Date : Thu, 19 May 2005 09:17:49 +1200 (NZST)
From : Michael Foot <mjfoot@...>
Subject: Re: RAM chips for Model B
Quoting Jules Richardson <julesrichardsonuk@...>:
> On Wed, 2005-05-18 at 13:56 +1200, Michael Foot wrote:
> > I still don't know which one is the video memory.
>
> Essentially, they all are :) Each RAM chip is only one bit wide, so to
> read or write a byte 8 chips are accessed in parallel.
>
> Pin 14 of each RAM chip is data out, so you could just put a meter on
> that pin of each in turn and see if one has a data bit stuck permanetly
> high.
Ahhhh I see. Since I'm getting vertical lines down the screen in graphic
modes, it could be because of this.
> That almost suggests that the fault might be elsewhere rather
> than RAM; maybe in the CRTC itself or in the buffer ICs (8 and 9)
> inbetween the CRTC and memory...
I'm starting to suspect it's something interfacing to the RAM itself now -
unless the original fixed RAM chips are used for the upper 16K in which case I
can't easily swap them.
> >Will touching each RAM chip to see which one is not warm
> >tell me which is faulty? I still don't know which one is
> >the video memory.
>
> It may be one of the tracks to the RAM chips and not the RAMs themselves....
Yes, I though of this just yesterday. I've tested the tracks on the underside
of the board and they appear to be good. I don't know about the tracks on the
top though.
> > In message <1115931071.4283c1bf13a10@...> you wrote:
> > I might need to do this. I'll search my other beebs and see if any of them
> have socketed
> > RAM I can borrow first though.
>
> Oh, further to last post... what happens if you replace the faulty
> beeb's CRTC chip (IC 2) with one from one of your good machines? Just to
> rule that one out...
I'll give it a go. It's not the VideoULA because I've tried that so I'll start
replacing chips where I can next.
Thanks,
Mike.
--
Michael Foot <mjfoot(at)paradise(dot)net(dot)nz> *
http://homepages.paradise.net.nz/mjfoot/
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