"Arfa Daily" wrote in
:

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




most of the time,when you have an intermittent,if you tap it,you end up
working the joint to a better connection and you don't see the
intermittent.
you have to leave it alone and wait patiently for the IM to show up,then
lightly tap around to find the area most sensitive.

--
Jim Yanik
jyanik
at
localnet
dot com