Date   : Wed, 18 May 2005 19:07:23 +0000
From   : Jules Richardson <julesrichardsonuk@...>
Subject: Re: RAM chips for Model B

On Wed, 2005-05-18 at 13:56 +1200, Michael Foot wrote:
> Will touching each RAM chip to see which one is not warm tell me which
is faulty?

Hmm, usually that's a good bet - particularly if one is significantly
hotter than the others. But they don't always fail in that way, so you
may find all of them are an equal temperature...

>  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.

There are two RAM banks; IC 53 to 60 make up one bank, and 61 to 68 make
up the other.

According to the service manual, on a 16K beeb only ICs 53 to 60 are
used.

On a 32K machine, 61 to 68 become the lower 16K of memory, and 53 to 60
become the upper 16K.

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...

cheers

Jules