Micke W wrote:
> My trusty old BH6 v 1.01 is having intermittent boot problems and
> spontaneous reboots.
> All capacitors look OK (no bulging, no leaking) but I'd like to
> replace them to see if it cures the board...
>
> There used to be a BH6 "kit information" (which ones to replace,
> and their values) om "Homie's" homepage, but that's removed now
> (and he hasn't responded to email), so I would like to know:
>
> * Should I change/replace all 13 "large" (470 and 1000 uF)
> capacitors located between the slot 1 and ATX sockets?
>
> * If I recall correctly, "Homie" suggested that some "caps"
> could/should be replaced with a larger capacitor (higher
> capacitance) than the orginal ones. Correct? If so - which should
> be "up-rated" and to what value?
>
> I'd be very grateful for any help!!
>
> / mw
There is a FAQ here about caps. Check out the seventh paragraph
down. Larger caps are not always the answer.
http://www.motherboardrepair.com/faq.do
There is aome good advice on capacitor selection, in the datasheet
for the Vcore voltage regulator chip on your motherboard. They
explain in the datasheet, about ESR, ESL, capacitor AC current rating,
and capacitance. All of those can be factors in a Vcore design.
To get enough current rating, is one reason for using a large
number of capacitors in parallel. The actual capacitance value
in that case, is not what they are focused on.
In terms of some of the other (non-Vcore) caps, those caps can
be sitting across the PSU power rails. A PSU has a limit as to
how much capacitance it can drive, and maintain adequate phase
margin. Using more than that amount of capacitance could lead to
oscillation. Not that I expect you could add enough capacitors
to do that. (One of the specs from formfactors.org has
recommendations for PSU manufacturers, as to how much
capacitance a PSU should be able to stand, and remain stable.)
The safest thing to do, is stick with the existing values.
And don't "overengineer" your replacement. Replacing all
the "ordinary" caps, with Sanyo Oscons, would be changing
the circuit parameters in a radical way, and may not have
the results you would expect. If you are replacing ordinary
electrolytic caps, use more of the same type, and don't
change to, say, a solid electrolyte.
If you need info on sources of caps, good and bad types,
this site is another possible source of info.
http://www.badcaps.net/
And Gary also provided suggestions on capacitor type.
http://www.motherboardrepair.com/part_information.do
For ordinary electrolytic caps, the properties are
proportional to the volume occupied by the part.
Withstanding voltage and capacitance affect how large
a part will be. When you see "smaller than normal"
parts on a motherboard, and cannot find a same-sized
replacement, for the same capacitance and voltage,
it makes you wonder how the smaller component "cheated"
to do the job.
If you've never recapped a board before, be aware that
some circuit boards, use tiny holes for the cap wires.
This can make it difficult to replace them. It is easy
to damage the foil while doing it. Also, the holes are
plated with metal, and there can be internal conducting
layers touching the plated thru hole in the board.
Drilling out any arbitrary hole in a motherboard is
a no-no. The only tools you are allowed, are soldering
iron, and solder removal tools like solder-sucker or
solder wick. You may find, after trying to do just one
of the caps, that the effort isn't worth it. Practicing
on a duff board first, to get the hang of it,
is important. While I've had the misfortune to do
a few of these, even with the proper equipment (vacuum
desoldering station), the job is still difficult and
frustrating. The job would be easy, if the board makers
used larger holes for the caps.
Paul
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