Hello Kevin
   While almost anything is possible especially when we're talking about
failure modes where one can make the mistake of tempting Murphy, one of the
precepts of electronic troubleshooting (or any troubleshooting of complex
systems for that matter) is "reduction of options/possibilities"
where one sets up a hierarchy of likelihood and each step should
progressively narrow the field until only one is left.  The only
alternative to the deductive logic/Sherlock Holmes method id the so-called
"shotgun approach" where one just starts replacing things until
at some point it works.  Besides being expensive the shotgun approach often
actually increases complexity by adding variables to be considered since it
has no clear path and therefore reduces the likelihood of narrowing to one.

  An example of hierarchical ordering is knowing which components have a
history of failure.  One fact gleaned over time is that capacitors are many
more times likely to fail than resistors and harder to spot.  Resistors,
unless subjected to way off-spec conditions, tend to drift higher in
resistance value rather than to short out or fail catastrophically.  Also
when they "go" they tend to change appearance, darkening in
color, sometimes blistering depending on packaging.  Capacitors, OTOH, can
and often do fail catastrophically and can either open or short rather than
merely drift.  Electrolytics are only visibly changed if one can see the
"designated weak spot" a sort of release valve that is simply a
weak spot often where there is a hole in the metal case covered with some
rubbery material that will allow pressure release short of explosion.

  As this applies to mobos and CPUs, it is fairly rare for a voltage
regulator to open up and deliver higher voltage than the design point. 
Most often it will deliver what it is supposed to or nothing at all.  One
of the reasons for this is thatbpart of most voltage regulator circuits is
the lowly resistor and since they rarely short but often drift up in value
(and the greatest load is directed less toward ground than the supplied
device) drifting up decreases available voltage.  While some balanced
circuits can swing inversely, when one goes down another is forced up, this
is also quite rare. Also either inherent to components or by circuit design
it is rare for polarity to become reversed and overvoltage or reversed
polarity voltage are the two most common causes of failure.   So while it
is extremely rare since the motherboard is what delivers the working
voltages it is possible for a motherboard failure to take out a good CPU by
delivering voltage higher than design or more likel

y, voltage where it doesn't belong. This occurs most often because of a
mechanical problem though, not due to electronic failure.  For example
exerting too much force inserting a card or in some other way flexing the
motherboard too far can cause a break in circuit connection which would
posibly remove from the circuit a component that bleeds off amperage or
polarity protects a component.  

  Because of the nature of what a CPU does, essentially choose between 0's
and 1's rather than regulate voltages in general CPUs don't have the power
to take out some other component and due to the fact that power is only
delivered to a system if the supply gets the "power good" signal,
which means everything has settled in within designed spec *before* the CPU
is powered up, motherboards and especially CPUs are remarkably free from
electrical problems and given the above, the occurence of a CPU taking out
a motherboard approaches absurd in hierarchical order of what is likely.

  In summary I never think twice about plugging in a suspect CPU to a known
good mobo, but primarily in consideration of some mechanical occurence
since the last "known good" I do consider somewhat carefully
utting a known good CPU into a suspect mobo.  It is worth repeating however
that if I know nothing severe of a mechanical nature has affected the mobo,
and definitely if the system posted at all with another CPU recently, I'm
going to plug in that test CPU.

  So given limited access to known goods and being confronted with an
unknown mobo and cpu and further given limited or no access to meters of
any kind it is still possible to diagnose logically and safely.  Already
built in to virtually every mobo/psu combination are some valuable
diagnostic tools, namely the self checking "power good", of
course full "POST" and the onboard speaker.

  Because a bad mobo can take out a good CPU test the mobo by itself first.
 I'd plug in the psu to the mobo and not install ram or any peripherals
except for keyboard and an old hard drive or a fan connected to a mobo fan
header.  If the unit powers up and the fan continues to run off the mobo
fan header it is safe to assume the mobo has passed "power good".
 Usually at this point I would just put the system together but if I was
being extra safe for some reason I might plug in the CPU but not the ram. 
One should hear a series of beeps denoting "no ram installed" if
the mobo and cpu are fundamentally good, assuming bios is good.  Then plug
in the ram and power back up and it should POST assuming the ram is good
since beeping showed the bios is likely good.

  It is really fun and also instructive to get a "beater board" a
mobo/cpu/psu/ram that you don't care at all about.  You can try all manner
of what might seem abusive tests and learn just how amazingly rugged PCs
really are.  Most people don't realize for example that it is possible to
hot-swap bios chips in order to flash a chip that won't boot, say from a
bad flash or that many items are "frozen out" such as ps/2
devices which will not be recognized w/o powering off first.  Interesting
stuff.

Jimmy


> Hi,
>  
> Can a bad CPU destroy a Motherboard?
>  
> OR
>  
> Can a bad Motherboard destroy a CPU.
>  
> If the answer is yes to either of the above, what would be the correct testing 
> procedure, assuming limited access to know good components?
>  
> Thanks
>  
> KevinJ.
> 


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