So far so good until a recent flood (about knee deep) and one was fitted low down and got drenched. It was unpowered at the time, so with a little patience and perhaps a hair drier - in theory, at least - all should have been well, because they ain't cheap! The problem with that last statement was the wod "patience" - I have little and thought it was dried out when it obviously wasn't....
Buzz click pop, bad smell and I let the blue smoke out. Because I'm in remote rural France I have to get everything online and delivery is - er, well - "Gallic" let's say. So as well as the cost, it's the inconvenience, and so I thought I'd open 'er up and see if it was maybe just a fuse ha ha. This is what a good one looks like inside - there is a LOT of stuff in there, but pay attention to the expanded section, becuase that's where the problem lay.
That problem was fairly obvious: in between the big 4.7uF cap on the left and the pale green vertical inductor on the right - and under the tough white rubbery goo keeping some of the high voltage parts safe - something (who knows what?) had gone "pop" in a fairly big way. When I say "fairly big", I mean a) so that it wasn't actually there anymore b) it has taken out the green solder mask underneath and with it, a fair part of the copper track linking it ot the 4.7uF cap. c) the smoked remains of whatever it had been were all over the cap and inductor and inside the case. Even the rubbery goo was blackened. The inductor and cap were so blackened that I didn't know at first what had popped. It was only when I removed them to see if they were still viable that I spotted the disaster zone*. Ironically, both were fine and fully "in spec" and cleaned up fine.
*Spotted the disater zone only after I had taken a scalpel to the white rubber goo to cut away enough to see "what lies beneath". Answer: an empty pad marked "FB1" and the aformentioned bare board and missing copper. All in all, not good, but I hate being beaten. I mean I didn't even know what an FB might be, but I knew I didn't have one "in stock". Turns out its a ferrite bead and it looked as if it was probably an 0603 size...
I'm okay with 0805s but 0603s strike fear into my heart. I had had some recent success with SOD-323 diodes on some blown Wemos D1s and they aren't much diffferent from an 0603, so...
I trawled ebay to try to find some 0603 ferrite beads...and then I thought "Maybe some of my old "junk" boards might have something...long story short - whole house search until I removed a modem card (!) from an ancient PC. It held not just the one bead I'd need but four - which was good news as anyone who has ever dropped / pinged / burned / lost anything that small. So - with the obligatory cocktail stick tip as reference - we have the blank spaces where the FBs used to be and my calipers showing each to be 2.3x1.3mm - actualy slightly(!) larger than the SOD-323, but this has to go into a space I can only just get into!
First, I cleaned off all the soot residue, removed some "erupted" copper trace, flattened the remainder, polished and fluxed and tinned it...I was only going to get one go at this...
Taking a clear photo of the result is pretty much impossible without an industrial microscope, as the scale is so minute, but my best offering follows. Think back to the "good" shot above, remind yourself of this size of this thing and where it has to go back into:
...and it's a marvel I managed at all! So even though it's a terrible shot, it allows us to see several things:
First, you can see some soot still on the white rubber goo and where I had to scalpel it out...Secondly (despite vigorous scrubbing with isopropyl alcohol) the white FB1 legend is still burn-stained. Third, how close I had to get to the 4.7uF cap with my (huge) track repair...
As I look back at this and the good "before" shot, I relaise the original was probably an 0402 size. I'm glad I never saw it - I'd never have dared. Also using the bigger(!) 0603 helps bridge the gap in the blown-out copper track...
No-one, dear reader, was more surprised than moi to find an instant power on, lights cycle, reconnect to the "other end" of the power line adapter and my SSID being broadcast strongly...all as I stood by with fire extinguisher at the ready...
