A PC clone computer is a box, and a monitor and keyboard which plug into sockets in the back of the box.
First: the box. The box can be opened with a phillips screwdriver. Amazingly, there seem to be only three or four different screw sizes and threads used in all computers. A small reversible phillips/slotted screwdriver is all you need to do surgery.
The case will generally slide off or lift off. Some go forwards, some move up; whatever.. just wiggle it till you figure out how it comes off.
Inside you will find a zillion wires and cables, other lumpy things, and a bunch of cards which are obviously plugged into the a larger circuit board. This large circuit board is called the mother board on PCs, and the logic board among MAC people. I'll continue with the PC herewith, because here is where differences set in for the MACs.
So here is what's inside the box:
A power supply. Either has a switch on the back, or a switch is wired out to the front of the computer. The old XT computers had a giant red switch on the right side of the box, built into the power supply. The power supply provides power for the mother board, and for peripherals, and occasionally for gadgets on the front of the box.
The power to the board comes via two six-pin (or one 12 pin) plug, which can't be plugged in wrong, for there will some sort of interlock. These are all red, yellow, and black wires (with occasionally a single orange or white wire). Almost all the mother board power plugs I have seen are interchangeable.
All these wires to the mother board look very complicated, but if you were to trace them into the inside of the power supply, you find out that they are all soldered to three different lugs. They supply 5 volt DC (red, black) and 12 volt DC (yellow, black). The black wires all go to a ground lug. So at least now you know that there is no high voltage anywhere on the mother board.
The power supply also has separate sets of wires and plugs (actually, "sockets" or "jacks") for peripheral devices (hard drive, floppy drive, CDROM, tape drive), and these wires also are in red, yellow, black -- representing 5 volt, 12 volt, and two black ground return wires. The peripheral sockets come in two basic sizes, small and large.
The power supply generally is held with four screw to the box, and often floats above part of the mother board. In older AT computers, the power supply also clipped into a cutout in the box.
There will be a fan which both cools the power supply, and draws air through the box for the purpose of cooling the board, the cards, and the peripherals. Fans occasionally burn out or wear out. But you will hear it straining before it stops altogether. If the fan does stop, you are likely to loose the power supply, and possible some of the peripherals or the board. But fans can be replaced.
The peripherals (hard drives, etc) are connected (as you saw) to the power supply, and are also connected with flat cables to whatever add-on card (or mother board socket) is used to control them.
Hard drives (IDE types), ATAPI CD-ROM drives, and most of the ZIP drives, use a 40 pin flat cable to connect to a HDC or to the motherboard.
Floppy drives and streamer tape backups use 34 pin connectors. The cable between the first and second floppy drive (often) has a twist in it.
Older floppy drives use slip on card connectors, but these are pin-wise interchangeable with the pins-only cables.
Older MFM hard drives use a single control cables for either one or two drives (also slip-on), with a twist between the two hard drives. Each drive also has a separate flat data cable to the HDC.
If a SCCI controller card is used for the hard drives, a 50 pin cable (or other count) may be in use. (On MACs expect anything from DB25s 30 pin, and 50 pin sockets and cables).
In many later motherboards you will find the floppy drives and hard drives connected directly to the board instead of to a plug-in HDC/FDC card.
All the flat cables are striped in red on one side (to indicate the connection to the number 1 pin of a socket), and some are indexed. There is little chance of reconnecting them the wrong way later. And if you do, things will just not work.
At any rate, all of these cables can be disconnected, both the power cables and the flat ribbon cables. At that point all the drives and other internal peripherals can be removed. They will have two screws on either side of the bay they are mounted in. In most cases they will just slide out.
There are some gimmicky designs which use sliding plastic inserts, or metal brackets which have to be removed from the front. That means also removing the front from the case, if it is separate from the rest of the box cover.
A better computer will have 8 drive bays, or at least three 5 1/4" bays and two 3 1/2" bays. Brackets are available for mounting 3 1/2" drives in 5 1/4" bays.
The mother board will be held in place by all those plug-in cards, which have to be removed one at a time. Each is held with a screw at the top and a tab at the bottom, and of course they are "plugged into" the expansion slots of the boards. Remove them screws now, and pull out them cards.
Which slot they come out of or go back into (mostly) doesn't matter. General practice is to keep the video board far away from the power supply, and have the FDC/HDC card close enough to the drives so that you can actually wend the cable from one to the other.
But watch out for Win95/98 with PCI slots. If you don't put the cards back where they came from, Windows will just up and die at the next reboot.
There are obvious exceptions to were the cards go. You can't plug a 16-bit card into a 8 bit ISA bus slot; and the same with VLB cards, and PCI cards and slots. You can plug an 8-bit ISA card into a 16-bit slot, with the exception of some older longer cards which may have a lip at the back which will run into the 16-bit extension of a bus slot.
There are some other exceptions. Older XTs would not take an I/O port card in the first slot next to the power supply, and even today you will run into mother boards where a card refuses to work in one slot, and works fine in an adjacent bus slot. Oh well.
OK, take the cards out. The only one which might have a yet another connecting cable, will be the FDC/HDC card, which might have a two pin or four pin connector to an LED at the front of the box.
Take a look now at the slots which have been emptied. Some are of greater value than others. The expansion slots, all mounted on what is called the "bus" move data from the CPU to the cards, and quite a few other signals as well.
At this point everything is disconnected from the mother board, except for a whole bunch of small two stranded and three stranded cables which plug onto a number of jumper pins. Most of these are window dressing; most don't matter if they are plugged in in reverse, or plugged in at all. These smaller cables all go the front of the box, and turn on LEDs, meet up with push-button switches, etc. The standard list includes the following (see the pins-out [text] file also for details):
If your box has one of those cute LED numeric displays which roll out a bunch of impressive numbers, one or more of the Turbo LED wires might go to it. Assuming you will always run the puter at full speed, you can permanently disconnect it. I'll show elsewhere (when I get to it) how to wire the Numeric Display to something significant, like the processor model, or the year, or an IP number.
There might also be a battery connected at the back of the mother board via either a 2-pin or a 4-pin plug. Don't disconnect this, for the CMOS chip will loose all memory of what was on board, as well as the date. Most add-on CMOS batteries are attached with velcro to the nearest convenient surface. Undo it instead from wherever it is hanging, and place it on the board.
Some CMOS batteries slip into a cicular holder, a few are soldered to the board.
At this point you will be wanting to remove the mother board, but will find that it still won't move. Look for some screws which hold the board down. It could be a single screw, or it might be as many as five. After removing the hold-down screw, most boards slide out. They are frequently seated on small plastic nubs, which need to slide (with the board) an inch toward the left (away from the power supply) before the board can be lifted out.
That leaves only the RAM chips to be removed. Do that after removing the motherboard.