Finally getting around to trying to get this machine working and what I've done so far is to disassemble and clean the score reels and replace the Zipper Flipper Latch coil as it was removed.
The start-up process is where the problems lie. When the start button is pressed the score reels reset to zero properly but the score motor continues to run, the Game Over relay pulls in and releases repeatedly, and the reset relay stays pulled in. To start a game I have to manually activate the Start relay which stops the score motor and I then have to manually pull in the Hold relay. A game can now be played and the playfield features work (zagger lane, kickout holes, zipper flippers, scoring, etc). Another issue though is that the ball count unit does not decrement during game play so a game doesn't end.
I've tried to chase down the problem using the online schematic (I have an actual one on the way) to no avail but I'm also far from experienced with reading schematics.
https://www.ipdb.org/files/1606/Bally_1968_MiniZag_Schematic_Diagram_continuous.pdf
start up sequence: 1. Coin is inserted into the game. The coin relay will energize. It will stay energized through its own hold-in switch and a score motor switch. If the credit button is pressed (instead of a coin being inserted) and there are credits, the credit relay will be energized which energizes the coin relay.
2. The coin relay will energize the lock relay (this turns the general illumination on). The lock relay will stay energized through its own hold-in switch and a delay relay switch.
3. The coin relay will energize the reset relay, through a game over relay switch (if your game won't start, try cleaning the contacts on the game over relay; a very common Bally problem).
4. The score motor will operate. This will energize the score reset relay(s). The score reset relay(s) will attempt to clear the score reels to zero. This is done by operating the score motor. Each turn of the score motor will operate the reset relay once, which in turns moves a score reel one position, until the score reel(s) are at zero. If the score motor continues to run when a game is started, there's a good chance the zero position switch on the score reel(s) is dirty or mis-adjusted.
5. The coin relay, through the score motor, will advance the total play meter.
6. The reset relay, through the score motor, will reset the stepper units (zero the ball count and player units).
7. The coin relay, through the score motor, will decrement the credit unit.
8. The coin relay, through the score motor, will energize the game over latch relay coil.
9. The coin relay, through the score motor, will energize the 100,000 relay latch coil(s) (if the game supports scores greater than 99,999).
10. If the outhole switch is closed (single ball games) or the ball trough switches are closed (multi-ball games), a ball is released to the shooter lane through the outhole relay (single ball game) or ball release relay (multi-ball game) and the score motor.
Any help is very much appreciated!

Extra info for above as per Sticky post in regards to asking for help:
Game never worked in my possession. I bought it off of eBay, checked it out a bit when I got it and saw the issues and stored it until I was ready to fix it. Game is very clean inside with most if not all labels still in place. Jones plug connectors are all clean
Only service I've performed is on the score reels and replacing a Zipper Flipper latch coil.
Nothing looks or smells burned other than the Hold relay.
Has new fuses and new fuse blocks.
Not much past work done other than someone removing the above coil and installing new fuse blocks and fuses.
I can solder and do have a multimeter.

@Howard- I should have mentioned that the start up sequence is a generic Bally one that I copied and pasted here.
@Mark-I'll look at those switches again and a proper schematic arrived today.
Regarding the Hold relay and it's circuit, I've checked the panel, plumb bob, and ball cheat anti cheat switches and they're all gapped and working properly as well as the Ball Count Unit EOS switch.
Thank you for both of your responses.

thanks gilles, i haven't cleaned those yet but they look really clean and the solder joints are in good shape. I'll get to them at some point.
so I checked and cleaned all the switches in the Relay latch circuit and the problem remains and now the credit unit decrement coil fires continuously even after the credit wheel reaches zero.
so here's what it's doing now: when plugged in the Start Relay pulls in. when left flipper button is pressed the A.C. relay pulls in and the lites come on. when start button is pressed the Start relay releases and the score motor and credit unit coil operate continuously. I noticed that on the A.C. relay there is a switch with two brown wires with white traces to one of it's lugs. When the A.C. relay is pulled in, that switch is open. However, manually closing just that switch pulls the Start relay in which in turn stops the score motor and credit unit. It seems as though that switch is backwards but that can't be right. I still have to manually pull the Hold relay in to start a game (I'm assuming a game would start because I have the playfield pulled out for now).
Pic is of the AC relay pulled in and it's switch mentioned above with pen pointing to it.

IMG_0998 (resized).jpg

Alright, you were right Mark. It was simply a matter of the ball count unit not being in the “sub-zero” position so the switch wasn’t closing. Now the score motor and therefore the credit decrement coil are not running continuously.
Now though, when the start button is pressed the Start relay and both armatures vibrate rapidly for a split second. Sounds like a mini machine gun.
Hold relay still not pulling in.

Also, if I manually add balls on the ball count unit to open the switch like it was, pressing start doesn’t reset the unit to close the switch and therefore the score motor spins continuously again.

Playfield is still out of the game if that matters.

Score motor switch 3A pulses the ball count unit reset solenoid and I checked it but I already knew it was good because it also pulses the replay unit decrement and total play meter coils and they both work.

Gonna put the playfield back in and report back. Checked resistance of ball count unit coils and they’re good.

Thanks Rolf, I checked and the ball is where it should be. Also I figured it wasn’t the tilt switches because I could start a game but I checked them again and I could induce a tilt on each one. This to me means that they were in an untilted or open state.
Got the playfield back in the ball count EOS switch does fire the hold relay and it stays in. Also, the ball count unit does advance properly but it’s reset coil still doesn’t pull in at game start and therefore no Hold relay pull in.
The only way to properly start a game is to manually reset the ball count unit.

The Ball Count unit will only advance as balls are drained. When the game is over, game over lite comes on and no more ball can be played so that seems correct.
If start is pressed right away, score reels reset but tilt lite comes on because Hold relay has not pulled in. So I have to manually push the Ball Count Unit reset plunger and then machine kicks ball into trough for the new game, tilt lite and game over lites go out, and ball one in backglass lites up.
I checked the Start relay and it looks like it’s a green/white make-break switch which I found. Odd thing is that a lug on it has no wire to it and it looks like there’s never been any solder???
Pic shows this switch with white pointer just above it.

D6EA5A60-8284-45B4-8532-AB1303E92257 (resized).jpeg

The play meter does advance. I’ll check and be back.

playfield is back out and did the test. I’m getting continuity and less than an ohm on the three test spots.
Also did the same test with the probe still attached to the score motor 3A switch. I lifted the switch stack as if the cam did it and other end was attached to the green/white make-break switch on the Start Relay. Also got continuity and less than an ohm.
Perplexing...

jeez louise-alright so the ball count unit reset solenoid was bad and I pulled an identical one from capersville to replace it. so the good news is that now it pulls in properly at game start. bad news is that when I try to start a game, the Hold relay deenergizes and the Game Over relay won't pull in. Once those are manually corrected, it's back to normal. Head is starting to hurt...

Didn't get much done today troubleshooting-wise. I took a break and instead soldered all the lamp sockets on the playfield and replaced a bunch of brass coil sleeves with plastic ones. The squarish lamp sockets in the back box look like a pain to so I'll research how to deal with that. I'll try and make some headway tomorrow on the start up issues.

Had to get some outside help to get the game fixed today. I found a local guy by the name of Curt that I'm sure all the pinheads in the PNW and beyond already know about. He definitely knows his stuff and was great to talk to about pinball in general. Anyway it was the Start relay that was the source of most of the trouble as most of the switches were not gapped correctly. Apparently these latch relays have to be adjusted just right with less tolerance for deviation as compared to other relays. I also want to say thanks everybody replied here to try to help me out, it is much appreciated.

thanks gilles68. just ordered some maintenance parts (mainly zipper flipper stuff, flipper rebuild kit, etc). need to attend to the lamp sockets in the backglass but i'm looking forward to seeing the new backglass lit up. after that, i'll install the AAB conversion kit.