Date   : Fri, 13 May 2005 08:51:11 +1200 (NZST)
From   : Michael Foot <mjfoot@...>
Subject: Re: RAM chips for Model B

Quoting Sprow <info@...>:

> In article <1f29d5694d%mjfoot@...>,
>  Michael Foot <mjfoot@...> wrote:
> > I've just got hold of an Issue 3 Model B. Functionally it works apart
> from 
> > the screen being a mess in all modes. In MODE 7, there is vertical
> lines 
> > everywhere and the text is not readable. In MODE 0, the top third of
> the 
> > screen is fine and the bottom part contains random dots. If I shift
> the 
> > jumper to make it use just 16K, then all is fine. I'm assuming 1 or
> more 
> > RAM chips are faulty, but which ones? Which RAM chips are used for
> video 
> > memory and where can I get replacement ICs from?
> 
> You sure it's the RAM and not the address multiplexing? I had a similar
> sounding problem on a machine which was fixed by replacing IC39
> (74LS283)
> which I sat looking at with a scope and saw it couldn't add up!

Good point. I'd just assumed it was the RAM since MODE 0 is partially correct 
and it works when in 16K mode. I check that IC.

> Alternatively, I think I have some spare RAMs somewhere,

I'll let you know if I need them. Do you know which RAM chips are mapped to 
the video memory space?

Thanks,

Mike.

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