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 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 >:-(
Arfa