(Originally drafted on January 21, 2018. Blogger just decided not to post it when I asked, for reasons I can't even begin to guess)
Before I get into my new series, I'd like to take a moment to outline some of the tools, equipment and processes that can (and should) be used for people wanting to undertake such things. Keep in mind that I'm more of an electrics person than electronics, so it's going to be a fairly "simple" list of tools - but don't let that fool you into thinking they can't get many, many jobs done.
First things first, however, I would like to caution people who may not necessarily be hugely experienced with electrics and/or electronics. While most parts inside an older computer are going to be low voltage, there will be some high voltage portions. Low voltage - especially DC - generally won't even make you feel uncomfortable, but mains AC can be lethal. Things like power supplies and CRT-style monitors can be even worse than mains, sometimes being into the thousands of volts, so don't even be tempted to investigate these things unless you're absolutely certain of yourself, your abilities and your next of kin.
Otherwise, do ensure you're always being careful. Older electronics can be quite robust but if you drop a wire and feed 12VDC into a 5VDC component you're quite likely to release the magic smoke. This smoke smells bad, lingers, and electronics don't run without it, so avoid letting it out. Old electrical devices can also have leaking components or even wildlife inside, so be mindful of what you're touching and what you're touching it with.
Before you start, it's important to have at least a basic understanding of what you're looking at. Schematics for many old systems are available online - often scanned from the original documentation. Just make sure you've located a match for your system, since many of these older units went through multiple revisions in their lifetime which can change what chips are in use, what they're labelled and where they are, among other things. It can also be helpful to see if others have experienced the same issues you have, or even if it's a common fault, as this can save you a large amount of time, frustration, and money, and doing such a search may help you to better describe the issue to yourself, giving you a clearer idea of what you're trying to fix.
With that said, to start with you're going to need a few basics. These will be things you use on pretty much every task, and not having all of them is going to put you on the back foot straight off.
Firstly, and most obviously, you're going to need screwdrivers. A multi-tip screwdriver will come in handy, but you can get by easily enough with separate screwdrivers if that's what you have. For older systems most, if not all of the screws are going to be Phillips' drive - usually a #0 or #1 internally, and #2 for the case. You'll want to try and keep at least one blade tip screwdriver on hand, though, for prying and lifting, and the occasional screw which needs it.
The second thing you'll need is a multimeter. It doesn't need to be a super-expensive one (we frequently use my brother's $20 unit), so long as it's reliable and easy to read. An auto-ranging multimeter is nice but likely superfluous, since most of the time you're dealing with voltages of 12V (DC or AC) or less, so you can stick to the 20V/40V range 99% of the time. Ensure it's got good batteries!
Next you'll want some pliers and/or tweezers - something to hold things with that might be hot, electrified, corrosive or living. They can also be used to remove stubborn connectors (avoid pulling on the wires as much as possible), remove pins from connectors and, in a pinch, bridging specific contacts - though do be mindful you're doing the latter on purpose. Pliers also often include a wire cutter, which can come in handy for certain more significant repairs.
Depending upon how far you want to (or are willing to) go, having access to a good soldering iron is also a good idea. While a bargain shop unit which just plugs the iron directly into the wall will suffice, it isn't a bad idea at all to at least get something temperature regulated; if not controlled. You don't want the iron getting too hot and you don't want it to cool down while it's in use, otherwise run the risk of doing more harm than good. Make sure to get solder (60/40 PbSn with a flux core is good) and at least a solder sucker while you're at it (the big ones work better, but are obviously bigger and more awkward to use).
You'll also want basic cleaning supplies: a toothbrush (manual is fine) and/or cotton buds, and some isopropyl alcohol or contact cleaner. A lot of problems can be remedied simply by cleaning the contacts between two points. It can also be used on potentiometers (pots), variable resistors and microswitches to bring new life to old components that are a little warn but not actually broken.
Lastly, you'll want a torch (flashlight). A small, hand-held torch is sufficient, but you could also use a head-mounted torch or a desk-mounted option - so long as it's bright and can easily be directed on exactly what you're looking at without casting too much of a shadow. Physical inspection is just as relevant as electrical.
Naturally, there's a few extra things which can increase your ability to diagnose or repair systems. You can do a number of the same things without them, but they'll be more accurate, faster and/or easier. If you're planning to do this a lot, it's likely one or more will be a worthwhile investment.
Perhaps what I use most commonly is a vacuum desoldering station, sometimes called a desoldering gun. These are essentially like a soldering iron but with a hollow through the tip, which is hooked up to a vacuum pump (either in the tool or a separate unit). The idea is that you can heat and "suck" the solder at the same time, with constant suction, as opposed to using a mechanical solder sucker which often requires you to remove the heat and only offers a burst of suction. If you're removing a lot of DIP (Dual Inline Package) chips to fit sockets, the speed and convenience may justify the extra expense of one of these.
Another one I find useful is an Oscilloscope, or "scope". These come in many different styles, prices and features, ranging from a few hundred dollars to tens of thousands. Mine cost just over AU$300 - new and delivered - and it's well and truly superfluous to task, so don't think you need to go way up the scale to get results. You might consider looking second hand here, as an old CRO (Cathode Ray Oscilloscope) will be similarly useful to a more modern DSO (Digital Storage Oscilloscope) for most purposes. These are great for analysing clock, data and addressing signals, which is essential for more advanced diagnostics.
The other optional parts are more miscellaneous, so I'll lump them together. Electrical solder flux (a pen or syringe applicator is nice) can help to have on hand (don't be tempted to substitute plumbers' flux - you will destroy parts in the long run). Plastic spudgers/lifters or for-purpose extractor tools are also handy, since you don't need to be as careful with them as you do with generic metal implements when opening cases or removing chips. One or more magnifying glasses (with lights, if possible) can also prove useful.
Above all else, however, you're going to need time and patience. Some issues can be solved quickly and easily, some can be figured out with a systematic approach, but some are going to require trial and error. You need to be prepared to take a step back when things don't work out and have a think over different solutions. You also need to be prepared to accept that some fixes don't make sense, and that not everything can be repaired.
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