On Wed, 9 Feb 2011 12:31:38 -0000 "N_Cook" wrote in
Message id: :

JW wrote in message
.. .
On Tue, 8 Feb 2011 17:50:39 -0000 "Arfa Daily"
wrote in Message id: :

A big heavy Quad sub. It's been so long since I've seen anything new with
the Quad name on it, I wasn't even sure that they were still going.
Anyway,
this thing always worked in that the amplifier kept producing output, but
there was, at the very least, a background 'rustle' and at worst an
alarmingly loud crackle, that can't have been doing the internal bass
driver SMD desoldering tweezers

For simple 2 land devices like Rs and Cs , apply the tweezers across the
long axis or short axis? flat good metal/metal contacts generally shortwise
but full-on solder contact lengthwise although spherical/flat contacts until
initial solder melt.
any good. Not affected by volume setting, so output or PSU. Nothing
really
conclusive to be seen on a 'scope. When it was doing it, you could 'see'
it
virtually anywhere, including on the output stage power rails, which were
bouncing around all over the place. It had a 'feel' of an intermittent
output transistor about it. Nothing physical such as heat, cold or
vibration
would provoke it, so I initially rejected thoughts of bad joints. There
are
four output Tr's in it - 2 x PNP and 2 x NPN , paralleled up, so I
removed
one pair. No difference. Still worked perfectly well, and still had the
random crackle. So I swapped the first two transistors back in. No
change.
Some time was then spent trying to prove where the noise was coming from,
without any positive results, Eventually, I decided that it might
actually
be a bad joint. There was no indication that the board was built with
lead-free solder, other than the fact that the joints were conical and
dull.

I set about doing a blanket re-solder, and quickly ran into trouble. I
don't
know what on earth sort of solder they had used, but even on small
joints,
my temperature controlled bench iron was struggling. I even wound it up
full
to 450 deg, and it still wasn't good. On larger joints like the smoothing
caps and the very chunky rectifier diodes, it wasn't having any of it at
all. Eventually, I had to dig out my 45 year old 140 watt Weller gun. At
least that made short work of reflowing the joints, but of course, you've
got to then be twice as careful about running joints into one another, as
the tip is really too big for this sort of job.

In the end, when it was all back together, it was silent, so I have to
conclude that it *was* a bad joint. But what makes it so frustrating from
a
commercial 'let's make some money from this repair' point of view, is
that
unlike conventional leaded solder joints when they go bad, you just can't
provoke lead-free bad joints to show themselves by any of the time-tested
and trusted methods.

I hate the stuff with a passion :-(

ObAOL: Me too! In the last few weeks alone I repaired several custom A/D
converter boards where they were inoperative until you put a slight
pressure a 44 pin PLCC dual port ram chip located between the micro and
the PLD. Looking at all the pins under a stereo microscope which has 25x
magnification revealed no bad solder joints, every joint looked flawless.
Yet when I resoldered the chip (with leaded solder) the boards worked
perfectly. I'd like to round up the folks who started this nonsense and
drop 'em in a large vat of molten no-lead solder, which is about the only
thing it'd be good for.

On the plus side, I have not yet seen any failures that could be
attributed to whisker growth.


have you tried cut finger-nail clipping test - oh no a reason to save nail
clippings. Sometimes running a point-ended clipping along the run of PLCC
pins , the "note" will change with poor soldered pins compared to the rest

No I haven't heard *that* one yet. What will people say at work when they
see a dish full of clippings? Perhaps a plastic probe of some sort
might also work...