BIOS/Clock battery replacement

[Philip Pemberton]

On 10/03/11 13:47, Mr Ian Primus wrote:
> I've used CR2032's as replacements before on similar machines, it's
> worked fine.
>
> Just remember - whatever you do, REMOVE the old soldered in battery
> from the board. It'll leak and corrode through traces.

I'll second this with one addition -- the electrolyte in NiCd batteries 
also eats some types of glass-fibre PCB substrate. What this means is 
that you not only lose the tracks, but the board itself basically turns 
to dust as well...

Get a pair of wire cutters (the "flush cutting" kind ideally) and cut 
the thing off the board, then desolder the stubs and clean the PCB with 
vinegar, then water (ideally deionised or distilled), then isopropyl or 
a standard PCB cleaner / flux remover.

This works thusly:
   1. The vinegar neutralises the alkaline electrolyte (usually 
potassium hydroxide)
   2. The water gets rid of the vinegar and any water-soluble dirt
   3. The IPA removes any alcohol-soluble dirt (e.g. solder flux) and 
helps the water evaporate more quickly.

 > I've
> desoldered them and installed CR2032 sockets in their place with good
> success. But if you don't want to go to the trouble, you can just
> clip off the old battery.

Usually there's a 4-pin header which allows you to install an external 
battery once you've removed the board-mounted one. These are usually 
designed to take 4.5V alkaline batteries and generally have a diode and 
series limiting resistor on the PCB.

The Acorn BBC Micro (actually the Master series) used three AAs in a 
shrink-wrapped package shoved in the gap between the keyboard and the 
case... IIRC there was a 120-ohm resistor and a 1N4001 diode inside the 
package -- the resistor limits current if the diode fails.

Earlier machines had a battery holder by the side of the motherboard. 
You could remove the old batteries and replace them with off-the-shelf 
AAs. Unfortunately the battery holder sits in the Tube co-processor bay, 
so you can't install an internal co-processor without replacing the 
battery pack with the shrink-wrap version. There's nothing stopping you 
installing an external cheese-wedge co-processor though...

(And with that tangent over and done with...!)

-- 
Phil.
classiccmp at philpem.me.uk
http://www.philpem.me.uk/