LCD Noise help!!!

rocksolidsr · 07-08-2007, 03:49 PM

I am getting noise in my LCD monitor and I'm wondering if anyone here can help, I'm going to attach a diagram to show the setup.

Things I've done.

I tested to make sure that the PSU and PC case were grounded...they are
I ran the computer off of house power (without inverter)...noise still there
another thing I noticed is that when the computer boots up during the BIOS post there is no noise...don't know why

rocksolidsr · 07-08-2007, 08:33 PM

one other thing to note was I tested the LCD with my laptop VGA out and there was no noise...

netman86 · 07-08-2007, 09:01 PM

Your diagram needs a little work, the wire going from the inverter to the psu shouldnt be black, its confusing. I realize that its a 120VAC line, so make it blue or brown or something...

The wire coming out of the inverter, going to a relay, then a distro block, then 'ground' just doesn't make the slightest sense to me at all... why would you have a distrobution block on ground? why would you relay ground? whats tripping the relay?

As for the LCD part, which ground in your factory harness are you grounding through? Why not just ground to the nearest metal part of the chassis?

rocksolidsr · 07-08-2007, 10:18 PM

I have re-uploaded a picture...

the relay is tripped by a start up/shutdown controller (i forgot that) 12V turn on

and the ground goes to a distribution block b/c I 2 amps and a capacitor grounded to that too

the factory harness is grounded to chasis

netman86 · 07-08-2007, 10:24 PM

so you're cutting ground to the inverter to turn it on/off?
correct me if I'm wrong, but shouldn't the relay be on the + side of the loop? Mine sure would be...
as for the ground distro block, is there just not much chassis to ground to?

Anyways, you mentioned that the lcd doesnt make noise during the bios post screen. That is an extremely low resolution. does it make noise when the operating system (presumably windows) loading screen is up?
I'd check your refresh rate, lcd's like it considerably lower than you might expect. Also, some LCDs really hate high resolutions.

rocksolidsr · 07-09-2007, 08:58 AM

I'm using the DSSC Startup/shutdown controller and at the time of wiring the manufacturing of the DSSC told me that is how I should hook it up...it has worked fine for well over 2 years

as for the distro block I have a 4 ga wire to chassis and then to distro block to make connections easier

my current resolution is 1024x768 and I tried that with my laptop VGA output and it looked fine...so I don't think it is the LCD

one other thing to note was that I had the LCD off and unplugged the power and ground and I heard a buzzing noise through the speakers

netman86 · 07-09-2007, 02:14 PM

How close is the LCD to the harness? LCD backlights have inverters, if the speakers from the car were close enough to said inverter i could see speakers getting interference from it.

How old is the LCD? sometimes the lcd inverter makes a buzzing sound when its getting ready to fail.

rocksolidsr · 07-09-2007, 02:45 PM

The speakers are not very close to the LCD, and the LCD is only 1.5 years old, the buzzing goes away once I reground the LCD

rocksolidsr · 07-09-2007, 03:50 PM

I checked the PSU, motherboard case, LCD, and VGA shield for proper ground and they are all grounded to my main ground to chassis in my trunk

the way I tested this was I used a multimeter and check to continuity from GND to each of those three things, and each one had continuity

netman86 · 07-09-2007, 04:31 PM

Ohh! I diddnt know it went away when you grounded it. What do you mean by re-ground, anyway?
Is there resistance between the ground in the harness and the chassis?

archaic0 · 07-09-2007, 04:52 PM

I had a buzzing sound coming out of mine the day I took it out of the box, but for me that turned out to just be the speakers in the screen and the built in amp turned up all the way.

That was plugged into the house that day with their power supply and the video was clean, but with no audio input to the screen, but the volume turned all the way up, the built in speakers were buzzing.

As for switching ground, this is in fact what your entire car does with it's electrical system. Every part of your car has a connected positive lead with a fuse, and a switched ground. This is how automotive power works.

Whether you switch positive or negative doesn't matter as far as I know, just that you break the circuit. Good planning will tell you to break the circuit as close to the power source as you can so you don't have a lot of potential for shorts while the switch is off, but barring a short somewhere, there is no opperational difference.

WHY do cars do this? I've been told it's to save wire although I'm not certain if that's the only reason or a valid one. I've also been told that it has to do with the fusing and switching techniques, but so far I haven't been able to draw a circuit in my head that would benefit from ground switching specifically.

Regardless, that's how the car as a whole does it, so why not continue the trend with your addon stuff?

Personally I switch the power line, but that's just out of habit, I don't have any real justification for it.

As for the noise problem, you said you used the laptop VGA and the noise went away, but even with the PC powered off house power there was noise on the LCD?

So the PC itself seems to be the source of the noise, whether it's from the PSU (most likely?) or some internal component is the only mystery left then, right?

What about that LCD plugged into yet another PC? Do you have a desktop PC that you can use as a test to see if there is noise there?

Also, the point that netman86 was trying to make wasn't that the LCD might be bad, but that the PC may be set for too high of a range. Yes, you tried the same resolution on the laptop VGA, but did you check the refresh rate? Pretty much all LCDs should be set to 60hz, although I've seen a couple that wanted 75hz. Regardless of what it's set to, try any options you have to see if it helps.

In the car, try 640x480 (if you can), 800x600, and 1024x768, all at 60hz and 75hz to see if any combination of those produces video without noise.

Any chance you can swap the video card and/or psu in the carpc? Elimination is key here. Whether we understand WHY a part is producing noise is secondary to just proving that it IS producing noise.

Good luck!

rocksolidsr · 07-09-2007, 06:23 PM

UPDATE

This seems to easy, but all I did was change the refresh rate from 60hz to 70hz and no more noise...

Thanks for all the help

netman86 · 07-09-2007, 07:00 PM

hehe, i knew if i kept stabbing i'd find it. lcds really dont like high refresh rates.

archaic0 · 07-10-2007, 11:09 AM

Actually in this case, it wanted a HIGHER refresh rate. He had it at 60hz and there was noise. Making it higher to 70hz cleaned it up.

We had one brand of LCD in an office I worked at that was like this too. Out of habit we set LCDs to 60hz but these were giving us fits. We actually needed to set these ones at 75hz for them to finally settle down! I even tracked down the product manual and it said nothing about this. It could have been the video card's problem I guess, but it was strange none the less.

I'm glad you're resolved!

archaic0 · 07-10-2007, 11:31 AM

Oh, and one more thing. I got a solid answer on the "switched ground" topic.

Cars initially only had dome lights and headlights in them as far as power goes. The dome light being the one that started this trend. In a real world wiring example, you can save as much as 50% of the wire used if you switch the negative wire instead of the positive wire.

This really only applies to items like a dome light where there is one device that is operated by multiple switches around the car. For devices that only have one switch, it's a moot point, but why have two different styles of wiring in a car? Because of consistancy and habit, the negative switch system has been the standard.

Ther are always exceptions though, of course, so if you have an example of a positive switched car that's great. I'm not saying it's the only way it's done, but only that it's the most common and only in recent years for specific reasons do car makers deviate from it as far as I know.

Any device or light that has multiple switches to turn it on where each switch needs to opperate independant of each other will benefit from a switched gound by way of saving the amount of wire used.

So do we need to care as CarPC installers? Nope. But it's consistant to switch the ground, not strange. And it's perfectly electrically sound to switch either leg.