Date   : Thu, 09 Nov 2006 18:39:25 +0000 (GMT)
From   : info@... (Sprow)
Subject: Grrr.....annoying master cmos ram

In article
<38370.82.71.113.222.1163085016.squirrel@...>,
   Richard Gellman <splodge@...> wrote:
> > I've recently replaced the batteries on my master's CMOS ram but it still
> > fails fairly regularly (quite often with "Bad Sum" messages 
>
> It is in fact a DFS error, but as the user guide implies, its never seen
                 ^ ADFS

> in a genuine context.

If you reset the machine while it's creating a directory (for example) that
will probably give a "Bad Sum", though I would agree it tends to occur due
to the ADFS workspace being corrupted rather than a genuine disc error.

> > Does the actual CMOS ram often fail? If so is it easy to replace.  Would
> > anyone have the correct chips to hand?

> I've not known the CMOS chip to be unreliable. Usually problems occur due
> to failing battery packs or a failure to CMOS reset.
>
> As for replacing the chip, I'm afraid on this chip (Motorola/Hitachi
> 146818) is:
>
> a) difficult to get hold of with visiting specialising sites (such as
> those run by various members of this list) as this chip effectively *is*
> the millenium bug* (its the chip that doesn't go past 1999, which was
> installed into millions of PCs, Master 128s, and a number of other
> systems) and thus is undesirable in the post-2000 era.

RS used to sell them, I think I have a single spare somewhere, but would
doubt that yours had actually failed. You'd want a Y2K fix for post 1999
 http://www.sprow.co.uk/bbc/doomsday.htm
as with most chips if the world supply runs out it can always be replaced by
a modern equivalent, eg. an Atmel micro with RTC tracked out in handy 0.6"
DIL format.

> b) soldered directly onto the board, and thus very difficult to swap. Some
> later (replacement only?) boards wered fitted with a chip socket for this,
> but I've only ever seen one such board.

As you don't want the old chip at the end of the exercise it can simply be
cut out with side cutters, then the legs plucked one at a time with a
soldering iron, a bit of wick to open the holes out, then solder in the new
part.

Also a hot air gun (not a paint stripper!) will desolder the part intact,
whack the board on the desk to open the holes, then solder in the new part,
Sprow.