Date   : Sun, 12 Feb 2006 06:45:28 -0000
From   : "David Harper" <dl.harper@...>
Subject: Re: B+ 64k won't respond to keyboard, Econet fitted

"neil f" <faz@...> wrote:

> Woohooo! Fixed it.
>
> I took a chance and snipped pin 21 on the System VIA. The stub was no
> longer shorting to Vcc, but pin 21 on the User VIA was still shorted
> by a few ohms. So I snipped pin 21 on the Userport and the short on
> the CPU /IRQ line disappeared. I solder linked the System VIA's pin 21
> back up again and powered up. Bingo. Double beep and keyboard action
> is back to normal. 'Course I won't have any Userport or Printer port
> working until I snip out and replace the User VIA, but that's a job
> for tomorrow.

Well done.

Just a comment from earlier. I still think it could be a motherboard track 
that is the main culprit. (You would have bypassed it by what you have 
done.) In my experience breaks can be incredibly difficult to spot - they 
are sometimes below naked-eye size. You have to find them by a continuity 
check on the track.

I cannot remember details, but I think this pin is usually held high, and 
only brought down for a (necessarily edge-sensitive) interrupt for a few 
clock cycles - typically no more than four, which would make it 2 
microseconds. Others may well know better about this.

Anyway, you have clearly got to the root of the problem, and it is up to you 
how you patch it for a permanent solution.

David Harper