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Posted by Arfa Daily on 11/05/07 09:24
> "Joe" <none@given.now> wrote in message
> news:none-0411071904330001@dialup-4.232.123.160.dial1.losangeles1.level3.net...
> I have a Philips DVD Player, model DVP 642.
>
> It will be two years old next month.
>
> It seems to have a power supply problem that is similar to what others
> have reported, and is likely due to a bad electrolytic capacitor.
>
> The symptom is:
>
> The unit is in standby mode, with a red LED lit, as it is supposed to be.
>
> Pressing the power-on button causes the red LED to turn off - as it is
> supposed to - but nothing else happens. After a few seconds, the red LED
> comes back on as though the power-on button had never been pressed.
>
> Pressing and holding for a few seconds the power-on button makes the unit
> seem to come to life and operate normally. No need to hold down the
> power-on button.
>
> The unit seems to function normally until it is shut off (into standby
> mode), and the the same symptoms happen when I try to power it up again.
>
>
> So far I have opened up the unit and removed the power supply.
>
> No capacitors look to be bulging.
>
> There is some rubbery pale-yellow stuff that seems to have been spread
> deliberately on the circuit board, near one of the capacitors, but
> especially along where a 7 conductor ribbon cable enters the circuit
> board.
>
> Is this the leaky stuff that some have called "yellow gunk" from a split
> open capacitor? I picked at the stuff, and it is definitely *rubbery* and
> not all all liquid.
>
> Or is this some stuff that is meant to be a strain relief from the ribbon
> cable to the PC board?
>
> Does anyone know where to obtain a schematic at least for the power
> supply? It would help to at least identify the caps on this board. Any
> constructive suggestions?
>
> A related question: If I parallel a pair of capacitors to get close to
> the needed value, do the bodies of the two caps have to be insulated or
> separated from each other?
>
> --- Joe
>
I repair many DVD players during the course of my working life, and the
symptoms that you relate, are pretty typical of bad caps. As JANA said, the
yellow gunk is almost certainly there deliberately as either strain relief
for the wires that you mention, or for the caps themselves. Also, as he
says, there is no substitute for an ESR meter when looking for bad caps. A
capacitance meter is of limited use in this instance. Many is the time that
I have removed bad, but perfectly serviceable *looking* caps, which have had
an ESR reading that is away with the fairies, but whose capacitance value is
spot on.
The bad cap(s), if indeed that's what the problem is, is likely to be a
small one on the primary side - typical value 1uF to 47uF, and possibly
located near to some other component that runs hot, such as the main
switching element, and its heatsink. You could also have a secondary-side
cap causing it, but most often, when one of these is to blame, it can be
seen to be bulging. If you have a 'scope, you can look for HF ripple on the
secondary rails. Another way that you might get to the bottom of the
problem, is by using a can of freezer, and a piece of cardboard to confine
the spray to specific components. If you have a decent can, you can get
single drips of freezer from it. Also, take the time to look around the rest
of the machine to see if you can spot any 'distressed' looking caps
elsewhere, or any located close to hot areas. Sometimes, the type of
shutdown that you are seeing, is caused by poor caps off the power supply.
As to your question regarding paralleling caps up, there should not be any
need to insulate the bodies, as they are usually pretty well insulated in
the first place, and even if not, the 'can' is invariably the "-" terminal,
if it is commoned at all. In general, I would not recommend this approach
when dealing with switchers. Electrolytic caps are 'poor' things in terms of
construction and stability, and work in filter applications by the 'shotgun'
principle. There is no guarantee that two caps in parallel, even if they
come from the same manufacturer and series, will have the same inductance as
a single cap of the right value, nor that the high frequency current that
they are filtering, will divide equally between them. All caps used in
switchers are usually pretty easily available, and you should replace them
with low ESR 105 deg types, specified for this application. Switchers are
pretty fickle things, and it doesn't take a lot to upset them, sometimes
with catastophic (literally) results. Best not to mess with the way that the
designer specced his parts in the first place.
And if you're not familiar with working on switchers, PLEASE TAKE CARE. They
can be lethal. And I mean that totally literally, as in dead ... If you have
an isolation transformer available, it should at least be connected to this,
whilst the covers are off.
Arfa
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