Classic Computer Magazine Archive COMPUTE! ISSUE 166 / JULY 1994 / PAGE 42

PC brain transplants made easy. (replacing a personal computer's power supply) (Hardware Clinic) (Column)
by Mark Minasi

I had just a few minutes before I had to leave for the airport. I was installing Freelance for Windows, and I removed one of the installation floppy disks from my PC. That's when I noticed that the floppy was a mite warm.

I reached behind the computer, and--sure enough--the fan wasn't blowing any air. The last time that happened, I had to replace the motherboard because the heat had weakened various components. I didn't relish replacing a 50-MHz 486DX, a local-bus accelerator, a 500MB hard disk, a VideoSpigot, a Sound Blaster, dual 16550 UARTs, and an Ethernet card.

I shut down the computer and left on my trip, cussing.

I wasn't cussing about the potential damage to the PC; I was pretty sure that I'd discovered the dead fan soon enough to shut down the PC before any damage had occurred. I was cussing about finding a replacement power supply for my tower PC.

Until recently, power supplies were fairly generic things. But towers and the newer desktop cases have power switches on the front, away from their power supplies. That presents the problem of how to put a power switch for the power supply away from the power supply.

The answer is often to build some kind of combination of proprietary case and power supply, which is unfortunately what the makers of my tower PC had done.

As a result, I couldn't get just a new power supply; I had to get a whole new combination--case and all. I had to do a PC brain transplant.

I popped down to my local PC parts place and picked up a new case. It has a turbo switch; turbo, power, and disk drive lights; and a reset switch on the case, as well as a power switch up front (can't argue with progress, I guess). In order to create a small footprint, it has only two halfheight 5 1/4-inch drive bays and a lone 3 1/2-inch drive bay turned on its side. Inside, there is another 3 1/2-inch drive bay that has no opening to the front.

The idea with the drive bays is that you're supposed to put a 1.2MB floppy drive in one of the 5 1/4-inch bays, a CD-ROM drive in the other, a 1.44MB floppy drive in the first 3 1/2-inch bay, and a 3 1/2-inch IDE drive in the last bay. It's not an optimal arrangement, in my view, as the 3 1/2-inch bay is off-center and rotated 90 degrees from its normal orientation, leading most users to assume that the 5 1/4-inch drive is the A drive. Also, there's no place for a second hard disk. But it's an acceptable case, and indeed, it's the case that's most popular among cloners these days for a desktop PC.

Much of the brain transplant was routine removal of boards and drives from the tower case and insertion of boards and drives in the new case. The tough part turned out to be connecting all the switches--which is what I want to explain this month.

If you're taking the time to mount a PC motherboard in a case, you may as well do it right and hook up all the lights and switches. I'll assume that you've got the documentation on the motherboard somewhere in the pile of papers that came with your computer. Find the connections on the motherboard for the reset switch, keylock/power-on LED, turbo LED, speaker, and turbo speed settings.

Before going any further, however, make sure that you've got the power wires hooked up right. It turns out that the new cases make it possible for you to burn down your house if you hook up these wires incorrectly.

As I've already said, many modern PCs do not use a power switch mounted on the right rear, as do older PCs with AT-type cases. Instead, newer PCs bring in the 110-volt line current to the PC front panel through a thick black cable. The cable extends from the power supply to just behind a switch on the front panel. The switch has four flat connection points, called spade lugs, on it, and there are four wires inside that cable. These wires are black, blue, white, and brown. Your job is to connect the correct wires to the proper contacts.

AC power doesn't include positive and negative wires. Instead, there are hot and return wires. Ordinarily, the white is the hot, and the black is the return. Older power supplies connect white and black wires to the power supply from the wall socket. The big red on/off switch mounted toward the right rear of the PC connects or disconnects both the black and the white lines simultaneously when you flip the switch.

If you put the power switch on the rear of the case so that you can incorporate it directly into the power supply, the big red switch is all that's needed--but the needs of the new power supply's front-panel switches change all that. (Actually, old power supplies also include a third wire--a green one--for ground, but it's not important for this discussion.)

In order to build a front-panel switch, manufacturers drag both the hot wire and the return wire to the front of the case, and then send them both back into the power supply.

* The white wire connects the hot side of the wall outlet to the power switch.

* The brown wire connects the hot side of the power supply's power input to the power switch.

* The black wire connects the return side of the wall outlet to the power switch.

* The blue wire connects the return side of the power supply's power input to the switch.

When you turn the switch on, you connect black and blue, providing a return AC connection for the power supply, and you connect white and brown, providing a hot AC connection for the power supply.

If you disconnect the black, blue, white, and brown wires from the frontpanel switch, you should be able to see from the previous discussion how important it is to diagram your connections. This is one situation in which, if you reconnect things backward, you could end up directly connecting hot from the wall socket right into return from the wall socket. That would cause a short circuit that could make your computer catch fire, or--if your fuses or circuit breakers aren't up to snuff--your house could catch fire.

If you look at the front-panel switch, you'll see four spade lugs where you can connect or disconnect the white, black, brown, and blue wires. You'll notice a very low ridge on the connector and two spade lugs on either side of the ridge. Before disconnecting the wires from the switch, notice that the black and the blue are on one side of the ridge and the white and the brown are on the other side. The ridge is just a reminder about which wires go with which other wires. Just keep the white and the brown on one side and the black and the blue on the other side, and all will be well. The way I remember it is that one side is black and blue.

If you have any doubts, however, it would be best to enlist the aid of a friend who's knowledgeable about AC power and voltmeters. You really can do some damage if you wire these switches wrong.

Once the power is hooked up, you can concentrate on the other connectors. It would be nice if there were some kind of standard wiring color for the turbo switch, the turbo light, and the like. Since there isn't such a standard, here's the approach that I take to figure out the keylock, turbo switch, reset switch, and turbo light. I'm assuming that you're trying to figure out which switch or LED on the front of the PC case goes with which wire-and-connector combination inside the case. Then you can plug that connection into the appropriate place on the motherboard that you're installing. Your best tool for this job is a voltmeter.

The easiest of the connections to identify is the reset switch. It will have only two wires attached to it. Find a two-wire connection; then set the voltmeter on Rx1. Apply the leads to the two wires. Nothing will happen if it's the reset connection. Then press the reset button on the front of the case. If the needle jumps on the voltmeter, you've found the reset switch.

At this point, you should mount the motherboard inside the case and connect the power and speaker to it. If you turn the PC on, you should get a series of beeps from the speaker. (The speaker connection is easy to find, as you can usually follow the wires back to the speaker. Speaker connections are usually keyed so that they attach only one way, but actually, there is no single right way for PC speaker connections.) You can now test the reset switch by attaching it to the motherboard: Turn on the PC, and when the beeps begin, hit the reset switch. If the beeps stop exactly when you hit the reset switch, you'll know you correctly wired the reset switch.

Next, attach the turbo switch. It's the only connector with three wires on it. Most motherboards have only two pins for this function, but the turbo switch has three wires--quite confusing. Just plug two of the connector's holes over the pins (either the two to the left or the two to the right), and the turbo switch will work.

What's the difference? Attached one way, the PC will be in turbo mode when the switch is in the in position and in nonturbo mode when the switch is in the out position. Attached the other way, it works in reverse.

Now you can attach the turbo LED. You can find it because there will be only two two-wire connectors--the hard disk LED and the turbo LED--and the hard disk LED will have a red wire and a black wire on it. Attach the turbo LED's connection to the motherboard, and examine the LED. If it doesn't light up, disconnect the connector, reverse it, and reattach it to the motherboard's turbo LED connection. You should then be able to click the turbo button and see the turbo LED change color. If this does not happen, however, don't panic just yet; some BIOS's won't allow the CPU speed to change until the system has booted.

The keylock/power LED is the last connector. It usually has four or five wires on it, and it's usually keyed so that it will attach only one way. Try plugging it in and powering up the motherboard, and you should see the power LED on the case come right on. Once you've got that done, congratulations! You've done a professional installation job!