Date : Thu, 19 May 2005 15:50:21 +1200 (NZST)
From : Michael Foot <mjfoot@...>
Subject: Re: RAM chips for Model B
Quoting Chris Johns <chris.johns@...>:
> On Wed, 18 May 2005, Jules Richardson wrote:
>
> > Can someone confirm this with their own copy of the service manual?
> That
> > sounds a bit iffy to me, as if Michael's machine is working when in
> 16K
> > mode it'd imply that ICs 61 to 68 are at fault - but in 32K mode
> > according to my manual it's IC's 53 to 60 that are mapped to the
> upper
> > 16K of memory (which is right where video memory is on a 32K machine
> I
> > believe). 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...
>
> If you set the link used to select 16K mode on a 32K machine to
> something
> else, it will use the other 16K bank. It might be one way for 32K, one
> for
> 16K and off altogether for the other 16K, but I can't remember the
> exact
> details now.
This is what I get when I shift jumper S25.
When in the North position (CAS 0 + CAS 1) I get 32K with the screen messed up.
When in the South position (CAS 1 only) it will not boot.
When removed (CAS 0) I get 16K and the screen is clear.
I'm assuming that the CAS banks I have specified are correct and that CAS 0 is
the original soldered RAM and CAS 1 is the socketed RAM. This would indicate
that the upgraded 16K is at fault.
I've downloaded the Service Manual and have had a look through it. It gets
confusing wherever it mentions CAS 0 and CAS 1 as it looks to be contradicting
itself - unless the OCRing has done that.
I also noticed that when in the North position when I press and hold down the
break key, the screen is clear and shows the ROM boot messages and a flashing
cursor if that tells anything.
I've replaced the CRTC also, but it hasn't fixed it. So I'm thinking it could
be one of these issues:
1) Faulty RAM.
2) Faulty track(s) joining the RAM chips.
3) Faulty 74LS51
4) Faulty 74LS139
Cheers,
Mike.
--
Michael Foot <mjfoot(at)paradise(dot)net(dot)nz> *
http://homepages.paradise.net.nz/mjfoot/
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