BIOS/Clock battery replacement

On 10/03/11 13:47, Mr Ian Primus wrote: 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. 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/