I have a 300W, 12VDC to 120VAC inverter. It's worked fine in my car for years, to power my laptop computer, a medical device I need when I am "on the road", and even some small power tools. Recently it started acting weird. Sometimes it wont turn on, (LED is lit). Or it keeps cycling on and off. Often, I have to plug it into the cig lighter in my car repeated times to get it to work.
I tore it apart the other day and after removing it from the aluminum case, I carefully checked for loose solder joints, using a magnifying glass, wiggling parts, looking for any burnt or other obvious bad parts. Everything looked ok.
With it out of the case, I plugged it in, and found it worked perfectly. I used it numerous times out of the case, and it worked each and every time. However, I was careful not to load it too hard, because the case is used as a heatsink, and outside the case, the transistors are not heat sinked, and I could feel them getting warm, but not hot, becuase the load was low.
After several days of working perfectly, I put it back in the case, and immediately it failed to work. I was careful to make sure the power cord was not applying pressure to the circuit board, and everything else was correctly mounted into the case.
One thing I noted is that the board slides into slots on that molded alum case, and there are solder joints extremely close to that edge.
My thinking is that even though it worked fine for years, one of them solder joints is touching the case and grounding out. But why it did not do that in the past makes little sense. If I could, I'd put some electrical tape along that edge, but it slides into a groove with no spare room.
Since I need this badly, for medical needs, I decided to just buy a new one . I got a 400W, and thought that would work fine. However, that one has a built in buzzer which is loud and extremely annoying. As soon as I plug it in, it "screams" at me until it's "settled". Besides annoying, I often use these inverters when I go camping, with a portable 12V battery, and having this think scream in the middle of the night is not acceptable. Needless to say, I'm returning it for a refund.
So, until I am able to find one without that annoying buzzer, I need to use my old one. All I can think is to leave it out of the original case, mount some heat sinks to the transistors and put it into a non-conductive case, made of plastic or wood.
I am posting this because I wonder if anyone knows what else could cause it to fail to work inside the case, yet work perfectly outside. The amount of work and cost of another case is almost not worth all the trouble to re-case it, but it seems that most of the new ones have those annoying buzzers. Why they have that, is beyond me, but I refuse to have to cope with that noise.