[Tfug] How many bulging caps -

[Bexley Hall]

Hi Chris,

> Subject: [Tfug] How many bulging caps -
> does it take to ruin a motherboard?

The short answer is *none* -- assuming one or more caps
are actually bad (i.e., a cap can be bad without any
visible signs) and still toast the board.

Conversely, you can have many "bulging caps" and still have
a functioning board.  :-/

What you are really concerned with is how wwell the caps
are performing their design function AT THOSE PLACES IN THE
DESIGN. E.g., a bulk decoupling cap (the caps that typically
"bulge" are large electrolytics -- most often used for bulk
supply decoupling) that has degraded -- or, even *failed* -- used
in the audio portion of the board could just result in some
low-frequency hum creeping into your audio (perhaps even only
affecting recording!).

OTOH, when used to decouple power to memory, you could end up
with spurious memory errors (which you probably wouldn't be
able to diagnose without ECC *and* BIOS support for logging
and reporting those errors!

When a cap fails, typically, it's ESR goes up dramatically.
This effectively renders the cap an open circuit.  If the
design is overly robust *and* only one (or a few) caps have
failed, other caps that are electrically proximate can
compensate (caps typically have wide tolerances so you often
"over design" where they are concerned -- in case the tolerances
work against you!)

If you have an RLC meter, you can test the caps to see if they
really are "shot" (chances are, if they've outgassed, they are
toast :< ).

If they are SMT components, replacing them can be tedious without
hot air tools, etc.  And, even through-hole technology can be
a problem on most motherboards!  The boards are typically 4 - 6
layer and, as is typical with multilayer layouts, the caps will
undoubtedly connect to buried planes.  If these aren't thermally
relieved, it can be hard to get enough heat into the joint to
melt all of the solder (i.e., all the way through to the component
side of the board) *without* lifting the pad in the process.
(lose lose)

Given your stated use for the board, I suspect it's not particularly
"precious" -- just replace the board/computer and save yourself
the grief.  I only try to rescue boards that have some special
value -- PC's are too much of a "commodity" to bother with.  :>

> I've been having some trouble getting an old machine
> back up and running...The video card is relatively new, and
> I starting having video problems from the get-go.  I thought
> it was the card, but I took a look around, and then I noticed about
> 6 bulging caps near the processor slot.

Note that you probably should also *NOT* try to salvage the CPU
and, maybe even discard the memory!  Without knowing where the
affected caps resided in the circuit (electrically), you can't
be sure that the processor hasn't been stressed while the system
was in-service (with the faulty components).  You might end up
with a CPU/memory that *appears* to work correctly (when installed
in some other motherboard) but you might encounter problems when
that system is stressed in some way (e.g., temperature, instruction
sequence, memory access pattern, etc.).  Again, these are such
commodity parts that it is easier just to toss them out and start
with a known "good" set of components.

> I'm guessing that this is the end of the road for the board...And
> this machine, for a time.  First mythtv box I ever made finally
> bites it... :(

Mark anything that you choose to salvage from the system so that
any future "anomalous behaviours" can be tracked to this possible
cause.