MAME Cabinet grounding

[Slider1385]

I believe I'm getting a ground loop hum coming from my CRT monitor. I didn't really notice it up until now because I was doing some troubleshooting with a second LCD monitor attached to my graphics card. And what happened was I plugged the LCD monitor into an external outlet (Not the power strip in the cabinet) and as soon as I plugged in the DVI connector the hum went away. I experimented a bit and as long as I touch the dvi cable to the connector on the graphics card, it goes away. If I unplug the LCD monitor from the wall and try it, the hum is still present.

So I'm guessing I have a ground loop somewhere. Question is should I have run a ground to everything (My PC is de-cased and mounted on a wooden board) and would I pull that ground just from a ground on the powerstrip itself? Should I have the monitor, PC and PC power supply all grounded externally to the wall power supply like this?

Thanks All!!

[bobbyb13]

Some pics or a diagram would be good to best offer advice.

I imagine that your CRT only has a 2 prong plug and the chassis and/or the tube strap should get grounded.

Authentic cabs often have a common anchor plate/bolt with loop terminal connected wires that bond all components together in a single location.

If you construct that and then bond it to your main supply ground you should be all set.

[Slider1385]

Thanks for the reply. I'll attach a couple pictures, hopefully they can clarify.  The CRT is a de-cased Samsung TV, so yes it only has a two prong outlet.

Would I ground the board as well as the CRT chassis you think? Probably same to one of the PC motherboard standoffs?

[bobbyb13]

Cool build.

Find a ground point on the TV chassis and also some metal part of the tube assembly and bond that to your power inlet ground and you should be good.

A regular arcade monitor chassis and tube is tied together groundwise through the frame and then linked somewhere in the cabinet to power supply ground- but a TV setup is lacking that of course so you have to make it.

In theory the motherboard should already be grounded through its power supply but if the noise continues then you would want all those components grounded to a single point.

[Zebidee]

Thanks for the reply. I'll attach a couple pictures, hopefully they can clarify.  The CRT is a de-cased Samsung TV, so yes it only has a two prong outlet.

Would I ground the board as well as the CRT chassis you think? Probably same to one of the PC motherboard standoffs?

Make sure that you are connecting DC ground to DC ground - never try to connect a CRT ground to mains power ground. This is because CRTs use DC rectifiers and/or isolation transformers to make a completely separate circuit from the mains. Also, some of the metal shields around TV chassis are actually active B+ voltage (e.g. the HOT). So check with your multimeter first for connectivity between your possible contact point and video ground at the inputs.

Try connecting to PC ground via a spare power connector, or even sliding the wire in behind one of the main ATX power connections. This will give a more direct return path. Even assuming the standoff pads are connected to circuit DC ground, it would mean a longer path and more interference.

Of course your hum could be a funky connection somewhere, so check all your CRT video connections again too, especially grounds. If you are using a transcoder (I see component inputs to TV there) that also means more connections to go funky, so gently wiggle/twist your cables and tap on things.

You could try a line filter for the TV power input to clear up noise from the AC.

[Slider1385]

So sounds like that is conflicting grounding spots? I should connect a ground to the monitor frame and chassis to ground from my PC power supply? Or to the earth ground from my power strip?

[Zebidee]

So sounds like that is conflicting grounding spots? I should connect a ground to the monitor frame and chassis to ground from my PC power supply? Or to the earth ground from my power strip?

DO NOT connect *anything* from your monitor chassis (or CRT tube) to earth ground on your power strip.

It is OK to connect from your monitor's DC ground to your PC's DC ground as they are (or should be) already connected via the video input. This is why I suggest you check all your video input connections first, because it might just be a loose connection somewhere.

How exactly are you getting component out from video card to TV?

[bobbyb13]

Thank you Zebidee for clarifying that.

I should have replied in better detail.

[Slider1385]

Copy that, I'll use one of the molex connectors on my power supply to pull DC ground.  What about the crt frame? Should I bother with a ground for that? Should that be an earth ground? What about any other metal parts, such as the coin door while I'm at it? Should that be earth grounded?

I have an audio authority converter for component.

[bobbyb13]

All the monitors I've seen have a ground wire running from the spring tensioned wire on the tube itself to a grounding pin on the chassis neckboard.

Andrew can comment on the function of it but it always appeared to me that it was acting as the cathode for the tube high voltage circuit.

[Zebidee]

All the monitors I've seen have a ground wire running from the spring tensioned wire on the tube itself to a grounding pin on the chassis neckboard.

Andrew can comment on the function of it but it always appeared to me that it was acting as the cathode for the tube high voltage circuit.

More or less. That grounds out stray voltage that "leaks" (for want of a better word) from the anode cap, and anything else around the tube frame/back. That's why it is attached to a spring-tightened metallic mesh wire that fits snugly around the back, and that greyish slightly rough paint (aquadag, conductive), to catch the voltages and drain them off. That should be attached, and although it is a "ground" it does not connect to mains ground.

[Slider1385]

Well, sorry for this, but it turns out its not my CRT. Sorry, the buzz carried, I assumed it was the CRT because it went away with the external monitor. Turns out the buzz is coming from my speakers/subwoofer. When I turn off the speaker system, buzz goes away, I traced it back to my sub humming.

I'm going to play around a bit with the connections, but its confusing to me I would get a ground loop when the whole cabinet is plugged through one power strip. Could it possibly be the motherboard needs grounded? Since it's de-cased it might need a grounding point on a standoff?

[Gilrock]

I know in the guitar world a lot of my amps always had a ground lift so removing the ground from the amp is what made the noise go away.

[Zebidee]

Try using another power strip, getting power from another part of your house (away from the freezer or whatever might be causing local line noise), installing a EMI (electromagnetic interference) line filter into your cab, or using a different speaker system.

Line filters are simple to install in a cab, and even come built-in to PC-style power plugs (where you can remove the power cord altogether). Looks professional, neat and tidy, 6-10A rating should be sufficient. Some varieties include a fuse holder. Power strips sometimes have line filters built-in already, sometimes they don't, and sometimes they aren't that great. Having an extra line filter won't hurt.

For example: https://www.amazon.com/Outstanding-Black-Socket-Connector-Filter/dp/B082F3MN42/ref=sr_1_3?dchild=1&keywords=emi+filter+socket&qid=1625771107&sr=8-3

CRTs do create high frequency line noise themselves - this is why I've always avoided using TV speakers when installing TVs into cabs. Too much noise from the CRT circuit creating hum on the TV speakers because they are on the same DC circuit loop, so no isolation at all. However using separate speakers of decent quality has usually been fine, even when plugged into the same power strip.

[Ond]

You may be on the right track with the connectors and the hum.  Are there RCA connectors plugging into the sub? If so move them around a little to check.  If these connectors get dirty or worn and don't make a good connection you WILL get hum in the sub.