At the bottom of one of my boxes with various Commodore 64 parts, I got a pleasent surprise. Someone (probably me) had left a C64 lightgun – The Defender 64. Still hyped on the find, I quickly inserted it into a working machine and loaded up Goosebusters as I knew this game was made for lightguns. The game finally loaded just to confirm that nothing happened when I pressed the trigger… After some sighing, I pulled the lightgun apart and started to do some measurements inside.
It soon came clear that when pulling the lightgun trigger, the trigger signal never made it through the cable to the C64. I therefore focused on the 4011 chip on the small pcb inside the lightgun. 4011-based lightguns connect to the joystick port (specifically the down/fire lines) rather than the joystick port’s paddle/analog inputs, allowing the 4011 to send digital signal pulses that simulate a joystick press when a hit occurs. The lightgun has a photodiode that reacts when the TV beam passes a bright spot on the screen. That tiny, noisy analog signal is fed into the 4011 chip, which is wired as amplifiers. The 4011 then turns the light spike into a sharp digital pulse. Finally, the C64 measures when that pulse arrives during the video scan and converts the timing into X/Y screen position. This is also why lightguns only works with old CRT TVs without modifications.
It soon came clear that something was not as it shoudl be. I therefore got some new chips and a new photo diode. I also changed the capacitors while I had everything taken apart. It was time to fire up the soldering iron.
It was finally time to test the repaired lightgun – round 2! I loaded up Goosebusters again and still nothing happened. It turns out, that my good old Bang & Olufsen design CRT TV from the early 90s has some sort of plastic screen in front of the actual tv screen. This causes the flash from the screen to ‘disapear’ leading to the light gun not receiving the signal. So I found a very old CRT TV that I could use for testing. This changed things a little. Now I got some shots fired! Whup whup!! Problem was, that I was continuesly shooting at the top right of the screen making me a very poor shooter. I may have destroyed the lightgun instead of actually fixing it…
While scavenging for an old CRT TV I purchased another lightgun identical to the one I tried to fix. I paid like 18 EUR for it, so it was not too bad. This lightgun worked flawlessly with the old CRT TV! Now I finally managed to get some decent shots fired with the replacement lightgun.
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