Your voltages are fine, nite the range given in the manual. Wall voltage is also very good, the typical issue there is when the voltage is low.

Check out pin wiki for more detailed steps to take. I’d recommend follow the test sequence recommended there. Very possible to have an issue at the solenoid driver or mpu board. It’s very common to have poor continuity given connectors are 40+ years old.

I don't think videos can be posted here but you can upload one to YouTube and link to it here.

Congrats on your Xenon! With the advice of the group here you'll be back in business soon

What do the pins of J4 on the SDB and MPU look like? Shiny, or grey or green? As others said, run thru the troubleshooting list on Pinwiki and report back.

Welcome to Pinside!

Looking at the Alltek schematic for the MPU board, the red overvoltage LED comes on when the 5V power to the MPU board is high, or more likely there's a lack of 12V power to the MPU. It's unrelated to your transformer.

Measure your voltages at the test points on the solenoid driver board and then the test points on the MPU board.
Report back with your findings.

Even though the boards were swapped with new, I would doubt the connectors were repinned. can you please upload some pics of the connectors - specifically J4 on the MPU and J3 on the Solenoid driver board. More than likely as the game heated up with extended play one or more of the connector pins expanded and went open.

'Better' depends on your perspective.

With the transformer wired as 115VAC input and your mains line actually at 122VAC it means the output voltages of the transformer will be slightly higher.
The difference in voltage is nowhere near enough to cause transformer damage. The game will be generating a little more heat in wasted energy but you might prefer the slightly extra power from the flippers.
I prefer to wire games to the proper mains line voltage.

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SW ILL BUS (Switched Illumination Bus) refers to the power voltage used for the lamps that the game can switch on and off. We refer to them as Switched lamps, Feature lamps or controlled lamps - all the same thing. With these old Ballys it's usually 5.4VDC or 6.5VDC depending on the game.

BUS usually refers to the power.
RET (Return) usually refers to ground.
ILL is short for Illumination.
G.I. is short for General Illumination (the lamps that are always on) which illuminate the general playfield area and behind the backglass.

Another useful thing, see below how the wire colors are indicated in the Bally schematics:

Centaur_MPU_Wiring.png
CoinDoor_WireColors256.png

It is really dumb and confusing that they list them 11.9v and 230v. They should either list both the voltage with the cap in circuit (15/230) or both without the SDB caps out of circuit (11.9/190).

I believe you, as mentioned, should jumper the transformer to suit your mains voltage by changing 2 of the jumpers on the transformer.

Easier to do with the transformer panel removed from the machine.

2 to 8 will stay the same
remove the jumper 3 to 6 and add a jumper from lug 4 to lug 6.
remove the jumper 7 to 10 and add a jumper from lug 7 to lug 11.

This will reduce the voltages.

I believe the Alltech guy once told me these boards only require the 5V.

8V is too high, as the CPU board is telling you. Switch the transformer to 120V. Power companies are allowed to go slightly above and below 120V. Delivering power isn't an exact science. More like a juggling act. They prefer to stay slightly above in part because it makes you use more power, causing bigger bills. Hot weather is usually about the only time most people will see their wall voltage drop below 120V.

The power infrastructure in our country has greatly improved in the 40+ years since that game was built. Unless you live in a neighborhood with sketchy power, switch it back to 120 and work on your other issues.

Don't clean the pins. They're relatively new and cleaning will remove the shiny tin plating.

It won't hurt the boards provided you're careful not to accidentally short your multi-meter probe somewhere else you shouldn't.
Plug everything back in. You might find that the game works now that the connectors have been reseated.

That measured 8V is the switched lamp voltage which the CPU does not monitor, it's not the logic 5V power supply voltage reading.
That 8V switched lamp voltage is high though.

I can only believe what the schematics say - check out the Alltek MPU schematic snippet below:

Alltek_Crowbar.png

Your solenoid voltage at TP5 is just about on the money. Spec is 43V DC. The upper saucer issue is a separate fault.

Yes, by all means. The voltages in the failed condition might better give you clues to the problem.

Is the game still powering on properly? Go through the diagnostic tests which are accessed via the little red push button switch inside the coin door to get some familiarity with them. The diagnostics are discussed in the manual. See whether solenoid test is working properly.

That Humming may be a stuck-on coil. Leaving it on will burn out the coil or transistor.
You said you had problems with the saucer. Disconnect one wire on the saucer coil and see if that was causing the humming. Turn it on just long enough to see if the humming is gone. If it still hums turn it off.

when you turn her on, do you notice a coil lock on, like a target reset coil or a ball eject coil or any coil? No coil on the playfield should be permanently on upon boot up going into attract mode.

to reduce or possibly eliminate any excess noise/hum, try removing the sound/speech from there mounting positions on the backboard, carefully let them hang and start her up and see if the hum is reduced if not gone? Express extreme care when leaving a board hang so that there isn't any chance of the rear circuitry touching something it shouldn't, if in doubt, as the board may move from your original resting position, isolate it with say a piece of cardboard?

Looks to me you may have a short in that drop target 1 light circuit.
Inspect the bulbs and sockets on that circuit. I think there are two bulbs in that circuit.

have a look under the playfield where the suspect lamp holder/s is, see if a related wire nearby has for instance a blob of solder causing a short?
Check the solder tab isn't touching anything other than the wire soldered to it. If you're unsure take some clear pics and upload them.

how confident are you with a multi meter? assuming you have one.

Search Bally Pinwiki and it will have numerous manuals you can check out that will have parts used in your machine.
https://www.pinwiki.com/wiki/index.php?title=Bally/Stern

Regarding humming, which appears to 'noise', with the machine off, try unscrewing the sound/speech board from the ground mounts, letting it hang and being super cautious the rear isn't touching anything, you could put some cardboard behind it, or with it on and one screw holding it in place, undo that one screw with one hand, holding the speech board in the other, pull it ever so slightly from the mounting points and hear if the humming is reduced or even gone?

The voltages at TP1 and TP2 on the rectifier board are unrelated to your overvoltage issue on the MPU board.

How are those voltages measurements requested a week ago at the test points on the solenoid driver board and MPU board going?

The U4 chip is a "4514", which is a 1 of 16 de-multiplexer chip. '7' in the schematic indicates physical pin 7. '4' indicates logical output 4.
'9' at the J3 connector means pin 9 on that connector.

4514_Pinouts.png