I finally got my Premium to a playable state (I hope). Never had a NIB with this many issues. I think I've spent about 15 hours messing around with it and about an hour total playing time.
Screen ramp:
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Had a lot of fun trying to get this working. Every time I thought I had it fixed (would work great 20 times in a row while testing), it would eventually hang and not go up every few games. Out of the box the left side of the screen was too close to the ramp, causing obvious binding issues. Did the fix recommended in:
https://pinside.com/pinball/forum/topic/stranger-things-club-join-our-party/page/393#post-7931646
and that got me most of the way there, but was still having problems. For me the final piece of the puzzle was noticing that the left side of the screen would sometimes flex very slightly inward whenever the ramp tried to go up. Even though it didn't look like it was binding anywhere it was adding enough pressure to stop the screen.
I took a washer, wrapped it in a couple of layers of electrical tape to fatten it up a bit and wedged it between the screen bracket and the left hand post just behind the drops to fully support the bracket. No flex anymore, and I have played about 10 games with no more ramp stoppage. Not an elegant fix, but it is invisible, should never move anywhere and gets the job done.
Projector focus:
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My projector focus seems reasonably stable now. I put the foam bracket back on so everything is buttoned up but I'll still have it go blurry about 75% of the time when I lift the playfield up and put it on the support rails. Banging lightly on the apron or just lifting the playfield and knocking it on the rails again will get it back into focus. I doubt this is normal, and am hoping Stern will get back to me and get me a different projector as I am worried at some point I won't be able to get focus back in, but playable for now.
Drops:
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My second drop target was never registering. Nothing visually wrong with the opto board for the drop targets, all the solder points looked normal. Tried cleaning the opto but no go. Swapped the board in from my Deadpool LE and everything works 100%. Just waiting to hear from Stern support, should hopefully send me a new board and I'll get both games up and going.
TK lock:
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I was getting major air balls off of the left ramp and missing some locks. I moved the left ramp cover over more to the right and knocked flipper power down and I get three balls locked no problem, luckily I didn't have to touch the diverter. Left flipper at 215, right flipper at 210 work for me, still plenty of power without things going out of control.