The PowerBook 550c has a single surface mount cap in the battery charge circuit, it should be removed at the very least, and replaced if you need battery charge function. It is a 100uf 25v cap. The 550c's LCD was made by Sharp and has a bunch of electrolytic capacitors that should be replaced. See the resources page for full cap reference!
The whole PowerBook 500 series suffers from very brittle plastic. It's literally turning to dust!
The main issue that arrises out of this on the 500 series is hinge cracking. The hinge design on the 500 series was not built to hold up under brittle plastic, as the hinges are screwed into standoffs on the top panel of the display. These standoffs crumble away, and the hinge seperates from the case, causing a whole myriad or problems. Most importantly though, this puts your display cable at risk of damage. You can 3D print new standoffs and superglue them in place of the old ones as a relatively easy fix, so it isn't the end of the world if yours are broken. You can also reinforce the original standoffs with a bunch of epoxy if they're still intact. If they've already broken though, I'd recommend the 3D printing route.
The front cover bezel at the bottom of the display housing is also prone to having its clips snap due to stress. New clips can be 3D printed, so again, not the end of the world if yours break.
The 500 series PowerBooks are some of the last ones to use SCSI hard drives. These drives are a real pain in the rear, because many of them suffer from the dreaded rubber bumper problem. Basically, the head mechanism in Quantum-made hard drives of the time have little rubber bumpers that are used as stoppers for the drive heads. These bumpers turn to goo, and well, the heads just get stuck. When this happens, you might hear a quiet ticking sound from the drive as it attempts to move the heads and fails. You may be able to manually dislodge the heads, but the issue comes back again the next time you start the drive. The only way to fix this is to tape over or replace these bumpers to stop the heads from getting them stuck. Some drives have the bumpers inside the voice coil assembly, but a select few Quantums have them UNDERNEATH THE PLATTERS. Such drives are virtually unrepairable.
The easiest (but most expensive) fix to this issue would be to replace the spinning rust in your PowerBook with a ZuluSCSI, BlueSCSI, or similar. These devices were designed just for your PowerBook, and they will be a lot more reliable (and faster) than a hard drive.
The PowerBook 500 series laptops use Nickel-Metal Hydride batteries as their main power source. Besides nearly all of them being dead today, they have a nasty tendancy to leak their guts all over the inside of your PowerBook. These laptops can have either one or two batteries, and if dead, you should check to make sure they aren't leaking. Even if they aren't leaking, NEVER leave them in for a long time, working or dead. If they are leaking, get them out and leave them out until you crack them open and remove/replace the bad cells!
The PRAM batteries in these PowerBooks are not known to leak commonly, but it's never a bad idea to remove them anyway as the PowerBook will run fine without them.
The 550c's LCD, like any other older LCD panel, is affected by Vinegar Syndrome, a defect where the LCD's polarizer film will become damaged due to a chemical breakdown in the adhesive that attaches it to the screen. This defect is accelerated by humidity, and as Japan has high humidity, a very large majority of LCD panels from this time now have this fault. For more information on it, see the linked page detailing it.
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