New Comet owner here!
A few months ago my wife was telling me all about the pinball machine her family had when she was a kid in the early 90s. After a little research, we eventually realized it had been a Comet. She made a few phone calls and we found that after her parents had parted with it it had been passed between family friends for a while before eventually ending up in the back of a barn at a walnut orchard. And there it sat for around 10 years.
Father-in-law unloading the pin
Having sat in a barn for a decade, with minimal maintenance prior to that, the machine was not in the best shape. The playfield was worn and covered in grime. The alkaline batteries hadn't been removed before it was put into storage so one corner of the MPU board was corroded. The rubber rings were filthy, brittle and falling apart. Ramps were cracked, and plastics were warped or missing. And those were just the issues that were obvious without turning it on!
Condition of the playfield when it arrived
The usual corrosion
Repairing the PCB was first order of business since that would let me determine what else on the game needs work. I've done some electronics repair work before, but nothing quite like this.
I ended up desoldering most of the passives in the power input and sound section of the MPU since they had signs of corrision. The corrosion on the PCB itself was mostly limited to the immediate vicinity of the batteries. I removed that with fine-grit sandpaper and then installed fresh components. Rather than reinstalling batteries I swapped the RAM out for NVRAM.
MPU board after repair, but before installing the NVRAM
Once the PCB was in good shape I was finally able to power the machine on. It booted up without any drama (phew!) and I was able to use game diagnostics to verify that the playfield electronics were in good shape (modulo the few dozen burnt-out lamps).
Next came the clean up of the playfield. This was all entirely new to me, so I roped in my 7yo daughter to help. The playfield itself was dirty but not as bad as I'd first thought, though the paint had worn away in a few high-traffic areas. There's a lot of planking, which I suppose is to be expected after being stored in an unheated barn for a decade, and some paint is flaking off behind the right flipper.
While replacing burnt-out / missing lamps on the playfield I noticed that none of the general illumination lamps appeared to be working. I probed around with a multimeter for longer than I'd care to admit, then thought to check their connection to the power supply. Low and behold, a pin had broken off the connector. The lights work much better now that they're getting power!
We installed a playfield protector to limit future damage to the worn and flaking areas, then replaced broken posts, installed fresh rubbers, new plastics, ramps and decals. It's finally starting to look like a real pinball machine!
Playfield under lights
Playfield at night
Pin all lit up at night
There's a few odds and ends left to do before I declare it done and put the glass back on. The player four score display doesn't work. The Dummy drop target adjustment bracket broke at some point so the target sits below the playfield when dropped, perfect to hang up the ball. The flippers are a little loose and probably need to be rebuilt. I'm sure I'll notice more things the more time I spend with it.
But the important part is that we rescued my wife's childhood pinball machine and restored it to working order! We've put a few dozen games on it in the last few days, and my wife is already plotting which pin we should pick up next 
Columbus, OH
