I'll start with this observation. The Start relay is an interlock relay with two coils, a Latch coil and a Trip coil. Based on the schematic and what you've described it sounds like the Start Trip relay is firing just fine to get the score reels to reset and such, and that it's the Start Latch relay that's not firing at the end of the reset sequence. If that's the case have a look at the switches that control the Start Latch relay coil:
MiniZag Start relay (resized).jpg
If you're not sure which is the Score Motor 7B switch, have a look here: https://pinside.com/pinball/forum/topic/sky-kings-power-free-play-and-top-hole-issue#post-4457680

/Mark

That switch is likely the Normally Closed AC relay switch shown in reply #4. (Refer to the wire color code on the schematic for wire colors.) Closing it lets the Start Latch relay fire which is what we're trying to accomplish. But your observation lets us eliminate the Score Motor 7B as a potential problem. That leaves the two switches on the left side of the schematic in reply #4 which are in the path that should fire the Start Latch relay in this case. While they may be clean, they may not be closing.

Can you verify that both of those switches are closed when the Score Motor is running and won't stop? The problem is likely either in those two switches, or in the devices that operate them.

It may, I have no idea. It may save you some debugging effort if you keep it in unless you need access to something way in the back.

Let's figure out why.
MiniZag Hold relay (resized).jpg
Ignoring the optional Add A Ball relay for the moment there are 5 switches on the left side of the schematic in the Hold relay circuit:
- The Ball Count End of Stroke switch should fire the Hold relay any time the Ball Count Unit advances
- the Hold relay switch keeps the Hold relay engaged once it has fired
- the three Tilt switches will bypass the Hold relay and make it relax again if it was active

If your Hold relay isn't firing it's either because the Ball Count Unit switch isn't closing, or one of the Tile switches is stuck closed or shorted somehow.

If your Ball Count Unit isn't advancing, you need to examine its circuit which is on the right side of the schematic.

When does the Ball Count Unit work and when does it not work? If you're convinced that the Score Motor 3A switch is working properly, all that's left in that circuit is the Make/Break switch on the Start relay.

When you start a game, does the total play meter advance? If so, it means that there's some kind of discontinuity in the green-white wire highlighted in red:
MiniZag Hold Ball Count unit reset (resized).jpg
With the power off, if you measure the resistance (not just the continuity) between the solder lug on the Score Motor 3A switch with the green-white wire and the solder lugs at the other three end points of the same wire you should read an ohm or less at each of those three points. If the total play meter advances but the Ball Count Unit doesn't reset that implies that either there's a break somewhere along the green-white wire or (less likely) the reset solenoid is bad. The discontinuity could be a bad solder joint, bad jones plug connection, broken wire, etc.

It could be that your solenoid is bad after all. You could try a bulb tester (as described at https://pinside.com/pinball/forum/topic/em-pinball-circuits-basics-to-not-so-basic/page/2#post-3184715) clipped across the Ball Count Unit Reset coil. If it flashes you know the coil is getting an impulse.

Or you could desolder one of the wires from the Ball Count Unit Reset coil and measure the resistance across it (with the power off). You should measure about 15 ohms across the solenoid with one terminal disconnected.