Re: Slowly dropping Vdd values

The message <q5okn1h7k23pcf5bitgpk9duqti7n4ms10@xxxxxxx>
from Jaimie Vandenbergh <jaimie@xxxxxxxxxxxxxxxxxxx> contains these words:

> On Tue, 15 Nov 2005 03:02:15 GMT, Johnny B Good
> <jcs.computers***@xxxxxxxxxxxxxxxx> wrote:
> >The message <9gbin1dv8ik5gfl9cumfdd40qdt6j4ojip@xxxxxxx>
> >from Jaimie Vandenbergh <jaimie@xxxxxxxxxxxxxxxxxxx> contains these words:
> >> On Sun, 13 Nov 2005 00:07:22 +0000, Steven Briggs
> >> <news@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> >> >Agree its the core voltage regs. Doesn't sound like electrolytic caps
> >> >though, more like a leakage path developing in the feedback loop.
> >> >(I'm a
> >> >PSU engineer BTW).
> >
> > That assumption might have been correct, I'd imagine contamination from
> >leaking caps could have had that effect but the presence of the solder
> >resist usually demands significant leakage to bridge solder points by
> >which time significant ESR values would likely have developed quite
> >capable of causing a similar symptom.

> "Bzzzt!" and the magic smoke escapes, you mean?

The electrolyte's main job is to act as a liquid conductor that will
conform exactly to the shape of the microscopically etched aluminium
oxide surface of one of the plates (I don't know whether it's the anode
or the cathode which has the oxide insulating layer). When electrolytic
capacitors dry out, the conductive properties of the electrolyte
deteriorate, producing higher effective series resistance as well as a
reduction of capacitance value.

> I can only guess that some of the dodgy caps were either on lines
> upstream of the core voltage regs, dipping the voltage by parasiting,
> or possibly they were on a line which pushed a multiplier at the PSU
> end over the wrong way.

This is a "Don't Analyse the problem! Just fix it!" type of repair. :-)

It's enough to observe the presence of obviously defective caps and
know that replacing them will restore that part of the circuitry back to
it's intended functionality. There's just no point in analysing the
problem beyond "Hmn, 'blown' caps, that's not good news, they need
replacing."

> Having seen the swollen caps, I didn't stop to analyse all the
> voltages.

Sensible. :-)

> >> And to wrap up the thread, it _was_ caps. Four of them had developed
> >> swelling, two with electrolyte leakage.
> >
> > Presumably, you cleaned any brown stuff off the board B4 fitting the
> >replacements. :-)

> Only one had leaked downward, kindly - the other had just been
> bubbling gently out the top. Bit of elbow grease and not even a mark
> left on the screen print. Whatever that green varnish is, it's damn
> tough stuff.

It's called the "Solder Resist Mask" because that's what stops the
solder wave machine from plating all the circuit traces with a coating
of lead/tin alloy thus confining the solder to where it's needed and
reducing the consumption of solder used in the solder wave process.

> >> Swap out for new 6.3v 1000uF or 1500uF, matching the original, Vdd
> >> rock solid after three hours game playing. Splendid.
> >
> > I'm pleased you managed to successfully repair the board. My point
> >origionally was to look for obvious signs of component failure and the
> >electrolytics have the charm of (usually) visibly failing, so well worth
> >looking at and putting right since they can be the cause any manner of
> >instability and about the only component that us mere mortals have any
> >chance of successfully replacing.

> You're not wrong. I've repaired mobos by replacing caps before, which
> is why I've got the bagful of spares.

Always worth keeping a stock for those types of fault that are almost
exclusively due to failing capacitors. Component failure other than
electrolytic capacitors usually results in sudden onset of symptom of
failure, usually catastrophically terminal failure. Faulty electrolytics
produce gradually worsening stability problems mirroring the gradual
deterioration of the dying capacitor or capacitors involved.

> >> Until next time some caps dry out, anyway.
> >
> > Well, that's always a possibility with any repair, especially on
> >something that's seen a few years service.

> Thing about this particular motherboard is that it was built in the
> months just before that bit of semi-succesful industrial espionage was
> discovered. The one that resulted in (probably billions of) caps with
> unstable electrolyte, since said spy hadn't managed to steal the whole
> recipe...

Well, even properly manufactured caps can fail prematurely if not
correctly specced for the electrical properties of the circuit and the
environmental conditions of use.

> Hopefully they'll either all die off or be retired, and I can put the
> soldering iron away again!

Even properly manufactured and specced capacitors can fail. You'll,
hopefully, start to see a reduction in the incidence of this type of
fault but this type of failure mode is never going to completely
disappear.

In a few years time, you might well look forward to doing this type of
repair job as a brief break from dealing with the more humdrum type of
repair work.

--
Regards, John.

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