Hi, i am workiing on an emachine T6420 for my brother and this thing starts up with the fans running and a black screen. my monitor that i connected to his computer shows a yellow standby light on the monitor which should normally be green. i even swapped my hard drive with his and still nothimg but a black screen. both hard drives power up and you can hear them running. all of the cooling fans are running. the only thing i can think of is that the onboard video which is permantly mounted to the motherboard, is fried.

youmight remove your graphics and check the socket for dust restart your computer this should activate the found new hardware wizard
if you run an extended desktop this will tell you if the monitor has gone bad
use a vga lead to attach it

click start control panel display change display settings adjust resolution this may vary depending on your operating system you should a no 1 and a no 2 with detect and identify keep an eye on

the screen when you select the number tabs this may vary depending on your operating system when you select the no 2 tab this should be the extended desktop always click apply after making any changes
i have read some reviews on your monitor
So the ex-hubby calls me with his woes - his eMachines T3124 is having problems. Seems that when he cold boots in the morning it hits the "E" splash screen and then just goes to black screen. No bios boot screen, no boot to windows. Everything sounds ok though. After a few restarts it boots ok. So, first trouble shoot is to replace the 180w Bestec power supply with a salvaged 300w Hippro from my T6420 (which was replaced with a 500w) - the one I replaced the mobo on. Works great ... for one night. Next morning I get the frantic call - now he gets nothing after the "E" splash but white wavy lines, but the computer sounds like it's running fine. Ok - ok ... think, think. First thought was maybe the onboard graphics have gone, next thought maybe the mobo is going ....

What is the next thing we do after trying a known, good power supply? Try another working montior. I had him try using a trusty older CRT monitor and Wha-Lah! After less than 3 years the E17T4 LCD monitor had failed. Most recent crisis handled. This little incident has made him realize though that he needs to burn all his picture and videos to CD. Scared him to think he could have lost them. Today's reminder? BURN your pictures and videos now! You never known when something might happen. And eMachines monitors are ****.

hope this helps

This is eMachines Support, T6420 desktop computer>Specifications, and downloads page for the User Guides,

http://www.emachines.com/support/product_support.html?cat=Desktops&subcat=T%20Series&model=T6420

The eMachines T6420 does not come with a graphics card from the factory.
It comes with Integrated Graphics.

Integrated Graphics is also known as OnBoard Graphics.
On the motherBoard

The GPU is soldered to the motherboard.
(Graphics Processing Unit,

http://en.wikipedia.org/wiki/GPU

OR,
someone has installed a graphics card, into the PCI Express x16 slot on the motherboard.

You can determine real easily if your computer is using Integrated Graphics, or a graphics card.

If it is using Integrated Graphics, the monitor will be plugged into the I/O area, on the back of the computer, and into a VGA port,

http://en.wikipedia.org/wiki/VGA_connector

(I/O area;

I/O = Input/Output

The I/O area, is the area that has the rectangular metal shield, and Input/Output devices are plugged into this area.

Devices such as a Keyboard, Mouse, Printer, and so on )

HOWEVER, with the motherboard used in an eMachines T6420, the I/O area could also have a white DVI port.

http://en.wikipedia.org/wiki/Digital_Visual_Interface

This was an option offered. The DVI port still connects to the Integrated Graphics on the motherboard.

The eMachines T6420 uses an MSI MS-7207 motherboard.
(Micro Star International, Inc.)

http://www.tigerdirect.com/applications/searchtools/item-detailsInactive.asp?EdpNo=2665522

The above link is to,
1) Show the motherboard
2) Provides a link to the Motherboard Manual

Looking at the photo of the motherboard, you will see two long White slots.
Above these two slots you will see a longer White slot.

The bottom two white slots are PCI expansion slots.
The top longer slot is a PCI Express x16, expansion slot.

This is where a PCI Express graphics card would go.

I would like you to scroll to the bottom of the page, to the Product Resources heading.

Under it is a link to the Motherboard Manual, for the MSI MS-7207 motherboard.
Also known as K8NGM2-L

Left-click on the blue - MSI K8NGM2 Series Manual.

You will be looking at a blank white page, as the PDF file downloads in the background.
Took 34 seconds with a medium speed DSL connection, for me to download it.

NOW,
No Signal on the monitor, means the monitor is Not receiving a video signal.

1) Check the monitor cable if it is removable. Use it on another working computer to insure it is good.

2) If the monitor cable proves to be good, the next area to check would be the graphics.

If the computer is using Integrated Graphics, and no one has installed a graphics card, the test would be to install a graphics card, and plug the monitor into it.

Computer off, unplugged from power, Anti-Static precautions are observed before reaching inside the computer, and handling the graphics card.

I don't think the problem is the graphics. Could be wrong, but we'll see.
Let's carry the diagnosis past the graphics for now.

I think the problem could be due to a bad Power Supply.
Weak Voltage power rail.

Enough power to light lights, and spin fans (Perhaps), but not enough to turn the Processor on.
(Fans may be spinning, or may just spin a few times, and stop)

1) ALL the lights use Less than 1 Watt of power
2) EACH fan uses 2 to 3 Watts of power.

3) A typical Processor can use 51 to 125 Watts of power.

Continued in an additional Comment.

has the monitor got a proper connection to the pc? also have you tried rebooting the pc?

three things i find to cause this,

one, the power supply is bad,
two, the power connector is lose or not push in
three, the heat sink on the processor has come off. This will cause the cpu to over heat and self shut off.

to test the power supply, pull the connector off the mother board and short the green wire to any black wire with a paper clip, this will switch on the power and see if it stays on.

if you need to replace it, you cannot use a regular atx power supply. E machines have a different pin out.

Change the order so that the CD boots first. Try to run Repair from the Windows CD. If unsuccessful, you'll need to reinstall Windows.

This problem comes due to problem with RAM and CMOS.

first you get RAM outside the CPU and clean properly by brush,and put into the RAM SLOT.

it work.
otherwise ,you have to change RAM.

No you cant, the gateway GX7018E uses integrated graphics like the T6420. You will have to buy a cheap PCI or PCI-E graphics card to test it out. 

Most likely your video card is broken. To check if the motherboard isnt broken either, I would suggest you test usb, sound, and other features of the motherboard.
 If your motherboard isnt broken then you have to buy a "pci-e graphics card" at your local store. Shouldnt cost much, about $40-70. Hope that helps.

ohk i also have an emachine make sure you have the moniter hooked up all the way i have had the same problem

Lets start with the basics. Does the monitor have power, do you get a light on the monitor when you try to turn it on? If you do then do you get any lights on the computer when you try to turn it on? If the monitor has no lights on it check the power connections and if still no lights then try plugging it into a different outlet preferably directly into the wall. If still nothing then the monitor is bad. If the computer has no lights but the monitor does once again try plugging it in to a different outlet and if that does not work there is a problem with the computer most likely the power supply, CPU, or motherboard. If both have lights but you get no display on the monitor then check the connection between the monitor and the computer. if the connection is good (I would ***** in the screws on the sides of the connectors just to make sure there is a good connection) then there could be a problem with your video card.