486DX266: CMOS Battery and PC Speaker

23Mar/170

486DX266: CMOS Battery and PC Speaker

After building this machine, I am finding that I am losing my BIOS settings each time I pull the plug at the wall. The original motherboard owner/restorer replaced the onboard battery with a coin cell socket, but it seems that the 3v cell I put in there isn't powerful enough to keep the charge!

PC Restorer has a great article on CMOS batteries and the best point to take form their site is that most BIOS' need 3.6-4.5v to keep their memory.

In the picture above, you'll see the 4-pin header for an external battery. Pin 2 is missing. On this motherboard, pin 1 is positive and pins 3 and 4 are ground.

Using a standard 4-pin header plug from Jaycar, I wired up a 4-cell AA battery pack, but only used 3 cells. This required soldering new wires to allow current to flow between only three of the batteries.

With this all soldered, a quick test with the multimeter saw ~4.89v on the pins... which was more than expected, but an acceptable value.

Installation was a breeze and testing proved that my BIOS settings finally held!

PC Speaker

The case I have installed this machine in didn't have a PC Speaker installed by default. Therefore I used the same 4-pin header to rig up an 8ohm speaker from Jaycar. There's no real need to worry about the polarity here as it's just mainly beeps and boops. Who cares which way the diaphragm shifts?