Hp compaq dc 7100 , my problem is that as i start my pc than 4 beep with time interval 0.2 sec with led than after 0.5 sec it start again , remain continously

hold it down intil it turns on or try charging it then tying again later

try these
http://h30499.www3.hp.com/t5/Business-PCs-Compaq-Elite-Pro/hp-dc7800-5-beeps-problem/td-p/1142014#.VJYGUvXpAg


HP Guide to beep codes:
http://h10025.www1.hp.com/ewfrf/wc/document?cc=uk&lc=en&docname=bph07107
(see your machine spevcs for which BIOS you have if you don't know)

http://h30499.www3.hp.com/t5/Business-PCs-Compaq-Elite-Pro/Boot-problems-with-dc7800-CMT/td-p/1080736#.VJYHL_XpAg

and many more generated by searching the internet for "5 beeps hp dc 7800"

The second reference I posted is possibly the place to start
20.December.2014

make sure both the 4 pin and 20 pin connectors are firmly installed. if the mobo light flashes how many times does it flash, do you hear any beeps?

If you need further help, reach me via phone at https://www.6ya.com/expert/joel_63c670a84e9f6a97

Hello there, this means the machine has an internal hardware failure. 4 beeps almost always indicates the memory (RAM) has gone bad in the machine. If you are unsure of how to put new ram in the machine yourself you are going to need to contact a local service person to do this, it should not be too expensive. If you need help locating someone in your area please respond to this answer with your zip code and I will find someone for you.

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Thanks
Tim

It's a critical hardware failure. You're better off getting it serviced. It would just confuse you if I tried to walk you trough how to find the problem.

It all depends how many beeps there were.
From http://h18000.www1.hp.com/products/quickspecs/12253_div/12253_div.HTML
Diagnostic LED Explanation Table Number of 1-second red LED blinks followed by 2-second pause, then repeats:
2-processor thermal protection activated
3-processor not installed
4-power supply failure
5-memory error
6-video error
7-PCA failure (ROM detected failure prior to video)
8-invalid ROM, bootblock recover mode

I have only really come across 4 beeps = order new Power Supply or 5 Beeps = reseat the memory and hope it works

Regards,

Oli

why wont you buy a new power supply or a plug that converts from a 20 pin to a 24 pin

normal white wire....?
white with red....?
brown....?

4 beeps & 4 flashes mean problem with the power supply. For my experience, most of the time the PSU is good, it is just overloaded.

For more LED and Beep messages, check this link:

http://bizsupport2.austin.hp.com/bc/docs/support/SupportManual/c00067072/c00067072.pdf

you need to replace your hard drive !!!