
> the crack. Second most likely is the chip itself.
>
> If all the keys in one column are faulty, suspect the decoder (7445).
>
> If
> all the keys in several columns are bad, especially suspect the counter
> (74LS161). If some keys in a column are good but some are bad, but all
> the
> keys in a row are bad, suspect the MUX (74LS251).
These ICs sound the most likely. I'm guessing that the track is cracked or a
solder joint is bad since the keys work sometimes. I'll take a closer look
around these ICs.
The things we do to keep these machines alive!
Thanks everyone!
Mike.
---
Michael Foot <mjfoot(at)paradise(dot)net(dot)nz> *
http://homepages.paradise.net.nz/mjfoot/
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