Date   : Wed, 29 Mar 2006 23:23:49 +0100
From   : Fragula <fragula@...>
Subject: Re: Model B NMI PITA.

Andrew Benham wrote:

> What's IC27 ?  If that's a 7437 too, then you're in trouble, because the
Nope.. that's a 38 ok, and was the open circuit bit I half-remembered. I
had a naff pullup on that at some point and it needed to be fixed.

Fortunately I was more into TTL back then, and had better eyes, and a
bigger workbench. (and it was before i started going senile)

The ones that are 37's are the FDC buffers. Totempoleness doesn't appear
to matter much there, though I guess they are fighting with any
pullups/terminators on the drive cable.

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) until I fitted the econet, so I guess its either ok to use '37s
as a throwdown for ICs 79&80, or I just got lucky.

> Econet and FDC interrupts to the NMI line rely on 2 open-collector
> outputs being wire-or'ed.  You can't wire-or totem pole outputs (or
> rather you can, but you run the risk of shorting the Vcc supply).

idd.. that's been thoroughly checked.

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 older these machines get, the more I think they have delusions about
becoming/being Analog.

This one is the Yellow screened issue 7 BTW. The ever troublesome white
screened (formerly Old Faithfull) has its board out now.. It will
probably get a bit more probing over the weekend. In between tours of
motorcycle shops, giving point and click lessons to a computerphobic
shop owners, the seemingly inevitable trip to Maplins (to get a half
dozen or so 7438s for starters!, though i bet they don't have any, and
offer my a lower price on my energy supply, double glazing, or
somesuch..) and if I'm really quick, maybe a visit to the pub.

Cheers!

M.