On maintenance method you had to check 1st where the power comes...on the A3 driver board (attached pictures of my stargate burned last week)
Find the transistor who active the right upper flipper, test one you are sure it works for knowing values...for testing a soldered transistor you had to test continuity between each legs and on both side because it works like a diode inside...you might have all same value on all transistor. Also have burned transistor without any proof of it at first look. Many burns but not until I shunt all ground wires.

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Sorry for this mistake I had not notice that yet. Thank you Zacaj

I confirm you must put your MM on DC= not AC~ and be sure the battery inside is good if you don't want to read faulse value...check values between every legs on one that work fine and do the same at the top flipper...if there are same the problem is the coil and if there are different, it comes from clamp or cut wires or connectors or bad soldered wires.

Unsold coil and test stripe wires to the ground. Put your MM on Ohmmeter or speaker and test stripe wires. Put one touch meter on one stripe wire and the other touch meter to a metal plate in the box who might be connected to the ground with yellow green wires...you might have 0 or no sound came from the MM

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Did you have schematic diagram wires...it's help a lot...you need to understand where wires are connected. Look page 55 and 56and if you don't have one you could up this one.
https://www.google.fr/url?sa=t&source=web&rct=j&url=http://www.gametronik.com/site/rubriques/pinmame/Jeux/Stargate/Manuels/Stargate%2520(Gottlieb)%2520(1995)%2520English%2520Manual.pdf&ved=2ahUKEwiFus7k1-bYAhWJfFAKHcvdApkQFjAMegQIDRAB&usg=AOvVaw1KLIMCBd2Bm6vpwHrRs1FH

Don't underestimate you...you'll be proud of you when you understand something new and so...logical that it just show you where you go...and I'm pretty sure you might have to look at this at any time. Page 55 and 56 for your flipper just follow links that are only wires...
For now you he to check if any + wires (those who give the info lol) is not touching any - ground who can blow your fuses. The ground is where all are connected except separate ground like in these case what is the wire on the bottom flipper who got the same colour? Black green I think...thanks to the shematic

Also try between black purple wire and green green (both soldered on coil) it may not beep

You might do it unsoldered