Bad CPU?

I have a homebrew PC with an AMD K7800 CPU.  The computer has developed a problem where it will poer on for 1 second then shut down.  There is no video display and no BIOS beeps.  I did some trouble shooting and jumpered the PS Pins 13&14 (ATX) and the HDD's and fans came to life.  I then started removing cards, ram, etc to see if they were the culprit.  No caused a different reaction except the CPU.  When I pulled the CPU and tried to power on, the PC acted just like I had jumpered the PS.  When I put the CPU back in and try to power on it comes on for one second then shuts down.  Would anyone here say this is a bad CPU?  Maybe MB?
firemanrobAsked:
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mjcoyneCommented:
"On power-on the CPU begins to execute the BIOS code => that's what the POST (Power on Self Test) is, and that's what generates the BIOS displays.   No CPU running ==> No POST test, no display, no beeps, etc"

This is incorrect.  The CPU does not run the BIOS, the BIOS runs (or at least detects) the CPU.  The CPU has nothing to do with the POST, except in as much as the BIOS needs to find one while it is executing the POST or it will fail.

firemanrob -- I'm leaning towards your MB being the problem, especially in as much as you can actually see leaking capacitors.  There is a possibility that the capacitor that's bad enough to be causing the problem is involved in a CPU-specific circuit, and thus this circuit is not active when the CPU is not in place.

I would replace the motherboard.  Look at it this way -- even if the MB is not the cause of this particular problem, it's not like you've wasted your money, because certainly your current MB will be the cause of your next problem, and perhaps might take out some other hardware with it when it goes...

You should do a complete OS reinstall when you change motherboards, because the installation you have now has configured itself to the environment it was installed under.  You can sometimes get away with just swapping in a new board (especially if it's got the same chipset, etc.), but I cannot recommend this practice.  Like many of WinXP's plug-and-play operations, there will be stuff left over in the registry from the prior environment that can reach up and bite you later, and be very difficult to track down as the source of instability.

Back up your data, swap in the new motherboard, boot from the Windows XP CD, do a complete format of the hard drive, and reinstall the OS.  You'll have a nice clean install as well, having eliminated all the detritus that WinXP collects (lost parts of old programs, registry lint from programs long ago removed, etc.).
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firemanrobAuthor Commented:
Also noticed som capcitors which appear to be bulging and have a bit of rust on them on the MB.
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Irwin SantosComputer Integration SpecialistCommented:
If you have like parts to test, you could check it out that way.

Should you NOT have any of these, the most prudent would be to take it in to a computer repair shop, at the very least, have your CPU tested.

You've got 3 potential problems... CPU, MOBO, and power supply.  one of things that I'm going to ask you is the reason why you have to jumper 13 & 14, don't you have a power switch attached to the motherboard?
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firemanrobAuthor Commented:
Yeah I just wanted to eliminate the switch as a problem so I did so by jumpering 13 and 14 to ensure the PS would power the components and wasn't the problem.
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Irwin SantosComputer Integration SpecialistCommented:
the switch should just be momentary and not a permanent connection
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firemanrobAuthor Commented:
The switch appears to be working as designed.  I just want to know if there is a way to determine whether the CPU or MB is bad.  If it is a CPU problem, would I at lest see the BIOS screen?
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Irwin SantosComputer Integration SpecialistCommented:
"If it is a CPU problem, would I at lest see the BIOS screen?"

No, you may just get nothing, depends on the degree of the fault.

Again..the only way to test is if you have 3 of the same components to test.. you may not use all of them, but by process of elimination you can identify what is at fault.
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firemanrobAuthor Commented:
Well since i don't have that capability and there are leaking capacitors and no obvious damage to the CPU, I would tent to lean towords MB.  What do you think?
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Irwin SantosComputer Integration SpecialistCommented:
"leaking capicitors"?  that would be bad.. is that K7 board 3-4 years old, perhaps time for a replacement
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_Commented:
I think mobo also. Bulging, leaking capacitors is not a good thing.
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Gary CaseRetiredCommented:
"...  som capcitors which appear to be bulging and have a bit of rust ..." ==>  This is almost certainly the key to your problem.   Either replace the motherboard or, if you don't want to have to buy other parts or reload any software, have it repaired -- that's not a bad option if you want to keep the rest of the system components.   You can have it repaired for $45 here:  http://www.badcaps.net/

r.e. a couple of your other thoughts:

"... If it is a CPU problem, would I at lest see the BIOS screen?" ==> Absolutely NOT.   On power-on the CPU begins to execute the BIOS code => that's what the POST (Power on Self Test) is, and that's what generates the BIOS displays.   No CPU running ==> No POST test, no display, no beeps, etc.

Motherboard vs CPU issues are the toughest to diagnose ==> the symptoms can be very similar; and of course if a motherboard problem is keeping the CPU from running it's virtually impossible to differentiate from an actual CPU problem.   The telling sign with your issue is the comment about your capacitors.   Notwithstanding whether or not you have any other problem, you DO have a motherboard problem -- so I'd get that fixed first.   Chances are that will resolve the issue.

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firemanrobAuthor Commented:
Hi Gary,

Thanks for the CPU Bios info.  I had a feeling the bios needed a good CPU to do it's thing.  If I just replace the Motherboard and use the rest of the original comonents, I should just have to load the software that comes with the new MB right?  Will windows XP do ok with this type of system change without reloading XP?
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Irwin SantosComputer Integration SpecialistCommented:
It would be best to reinstall from scratch...loading up the drivers for the new motherboard is prudent..however, at least get the NIC driver running so that you can download the latest drivers from the motherboard manfacturer's website and install.
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Irwin SantosComputer Integration SpecialistCommented:
...with all that said...I would opt for the following...

Since you are using a K7 chip...that by today's standard is old tech...replacing the motherboard (after several responses from experts) should fix your problem. However, why pay for an old tech motherboard to support an old tech CPU?

Perhaps it's feasible to replace the motherboard AND CPU (get faster).

Another consideration is that your system I believe is around 4 years old, and may warrant a newer model power supply.  If you go for the above suggestion, then make this part of your effort.

The only other inhbiting factor would be the memory that goes on the replacement board.

Lastly, you're really close to a brand new computer.
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Irwin SantosComputer Integration SpecialistCommented:
cool. thank you!
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Gary CaseRetiredCommented:
"... The CPU does not run the BIOS, the BIOS runs (or at least detects) the CPU..." ==>  Whoa !!  The BIOS is SOFTWARE (I've written a LOT of BIOS code).   The CPU executes that code.  A BIOS "chip" is nothing from than a EEPROM that contains the code for the BIOS (technically we call code that's embedded in a chip firmware rather than "software" -- that's because it's "firmly" embedded in the chip's memory).   There is a small ancilliary processor in the supporting chipset that does a few base functions (supports reset, sets initial voltages, etc.), but it has nothing to do with this.

"... The CPU has nothing to do with the POST ..." ==>  Same as above.   What processor do you think executes the POST code?  Again, the POST test is simply software -- it is executed by the CPU.

... I've seen a lot of WRONG statements on EE, but those two take the cake !!

firemanrob -- With a new motherboard you'll most likely have to do a full reload of XP and all your programs.  If you're going to go that route, I'd suggest (as was mentioned above) that you replace all of the main componenets -- motherboard, CPU, and memory and get a signfiicantly upgraded system.

... However, with what you described, it's fairly certain that a simple motherboard repair (replacing all the capacitors) will resolve your issue.   If you don't want to reload everything, and are otherwise happy with the system for the purpose you're using it, that's not a bad option -- and only costs $45.  The badcaps folks have a very good reputation -- www.badcaps.net


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Gary CaseRetiredCommented:
mjcoyne -- I still can't believe you said that !!   Unplug your CPU and see if your POST test tells you you're missing the CPU :-)  :-)  :-)     (at least I got a chuckle out of your statements)
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Irwin SantosComputer Integration SpecialistCommented:
@garycase...it's new technology from the 23rd century.
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mjcoyneCommented:
Okay, so maybe I screwed up -- but why, during a POST with a too-far overclocked CPU, does the computer respond with "CPU failed"?  How could the CPU have failed, and it still be operating to execute the POST?
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Gary CaseRetiredCommented:
As long as the CPU can execute any code at all, it can generate that message.   Remember, if you change the parameters in the BIOS (to overclock), they do NOT take effect until the system boots and the CPU begins to execute -- it then reads the data in CMOS and will make any parameter changes you've told it to (which can include changing multipliers, etc.   With a properly designed test, it will "fail safe" -- i.e. with a note indicating what happened.

Memory tests do this all the time -- they display what they're testing ==> and if the system suddenly fails you know what area it was trying to test.   (of course not all memory tests will cause a failure -- in which case it can display more details.

With a CPU test, it could write a "CPU Failed" note BEFORE doing a potentially destructive change (e.g. changing it's multiplier, etc.) and then immediately erase it after it's done it (this would take a few microseconds -- and you'd never see the message if all worked well => in fact the GPU would never even have time to start rendering it).

... exactly what you'll be able to see depends very much on the design of the testing code; and that is different for different boards.   I'd expect a motherboard designed for a lot of overclocking to have a fairly "fail safe" POST test so you'd get at least some indication when you over-stressed it.
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Gary CaseRetiredCommented:
... perhaps an analogy would be a car that you added too large a turbocharger to.   It would work fine at startup -- but the first time you floored it and let the turbocharger rev past the limits, the engine would destroy itself.   Your tach, however, if it was designed to hold the highest value (as some are) would show you how high the rpm's got before it died.   Not a perfect analogy -- but similar to what you've described.   Fortunately (unlike an engine) a too-much-overclocked CPU doesn't USUALLY destroy itself.
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