You have such a nice playfield. As others mentioned though 1200 dpi is not real. Just use the native resolution of that scanner which is 600 dpi. You can always donwsample at any time later. Also as someone mentioned, PhotoShop actually does a pretty quick and dirty job when it blends. It makes these odd masks that work great for a photo but are not meant for this kind of work. Let it try, but don't let it merge the file. Delete the masks and fine tune the positioning of the layers manually. One key thing, in PhotoShop use the measure tool to check some dimensions and compare them to the real PF. I bet you will find it is wrong. The factory only allows .050" in the keylines to be misaligned to the playfield before seeing bare wood. If your overall image is off by that or more, you have a problem.

I spend about 3 hours manually aligning things before merging layers. You only have to do this if you need it. Just keep the individual scans.

The 1200 dpi downsampled to 600 dpi is fine and will save space, no need to rescan. For printing, a really well created 150 dpi print is as good as you need. 300 dpi is plenty. I work at 600 dpi myself but did a whole playfield at 300 because that was the scan I was given and it looks great. 1200 just won't pay off unless you were printing oversized posters of the artwork.

I have used the Pinball Pimp stencils as well. I found it was best to do one side at a time so I could peel off the stencil at the right cure point (spray cans though, not automotive). It takes a while to pull the stencil off without part of it laying into fresh paint on you. I also spend quite a while with Naptha and a cue tip sitting on a stable stool touching up any little paint strings that occurred. I would say from spraying to finished with stencil removed is a full hour. In my case, 2-3 light colour coats, maybe 10 minutes cure, and 20 minutes pulling stencil, then cue tip fixes. Even on a head I stopped trying to do both sides in once session.

Honestly I preferred the material from the dreaded Twisted Pins guys but the quality of the cuts are far superior on the Pinball Pimp ones.

I have been following this saga for a while now and I recently bought a Future Spa myself. One question I have not seen tried is since we are talking about latching issues, have you tried your simple one socket test with 2 or even 3 incandescents loading down one SCR? Don't do it for long since the SCR won't last but if load and latching is still a concern this would eliminate load level as a thing.

Fascinating possibility, never thought of clock issues. I think the factory clock is really a flip-flop. Does it use electrolytic caps? They would be constantly changing over time.

Sounds like we are getting close to pointing at the transformer but its hard to understand how that is possible. Just to eliminate all variables, have you checked the power from the wall outlet?

I almost edited my wall socket question to ask if you had a dimmer somewhere very partially dimmed. I did not think about solar panels. It sounds like you also have standard grid power. Can you just power down your inverter?

Most UPS produce 'Modified Sine' wave output. It is a brute force simplistic waveform that most AC devices tolerate. It won't look at all sinusoidal. It it a square wave with small 0V gaps between the positive and negative going square pulses. A spectral analysis would show lots of spikes at various harmonics

Your inverter may do a better job of simulating a sine wave and a 20 kHz frequency seems typical. You may be able to filter that out with something you plug the machine into. More modern switched power supplies would probably not be bothered by that sort of thing.