Re: Desoldering SMT Chips?
- From: "Arfa Daily" <arfa.daily@xxxxxxxxxxxx>
- Date: Mon, 18 Jun 2007 08:22:58 GMT
"Madness" <policedept@xxxxxxxxxxxxxxxx> wrote in message
news:yEqdi.5817$0j5.2247@xxxxxxxxxxx
I need help here. I have an Apex AD-660 DVD that has extremely low
analog sound. I've pulled the board that contains the A/V jacks on it.
On the *bottom* of it, there are two 8-pin SMT chips that I believe are
op-amps (yet there's a mess of through-board discrete components on
top!). I was able to find equivalents for them, but how do I desolder
the damn things??? I don't think my $15 RackShack soldering iron is
going to cut it!
From experience, these are likely to be line out buffer amps, but I wouldhave said that it is unlikely that both or even one of them is faulty,
unless something nasty has been done on the jack side of things. I would
have thought that the most likely suspects are that the analogue sound has
been disabled or muted somewhere in software, or that a supply rail to the
opamps has gone missing, often due to a short circuit decoupling
electrolytic somewhere. I get faults like this on various manufacturer's
items on a regular basis, and SM chips are, on the whole, pretty reliable.
This exact symptom was very common a couple of years back, on a particular
Toshiba DVD chassis, and was caused by a short circuit cap. It's also very
easy to accidentally turn off the PCM analogue mixdown to the line out
sockets via one of the menus, often several layers down in the "Audio Setup"
menu suite.
If it's definitely not a 'soft' issue, you should first 'scope the inputs to
the chips, and analyze the DC conditions on their pins, but if you do need
to ultimately get them out, there are several ways. With a little 8-pin, my
preferred way is to use an 8-pin-shaped bit on my Antex temp controlled
station. That unsolders all 8 pins at once. Flood the pins with new solder
first, and before Smitty says so, add a little liquid flux to the equation
!! ;~} The Antex bit with a long sloping edge does a pretty good job too,
but only one side of 4 pins a a time. My next method is to flood the pins
with new solder, then wick it all off with new, good quality soldamop. I
then slide a piece of thin wire like rework wire or wirewrap wire under all
the pins on one side, and anchor its end by touch soldering to any
convenient joint. It is then just a case of pulling firmly away, and
slightly up from, the pins with the free end, whilst touching each pin in
turn with the tip of a fairly hot iron. Each one will pop up from the board,
and the wire will pass under the pin at the same time, removing any remnants
of solder. Be careful that the last pin to be left attached, doesn't twist,
taking the pad with it.
After that method, the final one is by using hot air. There are good people
on here who swear by all sorts of ways of doing this using paint stripper
guns and the like, but I've always found that this is tricky for the
amateur, and is best done with a proper hot-air rework station.
To solder in new ICs, make sure first that the pads are wicked absolutely
flat, then carefully position the new chip ( right way round of course ).
Whilst holding down the centre of the chip, touch one corner pin with a
fairly hot iron. The remnants of solder on the pad, and the tinning on the
leg, will normally be enough to tack it in place. Recheck the positioning of
the other legs, then tack the opposite corner. Now add some liquid flux, and
go ahead and solder normally. Don't worry if the solder 'blobs' between a
couple of pins here and there at this point, but the liquid flux will go a
long way to preventing this. When all pins have been soldered, check with a
magnifying glass, and remove any solder shorts with a hot iron and thin
gauge soldamop, applied to the vertical faces of any affected pins. Recheck
that you've removed the blob, whilst still leaving a fillet of solder
between the pad and the pin. Removal and refitting is not hard at this
level - it just needs a bit of care. If you have some scrap boards around,
practice first.
Arfa
.
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- Desoldering SMT Chips?
- From: Madness
- Desoldering SMT Chips?
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