BBC Micro #3 (Caps, sound and colour)

Mainboard capacitors

After completely recapping the power supply I decided to change the electrolytic caps on the main board, there aren’t many so a quick and easy job. While replacing I tested the original caps and found that they were all ok in terms of the specs.

Original electrolytic capacitors

Colour composite output

With the basic BBC working and able to play games from the SD card, see previous blogs, the lack of colour was disappointing, so, I decided to implement the colour mod to the composite output. This is a very easy mod, only requiring a capacitor to be added (C58 – which is already there on Issue 4 boards like mine) and link 39 to be fitted. The link itself is just 2 empty solder holes on the board, so I fitted some male header pins to allow the link to be made with a simple jumper.

C58 and Link 39

With the headers installed and a jumper added the composite output now displayed colour images, although I will say it’s not as sharp as the raw black and white picture. Looking at the output of Q9 with an oscilloscope it seems to show that this is the point at which some of the interference is added. I have ordered some BC239 transistors so I can try swapping it out in the future.

MMFS in colour over the composite output


During my “testing”…. ok playing games, I very quickly found the volume far too loud. The only way the volume can be changed is by adjusting the variable resistor, VR1, on the main board.

VR1 resistor to change the volume

With a game running I used a small screwdriver to adjust VR1 to an acceptable level – easy.

That’s all for now, cleaning next….