Hum Hum Hum!

[Deleted]

Hardware issue—strange mains hum when recording video

I’ve just upgraded to a new tower PC (roughly ¾ the size of a standard case). Everything runs fine—except when I try to record video. I get a mains hum, which you can hear in this YouTube clip:

The odd part: the hum only starts when I lift my hand off the PC. If I rest my hand on the case, the hum disappears. But of course, when I need both hands on the keyboard, the hum comes back.

Setup details:

• Using the same USB headset I had with my old PC (no issues there).
• Tried “earthing” the case to a nearby brick wall with a spoon—no effect.
• Touching the case seems to help, though it’s intermittent. Some days it happens, some days it doesn’t.
• I suspect it’s related to capacitance, since I’m only touching the painted surface, not bare metal.

Has anyone seen this before or have suggestions on what I could try next?

 

[Gasman]

Try earthing the tower.?
By you touching it, you will likely be earthing it.?

I would have hoped it was earthed via the mains though.?

 

[CJ_London]

Unfortunately when I need to type something on the keyboard I need both hands!

sock off, foot on casing?

 

[Mark_]

I'd double check to make sure how each connection is set. The "Hmm" sounds like an issue I get when my headset jack isn't properly seated. Improper connection some where could be causing a slight amount of interference.

I haven't heard of it with a USB device though, going by the sound.

if it IS a grounding issue, you may need to ground yourself/headset. If YOU are causing the issue, it may be that when you touch your case, you ground. Otherwise you could be causing the interference. As a simple check, try making sure you have skin to case connectivity when using the headset.

I would also look to current humidity / when last your furnace was running in case your getting an atmospheric charge.

 

[NauticalGent]

If you try CJ's idea, we are going to need pic and/or video. For the sake of science of course...

 

[Mark_]

Still waiting on that picture/video to see if your foot solved the issue...

 

[Galaxiom]

Are there any other mains powered peripherals connected?

 

[Galaxiom]

Could it be that the cover of the PC is not properly connected internally to the chassis and the light pressure from your hand is making it connect? Try applying a light pressure with something other than you hand.

 

[Minty]

To elaborate on Galaxioms second suggestion, if you have a multi meter, set to AC volts, measure from a bare metal part of the case (lid) to a known good earth point.

If you see a reading of more than a few volts then it's likely the case isn't earthed, or earthed properly.

Edit: I want to see that video as well...

 

[The_Doc_Man]

A "floating" ground can be a pain in the toches - literally, if the float amount is high enough.

When I was working my way through college, ungrounded or poorly grounded musical instruments OFTEN had this issue. Of course, back then, the two-prong plug was common-place for music amplifiers. Now, almost everything you can buy from a quality brand-name will have the three-prong plug with a positive ground. Further, the few modern items that still use two-prong plugs will have asymmetric plugs with one of the plugs being wider than the other. That "UL" sticker pretty much guarantees it.

If you have three-prong plugs and a multi-meter, you can do a quick CAREFUL test by comparing the ground socket to each of the other two sockets using the meter's AC Volts setting. In the USA, one of those leads will be "hot" and the other "not." But the one that isn't hot should be zero with respect to ground. If it is anything else, you have a ground fault somewhere.

If you have a simple power strip that you commonly use, start there and work backwards towards the wall to see where the ground fault originates. If the fault is in the wall socket you will need a licensed electrician to fix this. Or take off the wall socket plate and verify that all of the wires are connected to screw-type terminals on the socket. In the USA, that will be black, white, and green wires. You also need to know where your wiring enters the house and where the ground lead is attached to either a buried copper spike OR a metal part of the plumbing. Sticking a spoon in the wall won't work unless the spoon is touching the plumbing.

If you have a battery-based power conditioner connecting your PC to the wall, check THAT first of all - and if it shows up there but then the next leg (where the conditioner is plugged into the wall) does NOT show a ground fault, it is time to either replace the conditioner or exercise a warranty option.

You can also use the Multi-meter Ohms setting to verify good connection on the PC's power cable. If you've gone to the trouble of getting out the multi-meter, might just as well check everything.

 

[Vassago]

Also, when you look at the jack, does the jack appear that any of the metal ring is touching the case? That can cause feedback that might be correcting when you have pressure on the case. There are many possibilities here. Have you tried different peripherals? Do you have a picture of the jack?

 

[Mark_]

A "floating" ground can be a pain in the toches - literally, if the float amount is high enough.

I believe I need to refrain from asking what you've chosen for your seat...

Then again this is a good reason not to earth to your plumbing.

 

[Minty]

>>>I've connected an old speaker wire between a lug on the PC for attaching some sort of locking mechanism and the radiator<<<

Oops!

I think (but am not qualified) that it's part of build regulations to have your plumbing earthed in the UK...

 

[Minty]

The distinct tingle is a probably a result of the earthing being poor, allowing a potential difference (voltage) to build up.

In theory once earthed the problem will dissipate (pun intended)

 

[kevlray]

My guess is that the power supply is not properly grounded (or the motherboard has a bad ground connection). Hums in speaker systems a lot of times(back in the day) were caused by a floating ground.

 

[Galaxiom]

I've connected an old speaker wire between a lug on the PC for attaching some sort of locking mechanism and the radiator.

There is something wrong with the Earth connection from you power point, the lead or the computer power supply.

As I was preparing the end of the copper wire, I had attached one end to the PC, I was holding the other end ready to attach it to the radiator when I got the distinct tingle from it. In other words the PC case has some sort of voltage associated with it.

There are capacitors between the live and chassis, and the neutral and chassis designed to bypass high frequency interference to ground. If the chassis is not earthed the bypass capacitors will put small but detectible voltage onto the chassis.

Wherever the earthing has failed, you need to get this fixed.

 

[Mark_]

I'd also open the case and make sure nothing is loose and touching the case. Stray live connection can cause immense problems.

 

[Galaxiom]

I'd also open the case and make sure nothing is loose and touching the case. Stray live connection can cause immense problems.

If that were the problem there would be more than a tingle on the case.

I used to work as a electrical/electronics tech in the renewable energy industry and have first hand experience with bypass capacitors putting small voltages onto unearthed equipment.

 

[The_Doc_Man]

I believe I need to refrain from asking what you've chosen for your seat...

Cotton-blend pants on naugahyde seats leads to static buildup. In which case a poor ground will allow you to discharge it in the MOST unexpected ways. But sorry, no cameras were rolling at the time.

 

[Deleted member 147267]

maybe put a voltmeter between the end of the wire and the radiator?