The PowerBook 150, like other 100 series laptops, suffers from many different issues.
Other 100 series PowerBooks used a Lithium coin cell as a PRAM battery. Not the 150 though, it got a NiMH VARTA battery instead, and yep, these leak 100% of the time. It's absolutely a requirement to get one out of these, they're the number one failure in every 150. It's soldered to the inverter board, which luckily means repair is possible except in cases of a severe leak. Get it removed!
ABS plastic on just about anything made in the 1990s has become very brittle with age, and this includes the PowerBook 150. The most frequent place on the 150 where you'll see this issue is with the screw mounts for the screen hinges. These mounts will break and crumble away with use, and lead to further plastic damage if the hinges are used in this state. A tell-tale sign of this issue is a gap in the plastic on the back near the hinges. If you see a gap, you've got broken standoffs. In addition, the screw mounts for the drive cages, and really just about every one in the system commonly stress crack with age.
Multiple methods can be used to prevent and fix these weak screw mounts. The one I'd recommend the most is to replace the old brittle mounts with new 3D-printed ones. These new mounts include a large plastic plate that mounts behind the LCD, in order to allow extra area for the part to be glued to, and to relieve stress from the mounts themselves. These parts when applied correctly are very strong and last a long time. A link to the STL files for the new parts is available on the resources page.
Another method of repair if you don't have access to a 3D printer is epoxy. Lots of epoxy. This method works best if the original mounts are still intact, as otherwise they can be very difficult to reassemble. The idea is that you put a bunch of plastics epoxy around all of the mounts, in order to prevent them from breaking, and to add extra rigidity to the area. This can work when done right, but I'd still recommend the 3D printing approach if possible.
The Passive-Matrix LCD used in the PowerBook 150 seems to be a fairly problematic panel. In particular, they seem quite prone to developing Vinegar Syndrome, perhaps more so than other 100 series laptops. I've seen a record amount of them hit eBay with this fault, and one guy on the 68kMLA forums had all three of his become affected with all his others ok. Not sure why they're particularly bad about it, but they do seem to be.
If that wasn't enough, I've also seen a lot of them with large bad blocks of pixels, indicative of a failure of one or more of the LCD driver chips, embedded directly on internal ribbon cables that aren't replaceable. This, like vinegar syndrome, is an issue that can affect any LCD, but it appears more common on the 150's LCD than on other models from the time. Notable for these two issues is that the 150 used two different LCD vendors - Sharp and Casio. It's possible that one vendor had more failures than the other. I'm tempted to throw Casio under the bus for this one. I don't have a proper sample size to prove this, but it is suspicious that the only of the 100 series PowerBooks to use Casio screens happens to be the one with more failures. Also, oldcrap.org's PowerBook 190 (technically a 100 series, but more of a 5300 series laptop than anything else) has a big ol' bad block, and it's got a Casio screen. All still speculation though.
One issue that will effect both the Sharp and Casio panels is bad caps - they've got them in different spots, but both panels have 8 3.3uf 35v electrolytic caps that leak on a regular basis. Being a little bit newer than other models with them, failures are less frequent, but still happen. I've recapped mine and I think you should too, for longevity's sake. Further info and reference images are available from the resources page. There are a couple of other caps in the system as well.
As with any computer this old, it's more common to see a dead hard drive in one of them than one that works. Apple used three vendors in this time: IBM, Conner Peripherals, and Quantum. It seems the 150 mostly if not entirely got Quantum drives, and it's just its luck that those are the most failure prone. They've all got a rubber bumper inside of them that becomes sticky with time and gets the heads stuck. It's usually possible to fix them, but I screwed mine up trying so I can't say for sure. The good news is that the 150 switched to the much more common IDE drives, so finding replacements isn't challenging.
The main battery for the 100 series PowerBooks is NiCad based, so they leak pretty frequently. Don't leave an intact one in your laptop, and make sure to check any unit you get your hands on for one. Usually the logic board only ends up damage in the event of a pretty severe leak luckily. Also, if the battery has shorted (which is pretty common), the laptop won't start with it installed. If you have a known-good AC Adapter and your PowerBook is acting dead, there's a good chance that's why.
The original external PSU for these laptops uses ELNA Long-Life caps inside, which are well known to leak the worst out of just about any brand. They just love to barf their guts out all over the place in anything they're installed on and cause a real mess. They're what's behind the Mac IIsi PSU's awful reputation, and there are a couple of them inside the 100 Series PSU. So yeah, recapping them is pretty much required at this point. Getting one open is a challenge though, as you pretty much have to break it open. A vice and a couple of pencils is the best way I know to do this, and will usually get them to pop without much damage.
If you wouldn't like to bother with any of that, and I can't blame you if you don't, another option is to get an inexpensive replacement off of Amazon or eBay. The voltage and barrel jack requirements for these are so simple that replacements are still available.
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