I was having the classic Mist issue where the magnet would run back and forth and the mist ball would not populate.
This thread was the exact issue I was having:
After reading through that thread, I ordered the caps and the inductor mentioned in it with the intention of replacing L1, C7 and C10 on the 24 opto board. While I was waiting, I cleaned both ends of the opto switch with q-tips and window cleaner. That got switch 82 to work in test, and Mist would work perfectly for about 1 game, then the issue returned. This worked a few times, but always reverted shortly.
Today I got the caps and the choke, replaced them, and also re-flowed all the header pins on the 24 opto board.
Now at bootup, the problem remains, but after about 2-3 minutes everything seems to work just fine, and as long as the game stays on, everything works just like it's supposed to. As a test, I shut the game down and let it cool, then started it up and immediately put it into switch test on 82. It started off solid "open", then after a couple of minutes started "chattering" closed, and eventually settled down and started working correctly.
If I do a "quick reboot" just letting the game sit off for a few seconds, it comes back on working fine, which makes me think there is something that needs to literally "warm up" to an operating temperature in order to work properly. Any idea what I should be looking at?
More data: at "cold boot" I am seeing approximately 15VDC at the header pins of J118 on the driver board with the connector removed, so no load. For a 12V unregulated line, this seems fine to me.
I also saw this thread: https://pinside.com/pinball/forum/topic/bsd-dracula-mist-opto-intermittent-
and this one:
But in both cases the OP never followed up as to what ended up working.