Can you post pictures of said relay? And also the wires you want info on?

Sounds like someone may have put a transformer/power board from a Stern Galaxy into your game which uses a relay to switch the general illumination. Magic has no such feature.

That relay doesn't belong on Magic. Someone must have installed the power board from a later Stern game that uses that relay to flash the G.I. Magic does not support this feature.

BTW, on top of the transformer do you see it marked "16B-3" or "16B-6"?

Note the J1 pin header on your new rectifier board is 9 pins wide while the old rectifier board it was 8 pins wide. Make sure your J1 connector is keyed properly so you install it on the left 8 pins (the 9the pin on the right should be left exposed).

Also, the F2 fuse should be a 3/4 amp fast blow but your new one looks rather over fused. Confirm that it's the right rating fuse.
BTW, the fuses on the new rectifier board look like medium blow fuses, not fast blow. Care to post some pictures of the fuse markings?

Oh sorry, somehow I missed your reply last week.
The 16B-6 transformer is from a later game, there's probably a few that would love to trade you with the transformer you're supposed to have. You might notice the flippers/solenoids have more power than the game was originally designed for.

312 = AGC = fast blow glass body
313 = MDL = slow blow glass body
314 = ABC = fast blow ceramic body

The F6 3 amp fuse should be slow blow, not fast blow.

The F4 fuse should be 5 amp fast blow for your game. Ignore the 7.5A slow blow on the card above the rectifier board - as I previously mentioned your transformer/rectifier board panel has come from a later game. The 7.5A slow blow fuse spec is for games with 4 flippers (Big Game, Seawitch, Cheetah, etc)

For safety, the F2 3/4 amp fuse would be better as a fast blow, not slow blow. If something goes wrong with the high voltage circuit on the solenoid driver board, the transistors always blow well before that rectifier board fuse does. A slow blow fuse at F2 isn't acting as a protection mechanism for the high voltage circuit on the solenoid driver board so it should be a fast blow fuse. Bally recognised this error and changed their F2 fuse spec accordingly.

The sound board is just held by four round plastic clips like the clips holding the other boards.

Cheers. For your sanity, PinWiki reflects the correct fuse ratings - your rectifier board matches the first in the list "AS-2518-18".
Only F6 is marked as "SB" for slow blow.

https://www.pinwiki.com/wiki/index.php?title=Bally/Stern#Fuse_Values

There are six sounds:
10 point tone
100 point tone
1000 point tone
10000 point tone
Add Bonus sound
Pop Bumper chirp

Pay close attention to pins 3 and 4 on the 32 pin connector, those signals trigger your two missing sound effects.
I presume you have a newish 32 pin cable? What condition is the SB100 32 pin header in?

You've got some bad connection issue.

The solder joints on the SB-100 32pin pin header often crack. Suggest you pull the board out and resolder any suspect pin header joints you see under a magnifying glass.

Both barakandl and Great Plains have the cables on their sites for under US$15 (before international shipping).
Scroll down to the SB-300 board and you'll see @barakandl's cable:
https://nvram.weebly.com/new-pcbs.html

The dual 16 pin cable is the one from Great Plains to suite the SB-100:
https://www.greatplainselectronics.com/products.asp?cat=57

They are both activated through U4. It can be checked with a logic probe if you have one.

Maybe it's a difference with the Australian Leisure and Allied Industries built Sterns (presuming yours is a local). There are some plastics pictures on IPDB matching your art, and other Magic playfields with different plastic art.

Sounds like a switch matrix issue.
One you're getting the start button incorrectly activating and the other sounds like the slam tilt might be incorrectly activating (this one will cause the game to do a full reset like it's been powered off and back on). So if the displays come back on after 10 odd seconds into attract mode, this is the slam tilt. Not a 5V problem or HV problem.

How often is it happening?

Sorry I missed that bit. If the displays are not strobing then likely someone may have dialed down the voltage. Lower display voltage puts more stress on the high voltage transistors. Personally I dial it to around 180 volts - in a home environment you won't be burning out the displays.

Your switch matrix issues are likely to involve either the drop targets or coin door switches. Does the game report zero closed switches in switch test mode after you raise the drop targets and remove the ball from the game?

Power = voltage * current

There's about 0.2 amps current going to the displays.
240 volts input minus 190 volts output (Bally spec) = 50 volts across the HV 2N3440 transistor on the heatsink.
240 volts input minus 160 volts output = 80 volts across the HV 2N3440 transistor on the heatsink.

50V * 0.2A = 10 watts.
80V * 0.2A = 16 watts.

The heatsinked transistor has to dissipate more heat when the HV is dialed down because there's a greater voltage across it.

For clarity, J2 is a 9 pin housing:

SB100_J2.jpg