"N_Cook" wrote in message
...
Arfa Daily wrote in message
...
The first is the Warrior amp that I posted on here about, looking for
schematics. None were found, and as expected, the importer ignored my
pleas,
so I decided I would spend a half hour on it 'blind'.

It turned out to not be too difficult to get the main PCB out, complete
with
heatsinks and back panel. The wiring was long enough to allow the board
to
be turned over, without having to disconnect everything. The fault was
that
one of the two identical output stages was behaving as a pretty good half
wave rectifier, but only with a load connected. With no load, an applied
sine wave was perfectly symmetrical at the output terminals, and of
similar
size to the good channel. With a load connected, the negative excursions
disappeared almost totally. Nothing was burning, and the the output
protect
didn't even fire until the wick was turned well up, which led me to
believe
that the problem may well be back in the driver stages or earlier. As
there
are two identical amps, I figured that I would start with a few
comparitive
resistance checks between channels. Quickly, I found that at the base pin
of
one of the driver transistors, I had a reading of 3k or so on the good
channel, but open circuit at the same point on the bad channel. I
followed
the print back and took another reading and Lo! - 3k ...

So I went back to the transistor leg - open, but at the joint, 3k. I tell
you, I examined that joint with the strongest light and magnifier that I
have, but you could not see a problem with it. However, as soon as it was
resoldered, 3k on the leg as well, and the amp then worked normally. This
is
the problem with lead free. You can no longer spot bad joints by eye, and
they don't behave like conventional bad joints any more.

The second one was a Vox combo. This one was reported as "goes off after
a
while - tap top to get it back". It actually ran for about 2 hours,
during
which time I thrashed the output stage so hard you couldn't touch the
heatsink, and periodically knocked seven bells out of it with the butt
end
of a large philips screwdy. At no time did it show any signs of
intermittency. I was actually on the phone to the store that it came to
me
from, to check if they knew the owner, and whether he was savvy, or a
numpty, when it went off. Just like that. No provocation. You could then
lightly tap the top of the chassis just about anywhere, and it would come
and go at will. So easy was it to make it do it, you would have thought
that
the joint causing it would have been really easily spotted. I twisted and
wiggled everything I could, but nothing made it do it, but still the
lightest tap, and there it went.

Eventually, after a frustrating session of blanket resoldering that did
no
good at all, I came to a power resistor standing up off the board. It was
a
component that I had previously twisted. This time I pulled it, and one
leg
just came right out of the board. The joint looked perfectly normal - for
lead-free that is - but it had not whetted the resistor leg at all. How
the
hell could that take two hours to go bad, not be responsive at all before
that time, and then when it has gone bad, not respond to twisting, but be
so
tap sensitive that you could make it come and go with a feather? I HATE
lead-free with a passion.

If it ever finds its way into avionics, be afraid, be VERY afraid ...

Arfa




I'm thinking of making a tug-test tool for PbF checking. Probably based on
an automatic centre punch tool latch mechanism but somehow inverted in
operation. How many ounces or grams of pullout tension do you think a 1/3
W
resistor lead/wire link/TO92 wire should resist pulling out , leaded
solder
that is.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://diverse.4mg.com/index.htm



I would have thought pretty much to the wire breaking point. How many times
have you hooked a screwdriver or whatever, under a component in order to
unsolder one end, then heated the joint on the other side of the board,
whilst applying pressure to the link / component, only to then have it snap
on you, because you were heating the joint next to the one you were supposed
to be ... :-)

Arfa