A fair chunk of time had passed since I had done anything on my Nova 3. I had since moved house and it was, once again, stored at my parents place. While browsing the internet one day, I came across Tom Jennings' Nova 4/X site and my desire to work on the machine was rekindled. I had to have it back in my posession but space was lacking so I settled on just having the main CPU plopped on my desk.
After a couple of weeks of playing around with it, I noticed that a few of the bulbs had blown. I knew this would happen sooner or later and now I was faced with a major decision. Should I continue replacing bulbs every time they blow, or should I replace them all with LEDs and never worry about replacement from then on? It was a tough decision so I spent several days thinking it over.
In the end I decided to commit sacrilege and replace all the bulbs with LEDs. Several things influenced the decision. Firstly, most of the bulbs were busted when I originally got the Nova so I replaced them all with 12V lamps. It was quite a chore soldering them in so I wasn't keen on having to replace them regularly. Secondly, they lit VERY brightly and this caused me to see spots for a while after using the machine even briefly. The picture below is proof.
The last thing I wanted was to have an unpleasant time using the thing or the fear of temporary blindness. I studies the schematic diagrams and found that the modification would be rather simple. Each bulb has a 1k pull-up resistor to allow a small current to flow through it during its "off" state to keep the filament warm. This prevents the bulb suffering from thermal shock when it fully switches on therefore increasing its life. This small voltage was big enough to turn on the LED so I had to remove these resistors and then replace the bulb with a LED and a current limiting resistor in series with it.
I decided to begin with two LEDs to check that it all worked. Below is a close up of the two LEDs with their 680 ohm series resistors. I chose yellow LEDs so, to the outside, it still looks like there are incandesant lamps installed.
After making sure that it worked, I removed all the bulbs and pull-up resistors.
I then soldered in the LEDs with thier current limiting resistors.
Upon completing this, I reassembled the front panel and was able to enter a short program. All the LEDs lit up beautifully!