Wow. Sorry to hear. Did you double check all your work? Is it something simple like a fuse? Did you do any board work, or just at the grounding plate?

First things first, instead of guessing - get a DMM/Voltmeter and check all your supply voltages. In fact, it's probably not the ground mod at all, you might have disturbed a weak connection somewhere, or this could be entirely coincidence.

First verify that the voltages are present on the fuse outputs and bridge rectifiers. Then we can look downstream and see what might be getting lost - if any. Still could always be something else.

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Most are AC voltages. You'll measure across each transformer secondary winding by putting one probe on the side marked as a return. You can use the 3-digit wire color codes to find where they go. Then the other probe take the measurement off each side of the fuse on the hot side of the winding. This will verify you are getting power to the fuse, the return side is there and the fuse is good. Example: the 12 volt winding has its return on pin 4 of the transformer output, color code 644=blue with two yellow stripes for the first probe, F1 1/2A is the fuse on the hot side.

The center taps are a little tricker, basically measure from the center tap to either end of the winding. The center tap for 8.4VAC at the top is pin 7, but watch out for the center tap on the 6.3VAC because it is actually pin 12.

Change to DC to measure the bridge rectifier outputs. Negative (black) probe on the - side of the bridge (wire color 9=white) to the + side of the bridge rectifiers.

Pin out your ground mods with a Dmm on buzz tone. Go from the lugs you screwed to the metal or the metal itself. To the ground plains of each of the boards.

And this might help you with voltages.
And some other 80b issues.

https://pinside.com/pinball/forum/topic/hollywood-heat-sound-issues

The buzz tone is just an indicator that you can listen to for checking continuity. The ohms/resistance setting is another way to measure continuity. Low ohms almost near O is a good connection and has continuity. High ohms, there is not continuity. Don't be too hard on yourself...you just lack a lot of basic electronics knowledge. I went to two electronics trade schools and have a lifetime of electronics background. Its also not easy to teach basic electronics over a forum like this. It goes a lot smoother if you already have some fundamental knowledge of basic electronics under your belt. You might want to close up the game and do some reading or coursework on electronics before getting back into it.

I am by no stretch of the imagination an expert on Sys 80b games .... ..... but I'll throw in my thoughts ...

I recently had a Night Moves issue , the 5v was super low and so it wasn't booting. After some head scratching and metering I found that the ground connection from the 12v bridge was bad. Remade the connection and all good again.

I wonder if during your ground modding a connection has been missed, needs redoing ? It's straightforward enough to check continuity from bridge to the ground connection.

+12 is fed to the power supply board to create the +5VDC. Measure from pin 1 to pin 4 on J1 power supplty to see if +12VDC is present.

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I suggest measuring at the connector mentioned above, for simplicity sake. It's really easy to short out rectifier legs to other things you really don't want to short them to, unless they were intentionally mounted high on the PCB for airflow/clearance reasons. Even then, you'd really want a clip-type probe to grab the leg securely (In your hand is NOT secure...trust me. ).

Rule #1: Read Pinwiki. Seriously. That place is responsible for 90% of what I *do* know when it comes to troubleshooting games.

http://www.pinwiki.com/wiki/index.php?title=General#Testing_a_Bridge_Rectifier

One lug of the bridge has a single ground wire attached. Check continuity between the it and the grounding point. You can do this with power off.

Yes. Isochronic_Frost As wayout440 suggested above ...check at the power board first to see if you have 12v supply there.

The large cap (10,0000uf) is for the +12VDC, so if your 12 volts tested perfectly accurate, that is not your problem. You need to check these components on the 5VDC power supply.

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This below taken from http://pinwiki.com/wiki/index.php?title=Gottlieb_System_80#Recommended_Repairs_for_the_System_80B_Power_Supply_Board

*Note* the part about 5v readings being below 1v. It suggests grounding issues. That was the problem on mine when I had about 0.75v instead of 5v. The power board was perfectly fine.

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5.5.4 Recommended Repairs for the System 80B Power Supply Board

System 80B +5 VDC Power Supply. The adjustment POT is at the top of the PCB.

Step one to improving power supply reliability is to replace the original 500 ohm 1 watt adjustment pot. This procedure can be done easily without removing the large heat sink. A replacement sealed pot is recommended, but not completely necessary.

Next, remove all the old solder from the all of the .156" header pins, and reflow new solder onto the joints. The top header pins (J2) supply +5VDC to the majority of the associated circuit boards in the back box. Provided that the power supply is delivering acceptable voltages in the +5VDC range, these two relatively simple repairs are the only things needed for this board.

However, if you've adjusted the pot as much as possible in the appropriate direction, and the power supply is still delivering voltages higher or lower than around +5VDC, it's probably time to look at replacing the LM338K voltage regulator. While not a frequent occurrence, the voltage regulator will sometimes fail.

If the voltages measured are less than 1VDC, the grounds for the power supply (located at the transformer panel) will more than likely need attention. Once the grounds at the transformer panel are better secured, recheck the voltage at the power supply.

Note: When measuring the +5VDC provided by the power supply, first disconnect J2 on the power supply. Measure power at the J2 male headers first. Once within an acceptable range, turn power off and reconnect J2. Measure power across C1, the 100uf/10V capacitor that is next to J1 (power connector) on the MPU. The MPU may drag the +5VDC supply down a bit. If you find this to be the case, adjust the trim pot on the power supply until a steady +5VDC is seen at the MPU. Poor connections between the +5VDC power supply and the MPU board may also reduce the voltage measured at the MPU.

So you *definitely* have 12v going onto the power board ? - you measured this at connector J1 as described a few posts above ?

With J2 disconnected - what reading do you get at the J2 pins ? If you adjust the pot does the reading change at all ?

We've all done it. Glad its sorted.

A good reminder though that you can't check a fuse just by looking at it

I’m glad you got it working without to much hassle

Congrats. At least it was only the fuse. You might feel silly, but you learned, and it wasn't something massive.

Just checked back on this and had to LOL... any day you find out "it's just a fuse" is a GREAT DAY, because the stupid thing did EXACTLY WHAT IT WAS SUPPOSED TO!