Date   : Thu, 30 Mar 2006 10:48:33 +0100
From   : Mike Tomlinson <mike@...>
Subject: Re: Model B NMI PITA.

In article <442B08F5.9040003@...>, Fragula
<fragula@...> writes

>I seem to remember that I thought I knew what i was doing at the time
>(1984 IIRC). Took me a few days to find S9 though. It worked almost
>flawlessly (with a very hot 8271, but some people reckon thats not
>unusual)

IME, it is unusual.  There's two versions of the 8271, one in a ceramic
package and the other in plastic.  The plastic one runs cooler, but
neither version runs _hot_.  I'd replace those 7437s pronto and check
for wiring shorts.

>What I now don't understand is how the econet one I just fixed worked in
>the first place.. I just cleaned a few shorty looking bits of flux from
>the bottom of the board, kinda wiggled IC91 out and in again, and voila.

The model B Econet upgrades (the official kits from Acorn) were very
difficult to install right.  Great care had to be taken in identifying
all the resistors correctly, particularly as many of the ones Acorn
supplied in the kits were 5-band and had 2% tolerance.

It was also important to solder neatly and to clean the board of flux
afterwards.  All sorts of strange and intermittent faults could occur
otherwise.

>and if I'm really quick, maybe a visit to the pub.

Sounds like a plan :)

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