Commercial switch-mode power supplies in which the ferrite transformer halves are glued together with an epoxy that absolutely cannot be softened by heat or any available solvent. Thus making the transformer and the whole supply a lump of junk that cannot possibly be modified or reused for anything else.
Especially when the core halves have molded clip-points for metal springs to hold them together, but no, the power supply manufacturer used epoxy instead.
Like on this little 40W 18V laptop charger, that I wanted to change to 12V. Traced the circuit, it's a good design and would be easy to change - if I could get the transformer apart. But it's not going to happen. Glued together with an epoxy that completely ignores acetone and MEK, and doesn't even go soft at soldering iron temps. PCB and transformer at top in pic.
Frustrating, after the effort of schematic tracing. (pic 2)
I'd ordered a 12V 40W supply online as a fallback plan. It arrived... (bottom in pic) but the design is so cheap-arse shitty I'm not sure I want to use it. Not without modifying it somewhat. For one thing it has zero filtering on the mains input. Straight in via fuse, to the bridge rectifier. It looks unlikely this thing could put out 3.3A, which is what the markings claim. Next: trying this bit of crap.
Grrr... one way or another I need a 12V supply that fits in here. (pic 3, green arrow.)
Edit to add pics: load testing the el-cheapo 12V supply. Hey, it actually can put out a bit over 3A - briefly. Temperature of the heatsinks headed for meltdown, and that's without the plastic case. Also the output regulation is poor, and there's lots of ripple and HF noise - because there is no inductive filtering on the output, just a cap.
Such cost-cutting rubbish. Considering I recently bought some nice 12V 4A supplies in neat metal cases, MUCH better than this for Au$12.79 each, for Au$16 this is disappointing.