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LS855 motherboard-Turns on for 3 secs and then shuts down
http://www.motherboards.org/forums/viewtopic.php?f=3&t=123208
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Author:  isend [ Thu Mar 10, 2016 8:53 am ]
Post subject:  LS855 motherboard-Turns on for 3 secs and then shuts down

(This is a follow-up story of http://www.motherboards.org/forums/viewtopic.php?t=123204 .)

Our LS855 turns on for only 3 secs and then shuts down. It is a microATX with a 20-pin power socket. We forced it to turn on by shorting pin 13 & 14, and measured its pin voltages—all are normal.

The motherboard has some FET chips PHB32N06LT. When powered off we measured the resistances of one of these chips and got a drain-to-ground resistance of 83.2 Ohm, and a source-to-ground of 51.9 Ohm. We also measured another and got drain-to-ground 83.5 Ohm, and source-to-ground 31.7 Ohm, which is slightly different from that of the first chip. Is this normal?

Is it possible to get a detailed circuit diagram of LS855 or a similar model? (I only found its product manual http://docplayer.net/2823813-Endura-ls855-product-manual-www-radisys-com.html . I also managed to access Radisys' official document library but couldn't find anything more useful.)

(We also checked the ATX power supply. The power supply is normal.)

Author:  Karlsweldt [ Thu Mar 10, 2016 4:46 pm ]
Post subject:  Re: LS855 motherboard-Turns on for 3 secs and then shuts dow

There is a green wire to the left of the latch on the main ATX power plug. This is the control lead to turn on the PSU. Bridging it to a black lead turns the PSU on. If the motherboard comes to full life in a few seconds, should be no damage. But if no life signs, undo the bridge. Probable issues in the motherboard circuits.
You mention bridging pins 13 and 14, that is for the 20-pin ATX power plug, correct? Or the front panel header pins?
Likely the POST process found a critical error, and shut down the board to prevent serious damage. Could be a mismatched CPU or memory, or other critical error. Even the speed of the CPU cooling fan, or sudden rise in CPU temperature.
Is the BIOS battery good, and the CMOS reset jumper set as 'normal'?
There really is no way to get a diagram of a motherboard. Could be three to five layers of traces, and the design is copyright property of the company that made it. A "flow chart" of a motherboard is otherwise easy to find.

Author:  isend [ Wed Mar 16, 2016 5:09 am ]
Post subject:  Re: LS855 motherboard-Turns on for 3 secs and then shuts dow

To Karlsweldt,

Thank you so much. We really appreciate your comments. However, our technician still had trouble identifying the broken part of the motherboard, so we decided to send the machine to someone who claims himself to be a professional (a previous HP/Agilent technician). He may charge us less than Keysight/Agilent did. We will see.

Author:  evasive [ Tue Mar 22, 2016 8:18 am ]
Post subject:  Re: LS855 motherboard-Turns on for 3 secs and then shuts dow

Sounds more like the capacitors went bust. If your HP technician is any good he will find and replace those.

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