I have a Turbo as well, mine also has the smaller monitor. It's actually a 16" according the the manual my game came with. I do know that the games also came with 19" monitors, I assumed it would be easy to swap one in, I've had that in the back of my head as something to do one of these days. But I didn't know there were funny voltage issues to consider, so maybe I'll just leave it as-is.
The sound boards are definitely flaky, I have a working one in there now, but 2 others are missing sounds, one has all but 1 or 2 sounds, the other has different ones missing..
One of these days I'll poke around with them and see if I can isolate the part that's faulty on the ones with missing sounds, but I'm not really an electronics guy, so it'll be more blind luck than anything else if I get them working again. I figured I might try replacing caps, which is said to fix similar Zaxxon issues, another Sega game from the same era.
My 10" woofer was completely shot, I found a replacement 8-ohm online, made a huge difference. The bass from the motor as you accelerate through the tunnel is awesome and worth the trouble of tracking down a suitable replacement.