Boots up, and plays, but incorrect sprites – most noticeable little bug changes into ‘END’ at one point. Check 74ls138 at 8n, controlling graphics banking, and pins strobing. Replace chip and now uses correct sprites.
Quiet missile sound and explosion sound. Both of these effects use the op-amp chip, LM324 at 7T. Replace this chip and noises better, but very noticeable hum all the time. Check voltages on chip – GND floating at 2v, follow trace and there was no connection to ground at any point. Adding a strap to the nearby ground trace and sound was back to normal.