Been pulling my hair out trying to troubleshoot and adjust a Stern drop mech from Flight 2000 (a 9-wide mech populated by just 5 targets). The coils worked, the targets worked, and they would all reset and drop "by hand" but every time I installed it in the game, one or more would not drop or reset properly and/or reliably. This difference in reliability whereas my "hand resets" worked 100% of the time when it was out of the game, versus the electrical pulse of the game, was maddening.
This heavy bulky mech was annoying to remove time after time, and the game's self-test is too primitive, requiring to let it cycle all the solenoids to return to the drops: I was going to hear it in my sleep. And was also worried I might strip the holes in the playfield, bad enough just 4 screws hold this thing in.
Finally I remembered that in the box of power adapters I never throw away, were at least two from old desktop printers. Many of those are around 30V DC... just enough to fire a coil - and test a full mech - at the bench without having to install in the game:
Just identify the + and - leads from the adapter, wire in some insulated alligator clips, then carefully and quickly touch them to the coil lugs: POW!
This worked great! With full visibility of the mech topside I could finally see that the pawls on the small coils were ever-so-slightly out of individual adjustment, in response to the coil's rapid firing (vs. my quickest "hand flick"), requiring various tweaks. Those adjustments - probably due to 40 years of wear on the parts - were stupid finicky and took several more tries at the bench, which easily saved me over an hour of back-forth-in-and-out-of-the-game-and-test-mode-again nonsense.
So if you have an old printer, don't throw the power brick away...
You could even plug it into a game's service outlet to test mechs in-game, with caution.