Power led is lite, fan at power supply and processor also running.but my monitor is totally blank.

Bad Power Supply.
Weak voltage power rail.

Enough power to light LED lights, and spin fans, but Not enough power to turn the Processor on.

1) ALL of the LED lights on at one time, use less than 1 Watt of power.

2) EACH fan uses 2 to 3 Watts.

3) A typical Processor can use 51 to 125 Watts.
Just depends on what Processor it is.

No Processor operating, No computer.
No computer operating, No signal on the monitor.
NO video signal to the monitor from the computer)

Need guidance in replacing your Power Supply, please state in a Comment.
Need guidance in choosing a correct Power Supply, I would be happy to give a few references for you to look at.

Regards,
joecoolvette

(Do you have a KNOWN to be good, Compatible power supply around that you can use for a test?
Perhaps in some computer tucked away in the closet, etc.
Maybe a relative or friend has upgraded to a better computer, and has an unused one you could borrow the Power Supply from)

1) If the monitor has a removable monitor cable, and you have a Known to be good one to use for a test unit, try it. If it isn't removable, try another Known to be good working monitor.

2) If the monitor cable, and the monitor proves to be good, the next thing to suspect would be bad graphics.
Bad Integrated graphics on the motherboard, or if a graphics card is used, a bad graphics card.

3) I suspect it is a bad Power Supply. An issue that comes up so much, that I would skip 1 and 2 above, and check the Power Supply.

A) ALL of the LED lights, mumba_mwaba, use less than 1 Watt of power when in use.

B) EACH fan uses 2 to 3 Watts.

C) A typical Processor can use 51 to 125 Watts. Depends on what Processor it is.
(Older processors such as a Pentium II for example use less)

Enough power to light LED lights, and spin fans, (Or spin a few times and stop), but Not enough power to turn the Processor on.

This is why you may see LED lights lit, and fans spinning, but the computer isn't working.
The computer isn't sending a video signal to the monitor.
No Signal.

Do you have a KNOWN to be good, compatible Power Supply to use for a test unit?
May sound far fetched, but perhaps there is someone who has upgraded to a better computer, and has a computer they're not using, and you could borrow the Power Supply for a test unit.

verify that the monitor is working if you can. Is the hard drive lite lighting up, If not try removing and reseating the ram. if the hd lite in on then it would sound like the graphics card is out, try replacing it.

One of two things are happening here. Either there is a problem with the cord and/or electrical connection or there is too much dust in the ventillation holes on the monitor and its over heating. If you have not changed the power cord, then try relocating to a different power outlet. Then clean out the ventillation holes with a can of spray air and dusting.

Indicates you have a bad Power Supply, to me.
Weak voltage power rail.

1) ALL of the LED lights combined use less than 1 Watt of power.
2) EACH fan uses 2 to 3 Watts
3) A typical Processor can use 51 to 125 Watts.
Depends on what Processor it is.

(The Pavilion a6750f uses an ATX power supply, that has a maximum rated wattage of 300 Watts.

http://h10025.www1.hp.com/ewfrf/wc/document?docname=c01619697&lc=en&dlc=en&cc=ca&lang=en&product=3872428#N934

The AMD Phenom X4 9650 processor can use up to 95 Watts,

http://en.wikipedia.org/wiki/List_of_AMD_Phenom_microprocessors#.22Agena.22_.28B2_.26_B3.2C_65_nm.2C_quad-core.29

(In the chart under the heading TDP)

Enough power to spin fan's, but not enough to turn the Processor on.
Not enough to also light that Power On LED light.

No Processor operating = No Video Signal from the computer.
Hence the monitor is blank.
(No Signal)

http://h10025.www1.hp.com/ewfrf/wc/document?docname=c01356113&tmp_track_link=ot_recdoc/c01619697/en_ca/c01356113/loc:3&lc=en&dlc=en&cc=ca&lang=en&product=3872428

NOTE*
Computer unplugged from power.
Anti-Static Precautions observed.

Anti-Static Precautions:
Your body carries Static electricity.

Static will fry out (Short Circuit) the hardware components inside a computer.
(You may not even see it, or feel it)

Computer unplugged from power, computer case open, working on a table;
TOUCH the metal frame of the open computer case, to relieve your body of Static.
Should you leave your computer in the middle of working on it, upon your return be be SURE to Touch the metal frame again.

1) Computer isn't working, due to the Processor isn't working, due to a bad Power Supply.
Weak voltage power rail.

Enough power perhaps to light LED lights, and maybe spin fans, but not enough power to turn the Processor on.

Computer Power Supply's are rated in Wattage.
Volts times Amperes = Watts.
(Average computer power supply in a fairly new computer, is rated at around 300 Watts)

A) ALL of the LED lights combined, use less than 1 Watt of power.
B) EACH fan uses 2 to 3 Watts.
C) A typical Processor may use 51 to 125 Watts.
Depends on what Processor it is.

Power Supply with a weak voltage power rail, can have enough power to light LED lights, spin fans, but not enough to turn the Processor on.

Solution?
Test the voltages of the Power Supply,

Or, use a KNOWN to be good Power Supply, that has the correct amount of power cables, the right kind of power cables, the correct size, and shape of the case, and enough Wattage, for a test unit..

A bad Power Supply is one of the leading causes of computer failure.

I suggest you check the Power Supply. Bad Power Supply, weak Voltage power rail.

1) ALL the LED lights use less than 1 Watt of power.
2) EACH fan uses 2 to 3 Watts
3) A typical Processor uses 51 to 125 Watts. Depends on what Processor it is. (Older one's use less)

The HP Pavilion a1540n specifications state it uses an AMD Athlon X2 4200+, processor.

(2.0GHz. Dual Core. Built upon the Windsor Core. Uses a Socket AM2 processor socket)


HP Support > Pavilion a1540n desktop computer > Specifications,

http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00679533&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&product=3184140&lang=en

Can use up to to 89 Watts,

http://en.wikipedia.org/wiki/List_of_AMD_Athlon_64_microprocessors#.22Windsor.22_.28F2_.26_F3.2C_90_nm.29

Enough power for the LED lights, and fans, but Not enough for the Processor.

No Processor operating, No computer.
No computer, No Signal to the monitor.
(Video signal)

The Power Supply used is an ATX style, and 300 Watts.

A computer only uses the power it needs, and No more.
If you install a Power Supply with more Wattage than the original, it will be fine.

You either have a dead CPU or a dead motherboard or a partially-dead power-supply -- the power-supply should supply different voltages to different computers, i.e., one voltage to spin fans, and a different voltage to power the RAM, and a different voltage to power the motherboard. So, it can be "partially" dead.

Borrow a power-supply, and try that first.
If all else fails, take the computer to a qualified technician.

Well.... The motherboard could be dead... or check the power supply unit. Check the memory chips, see if the capacitors around the processor looks blown up (then motherboard is kaput) or make sure the monitor is properly connected to the motherboard monitor port... It could be numerous reasons ... See if the processor cooler fan is running and check if the processor's cooling paste is OK which would cause the processor to run TOO HOT and would prevent the PC to start-up etc etc....

fan o. k
get the eq wit a bootable win. disc and run into the bios for a new set in xp