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Bad CPU?

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April 7, 2010 6:43:59 PM

I have a PC that someone changed out the PS and nothing worked after. After working on the switch, it starts. I get no POST beep and no video. The HDD spins and so does the DVD. The case fan and the CPU fan both have power. The PC does not even beep without RAM installed. Does this mean that the CPU is bad?

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April 7, 2010 7:05:08 PM

It's still probably the PSU.

System specs - including both PSU's?

You have worked through our standard checklist and troubleshooting thread:
http://www.tomshardware.com/forum/261145-31-read-postin...
yes?

I mean work through, not just read over it.

Breadboard - that isolates any kind of case problem.
http://www.tomshardware.com/forum/262730-31-breadboardi...
Breadboard with just motherboard, CPU & HSF, case speaker, and PSU.

You can turn on the PC by momentarily shorting the two pins that the case power switch goes to.

You should hear a series of long, single beeps indicating memory problems.
Silence indicates a problem with (in most likely order) the PSU, motherboard, or CPU.

So, the best bet is to replace the PSU with a known good one of similar power capacity. Brand new, out of the box, untested does not count as a known good PSU.

Next best thing is to get (or borrow) a digital multimeter and check the PSU.

Yellow wires should be 12 volts. Red wires: +5 volts, orange wires: +3.3 volts, blue wire : -12 volts, violet wire: 5 volts always on. The gray wire is really important. It should go from 0 to +5 volts when you turn the PSU on with the case switch. CPU needs this signal to boot.

You can turn on the PSU by completely disconnecting the PSU and using a paperclip or jumper wire to short the green wire to one of the neighboring black wires.

http://www.youtube.com/watch?v=5FWXgQSokF4&feature=yout...

This checks the PSU under no load conditions, so it is not completely reliable. But if it can not pass this, it is dead. Then repeat the checks with the PSU plugged into the computer to put a load on the PSU.
April 7, 2010 11:02:01 PM

I will check all of this and reply back. Thanks for the help.
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April 8, 2010 7:17:33 PM

System specs:
eMachine W3107
AMD Sempron processor 3100+ 1.8 GHz
100 BG ATA HDD
512 RAM
Onboard video

Power supply: Bestec
Model ATX 300-12E
Input: 100-127V - 6A
Output: +12,+5, +3.3,-12,+5VSB

I don't have the specs for the first power supply.
I have checked all the wires with a multimeter and all are exactly what you told me they should be. I breadboarded the machine, PSU,MB,CPU, fan and speaker. I didn't need to short the power switch pins. The PSU just started on its own. After starting there were no beeps. I do not have any RAM in the MB.

Thanks for any help.
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April 8, 2010 7:42:33 PM

It could be a bad CPU, Mobo, PSU. Do you have any other PSU's you could test with, or borrow from a friend to test with?
April 8, 2010 7:47:03 PM

I just tried it with a different power supply that I took out of an old machine. It wirked fine when removed.

Echostar

450Watt PSU

Outcome is the same. Fans start running without having to short the switch and there are no beeps.
a b à CPUs
April 8, 2010 7:53:24 PM

sounds like its either the Mobo or CPU then. In either case, you'll probably need to replace both since it probably won't be worth it to replace just one.
April 8, 2010 8:02:01 PM

Thank you so much for your help. I will do a little shopping.
a b à CPUs
April 8, 2010 8:17:32 PM

no prob- sorry its having trouble:-(
April 15, 2010 12:23:54 PM

Hey there, I bought a used mobo on ebay so that is all I bought. Replaced the mobo and breadboarded it. Must not have had the speaker plugged in because I didn't get a POST beep but the splash screen came up and things are running just fine.

Thanks for your help. My problem has been resolved. (so far)
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April 15, 2010 3:37:48 PM

Great man- glad you got it back up and running!
!