[mdlug] [Fwd: Re: bios battery]

[Michael Rudas (computer)]

Raymond McLaughlin wrote:
> Garry Stahl wrote:
>   
>> Wojtak, Greg wrote:
>>     
>>> If you are just going to pitch it, what's the harm in attempting a fix?
>>>
>>> * Gets ready to draw his soldering gun at ten paces... *
>>>
>>>   
>>>       
>> Depending on the availability and cost of the caps I'll give my wife a
>> try at it.  She can handle a soldering gun.  Getting the bloody things
>> is likely the issue.
>>     
>
> I'm pretty sure the voltage is more of a maximum rating than a rigid
> spec, so a little higher should be OK, I think.
>
> What about the capacitance? My understanding is that these capacitors
> serve as "power smoothers" i.e. by holding a charge, at an applied
> voltage (not likely to be exactly 6.3V), on a DC power circuit, they
> compensate for microsecond dips in the supplied voltage as power usage
> fluctuates among the components. If this is the case then you should be
> able to get away with a little over capacitance, but not under.
>
> No doubt an exact match if probably best, and these are probably not
> that hard to find if you really look, so I'm just wondering.
There are many different styles of electrolytic capacitors, using 
different electrolyte formulae.  In this case, specs must be matched to 
function.  Low-ESR 105-deg C rated caps MUST be used here.  There's a 
reason that the markings are printed in gold-tone...

"ESR" in this case means "equivalent series resistance".  The bad 
chemistry of the defective caps causes resistive heating, which either 
dries out the cap (the boric acid electrolyte solution) or blows it out 
due to outgassing.

I have probably replaced a hundred or more of these caps due to bad 
chemistry-- here's one discussion:

<http://techrepublic.com.com/5208-11183-0.html?forumID=5&threadID=184286>

 I use the Panasonic FC-series low-ESR 105-deg C caps.  I slightly 
prefer them to the Nichicon and Rubycon equivalents.  They are of 
excellent quality, though they may be a bit bigger than the originals.  
They are available from Digi-Key.

The exact one you want is on page 1263 of the current catalog (pg. 12 of 
the caps-only PDF-file).  the Digi-Key SKU is P10202-ND at 73¢ each or 
45¢ in 10-up quantities.  I usually buy 10 to have a couple for spares.  
Digi-Key has no minimum order, but there's a $5.00 service charge below 
the standard minimum.

The actual replacement process takes a bit of finesse.  A mobo has 4 or 
6 layers of traces-- one of them is a ground plane, which acts as a heat 
sink.  The trick, then, is to use a hot iron (30 watts minimum) and get 
on-and-off the joint quickly, yet ensure that there are no torn traces 
or cold solder joints (45 years experience helps here-- heh).  If you 
don't have a solder-sucker or solder wick, a toothpick can be used to 
clear the holes.

It's important to practice, practice, practice!  Find an old mobo 
somewhere and practice soldering on it until you feel comfortable (or at 
least competent) doing so.  Use a 60/40 (60% tin/40% lead) or, better 
still, 63/37 eutectic alloy solder to reduce problems with cold solder 
joints.

-- Mikey