I am restoring this SSII and looking into this buzzing noise (see video). Where would I start diagnosing this buzzing noise that seems to pulse with the lights in attract mode?

https://www.dropbox.com/s/mz4ayz7644elibg/20170103_154814.mp4?dl=0

Thanks, I am actually reading through some of this. But, if anyone has any more pinball related advice I am hoping they will post here. The noise goes away when I unplug the MSU1. Also, the noise gets louder with the volume pot...

I have been scouring the www including pinside threads regarding pinball noises, buzzes, and hums. The one conclusion is that all the fixes seem to be different except for maybe the Williams system 9 games (which I have experienced on my Sorcerer pin) where cold solder joints are an issue on the PSU. I am hoping for a silver bullet on my SSII but I know it is not likely especially since this machine is so rare with only 600 produced. I think I do have one thing going for me in that the SSII uses a MSU1 board with its own processor including a diagnostic LED flash sequence. So, I have already noticed that I am getting a flaky first flash indicating a power issue (PSU/MPU2 molex header connector pins maybe). I am hoping that I can figure out how to boot the MSU1 board on my bench and hook it up to a speaker to see if the noise is generated from this board. Also, the volume pot is separate from the MSU1. I plan to remove the pot from the circuit to see if this is the noise source. When I turn the pot up and down I get a poor step mode type of response. So, maybe the pot is bad. I definitely agree with your comments regarding the running of signal wires next to the power wires since I ran into similar issues when building car audio systems back in the day. The speaker wires are running right next to the ground wire but this is all by design (not saying it is not the issue though). Thanks again for the info. I will update this thread with any progress I make just in case someone down the road can benefit from this work.

Ok, I updated this thread title from "Diagnosing a Ground Buzz" to "Watch a Novice Diagnose a Ground Buzz!" What I found with brief research is that buzz/hum noises from pinballs are due to something different "almost" every time. I found a couple of good threads or www blogs referencing a sure-shot fix for specific titles (eg Williams Sys 9) but for the most part the fix for a noise is always different or sometimes the noise is just inherant to the machine and that's the way it was off of the production line.
Keep in mind, I am a novice. I am a mechanical engineer by education and profession but I did ace both of my circuits classes in my undergrad! Honestly, the more I dig into trying to restore this pinball, the more I am intrigued by the elctrical portion of the machines. I already completely repaired a battery-demolished MPU2 board under the guidance of Viperrk who no longer frequents pinside but who was kind enough to walk me through the repairs despite my stupid questions and novice errors while learning to solder on this old board, lifting pads and traces along the way.
Well, here goes. Feel free to offer advice or tell me I am going to kill myself if I do that, etc, etc. Enjoy watching me learn, learn with me, or join in and we can learn together!

So, I am afraid that this hum/buzz noise is being caused by a ground loop. I have 3 little kids that play my pinballs and I do not want them getting shocked. I have been to arcades where you touch 2 pinballs next to each other and you can feel a tingle. Actually, this happens between my side-by-side TZ and Sorcerer running in my gameroom right now. I never noticed it until my daughter mentioned it and I re-checked. I think the kids' skin is much more delicate and sensitive so she was able to detect it much easier and it was actually uncomfortable to her. I'm thinking that "if" this sharpshooter II buzzing noise (reference video in original post if you want to hear/see what I am talking about) that it might be a serious one which could result in a larger delta V being transferred across a host (which I do not want to happen).
When I got this pinball, the safety shields for the transformer and PSU were sitting in the bottom of the cabinet. So, I know that work was done on the PSU. Duh. Also, when I first got this pin, I checked all the PSU test points and found that the +7vDC, +12vDC, and +24vDC to measure +3.5VDC, +16vDC, and +33vDC. I swapped out the lower 2 bridge rectifiers on the PSU (could not get to the upper one since I was in a akward position with the PSU in the head) and there was no change. I also replaced the electrolytic capacitor just for good measure. At that point I moved on to repairing the MPU2 and restoring/rebuilding the playfield. 6 months or so later, here I am with a 100% working (but noisy) pinball! Anyways, I want to start from the beginning with this potentially harmful to humans ground buzz. So, last night I pulled the PSU cradle with the transformer out of the head and hooked it up on my bench for closer examination. Below are some pictures of the unit on my bench and the method I used to power it up (which I borrowed from this thread: https://pinside.com/pinball/forum/topic/game-plan-power-supply-bench-testing-made-easy#post-1525425).

20170105_000632 (resized).jpg20170105_000639 (resized).jpg20170105_000706 (resized).jpg20170105_000733 (resized).jpg

My first observation was that the bare metal wires circled below on the transformer were touching the inside back of the cabinet. There are black marks that line up with a couple of these bare wires. Usually bare wires are ground (atleast this is so in circa 1960+ residential 120vAC wiring!), so maybe this is ok? Maybe not since there are black marks on the ground plate? Remember I am a novice. Anyways, something to figure out. Maybe someone reading this (if anyone is reading this) can help.

circled wires in contact with ground in back of head cabinet (resized).png

So, when I started measuring voltages I got the same result. No surprise there since I didn't change anything. Well, maybe a slight surprise that I powered this thing up on my bench without zapping myself and get the same readings. Yay me!
I still wanted to swap in that third bridge rectifier that I bought to I just decided to take mesuerements at all the test points before and after to see if anything happens. So, now my 7vDC that was measuring 3.5 vDC is measuring at the full 7vDC. So that is good I guess. Not sure what the 7v powers (I will dig into the schematics a bit later) but at least it will get the correct voltage now. I still have the high +12v and +24v though so I decided to shotgun check all the resistors with my meter since I have this thing on my bench. All the resistors measure good. So, no luck there. I reluctantly measured the DC voltage off of the voltage regulator and read +5v and +16v. Hmmm, my feeble mind is thinking that both the +12 and +24 come off of this voltage rgulator that is outputting +16 (that's why I am getting +16 and +32ish). The part number of the voltage regulator on my machine is SG323K whereas the PSU layout calls for LM323K. These are both 5v regulators though so maybe this is not the reason I am getting 16v instead of 12v. So, more investigating to do (like, what converts the 120vAC to 12vDC?). Or, maybe someone here can help (if anyone else is here).20170105_000618 (resized).jpg20170105_005343 (resized).jpg

Ok, so it sounds like the +16 and +5vDC are the feed and resulting voltage into and from the regulator. Makes sense. So where does the 24vDC come from? Directly off the transformer? Is a 32-36vDC reading on a 24v circuit within a reasonable range? I did not measure the voltage at the coils but I will when I put this thing back together. When I play it though the ball hits the glass out of the saucer eject and off the drop targets every time. Seems like maybe the coils are getting too much voltage. Assuming the 24vDC should come unregulated directly from the xformer I should be able to measure it directly from those lugs (one of which was touching, or did touch at some point, the inside of the head ground surface).

Just making light of other pinside discussions I have seen. In general, I agree with you and am hoping most will be drawn to the first of the options I presented which was offer help.
I did tone down my sarcasm in my intro post.

Just went back and reread the transformers section on pinrepair (it didn't stick before). Looks like (assuming the transformer is the same as the cocktail pins), the 4 lugs off the back of the transformer brick should read as follows:

"8 volts AC - converted to 7 volts DC through a bridge rectifier for the CPU driven feature lights. The regulator board fuse for this is 5 amps SB in cocktail tables, and 15 amps SB for standard pinballs.
28 volts AC - convert to 24 volts DC through a bridge rectifier for the solenoids. There is also an 82 volt MOV in this circuit for surge protection and a 10 amp fuse. Note 28 volts AC is also fed else where in the Gameplan circuit for the MPU board zero cross circuit, solenoid driver board, and the playfield.
10.5 volts AC - converted to unregulated 12 volts and regulated 5 volts through a bridge rectifier. The 5 volts is regulated through a LM323K transistor mounted on the underside of the aluminum transformer/rectifier board mounting plate. There is also a 11,000 mfd filter capacitor for the 5 and 12 volt circuits. One 5 amp fuse is used for both 5 and 12 volt DC circuits.
6.3 volts AC - for the General Illumination lights. Goes through a 5 amp fuse in cocktails and a 15 amp fuse in pinballs."

Should be easy enough to check tonight.

Ok, so 2 steps forward and 1 step back last night. Here is what went down:
After getting a shotgun transformers 101 lesson from my friend upstairs in the EE department and the help I recieved from pinside here, I went home and took a closer look at the transformer. There is actually printed text on the sides of the winding that I didn't notice the other evening since the light was bad and the text is faded (and because I have a bad habit of not noticing things right in front of my face as my wife constantly reminds me). But, the 10.5, 28, 6.3, and 8vAC markings are there pretty clearly on the output side of the transformer. On the front (input side) there are also markings that say 4A, 3A, 1, and 2. Pics are shown below. I assume that the A is for amps. The weird thing is that my transformer shows that leads were cut from the 4A input and only the 3A input. Also, the previous post on pinside that I used as a reference when hooking up my transformer/PSU cradle on my bench showed that his leads were connected to the 4A input.
I wanted to move the 120vAC input leads on my transformer to match my reference and see what changes. So, first I measured the AC voltage off the back of the transformer and got readings as follows:

Spec / measured vAC:
10.5 / 12
28 / 38
6.3 / 6
8 / 8

The measurements align with the high voltages I am seeing at the 12vDC and 24vDC test points on the PSU. Since the 10.5 and 28vAC are converted to 12 and 24vDC through the bridge rectifiers, I am measuring high DC voltage since the transformer is outputting high AC voltage.

xformer front (120vAC in) (resized).png
xformer rear (multi vAC out) (resized).png
xformer rear (multi vAC out) (resized).png

Next I swapped the 120vAC input lead from the 3A lug to the 4A lug and did the same measurements.moved the 120vAC input to the 4A lug (resized).png

The 10.5 and 28vAC outputs are now measuring correctly at around 11 and 28vAC. I went on to measure the DC voltages at the TPs on the PSU and was pleased with the results. I am now measuring 14vDC and 26vDC at the correct test points. Nice! After some thought, I wonder if the 4A and 3A are actually division factors for the line voltage. 120/3 = 40 (close to my 38 measurement) whereas 120/4 = 30 close to my 28 measurement...
TP 4 now measures 26vDC (resized).png14vDC at the 12vDC test point (resized).png

The voltages are a little high but within the ranges mentioned on pinrepair.com so I am happy. Now I am ready to put this PSU back in to the head. Here is a pic of the burn marks I mentioned before. I am thinking these were from a previous "incident" before I got the machine. Not sure what happened, but I found a receipt in the bottom of the cab that listed "transformer repair". Could mean lots of things. Just to be sure though I tucked the vAC output lugs on the back of the transformer in and shielded them with some electrical tape (I know it looks jimmy-rigged).burn marks inside head ground wall (these line up to the transformer vAC output lugs) (resized).png
output lugs on transformer with custom "shielding tape" (resized).png

So, did this have any effect on my ground buzz noise? Yes! The noise is still there but I can say it isn't really at the scary level it was before. I plan to jump into checking out some other things that may be contributing to the noise soon, but, I mentioned a few posts back that I took one step back. I had a couple of reset issues last night where my MPU2 would not boot. I took it out of the head and got it to boot on my bench though. After messing with it and playing a few games I think I need to take a look at the reset section which I did not look at closely during my MPU board work. So, I am going to tackle this MPU for the weekend and see where I can get and get back to the ground buzz diagnosis shortly after.

Hey where did you find that diagram? Thanks! Yes, looks like I connected it correctly for 115vAC. whew!
I am not sure if they were touching but they were definitely close. I bent the lugs in a bit and made sure there was more than 1/4 inch or so of clearance to the ground wall...

I am using the original paper schematics that came with the machine. Thought I had the complete set but I guess not. Yes you are correct it is when the GI is on and the displays are pulsing in attract mode. I mis-spoke in my OP. Thx again!

Hang tight guys. Lost my clock signal on my MPU board. Waiting on some parts and will give this idea a whirl went I get it back together!

Hey, my board took a crap on me. I had to send it to a local board expert. Dropped it off Monday. So, hopefully I can get back on this noise issue soon!!!