THIS OLD 486

COPYRIGHT 1996

Anthony Olszewski


These notes are only meant to illustrate how I go about working on a machine. I'm not encouraging anyone to work on their own system. In any event, if you do any damage, direct or incidental, you are assuming complete responsibility and liability!

It's seems like just yesterday, doesn't it? You just were not going to inflict that old 286 on yourself any longer. Time to shop for a REAL computer. A 486 that would run WINDOWS 3.1 like it was meant to be run. A system that would be good until the year 2000!

But technology had other ideas. Did the clerk that sold you WINDOWS 95 laugh when you described the hardware waiting at home? It might be time to think of a processor upgrade, especially for the 486SX.

Before you start putting creases in the credit card, you'll need to answer a few questions about your present hardware. First off you need to figure out what kind of 486 socket that you have. There is a range of sockets with different pin counts and voltage characteristics. If there is only one socket on your motherboard (the one containing the current 486 chip), then you'll need a REPLACEMENT chip. If the motherboard has a second (now empty) socket, an OVERDRIVE chip will probably be needed.

On older motherboards, the processor used 5 volts; the newer ones work at around 3 volts. If the board is now working at 5 volts, then either get a new chip that is the same voltage or use a voltage regulator. The voltage regulator is a miniature PCB board with pins below and a socket above. The pins plug into the CPU socket on the motherboard. The new 3 volt CPU then plugs into the voltage regulator. If you put a 3 volt cpu onto a 5 volt motherboard, it will fry and die unless a voltage regulator is installed first!!!

Three Volt 486 CPU. In Order To Work In An Older Motherboard, a Voltage Regulator Will Be Required.

To clearly state the obvious, if the original processor is soldered to the motherboard and there is no upgrade socket, your system is not upgradable. If this describes your motherboard, do not attempt to pry out, desolder, or in any other way remove the original processor! You'll only destroy the board.

The 486SX CPU Is Soldered Permanently Into This Packard Bell Motherboard. To Upgrade This Motherboard, An Overdrive Chip Is Used.

Almost for sure, there will be a series of jumpers that must be set so that the new processor will be recognized and run. With luck, these jumpers will be IDed by a silk-screen on the motherboard. Otherwise, you'll need the manual or the manufacturer's Web Site or tech support. For a no-name motherboard, there's no hope without the manual. I've seen manuals that were simply wrong. Then you'll have to resort to trial-and-error.

These Plastic Caps Are Jumpers (Magnified). Installed Over The Metal Pins Is On. Not Installed is Off.

If you're tempted to use a PENTIUM or one of the exotic 586s, then investigate if your system allows write-back cache. The 486s used "write-through" cache where the CPU writes data to memory external to the processor. With write-back cache, the CPU uses memory designed into the chip - a much faster method. Some 486s have a jumper to enable write-back cache. It's not unusual for this jumper to fail to work!

Intel sells an interposer to support the Pentium OverDrive upgrade in 486 systems with faulty write-back. The interposer plugs into the motherboard's processor socket. The CPU then goes into the interposer. The interposer disables the improperly designed write-back mode.

For a motherboard that does not allow for write-back cache, there are two options. First, just get used to living with a white-bread 486 as the only upgrade option. The other option is to look for an upgrade kit with a voltage regulator that disables the write-back cache feature of the CPU. This will not be as fast or powerful as write-back, but at least it will work with the motherboard that's there now.

CPU Fan. Best $9 You'll Ever Spend!

When installing a more powerful processor, unless definitely impossible, put in a CPU fan. At the very least, be sure to use a heat sink. It's a good idea to use both a heat sink AND a CPU fan!

Kits are available that include the processor, the voltage regulator, the heat sink and/or CPU fan. This is generally the best route to take, for all the items are tested, and guaranteed, to work together.

Upgrade CPU Sockets
Original 486 SocketSocket 1 Socket 2 Socket 3 Socket 4 Socket 5 Socket 6 Socket 7
Number of pinholes168 169 238 237 273 320 235 321
Voltage5V 5V 5V 3V/5V 5V 3V 3V 3V
Original CPU486DX 486Sx, 486DX 486SX, 486DX, DX2 486SX, 486DX, DX2, DX4 Pentium 60, 66 Pentium 75 - 120 DX4 Pentium 75 - 133
Replacement CPUDX2, DX4 DX2, DX4 DX2, DX4, Pentium OverDrive DX2, DX4, Pentium OverDrive - - - -
Socket types 1 - 7 are ZIF (Zero Insertion Force) sockets. Before counting pinholes, examine the socket. The socket type number might be printed on the socket.

ZIF Socket. Lift The Handle And The Pressure Is Released.

The Pentium OverDrive processor upgrade can't be done in a 486 system that has a socket with 17 pinholes per side (168- or 169-pinhole socket) . In order to install an Intel OverDrive CPU, the socket must have 19 pinholes per side (237- or 238-pinhole socket). You can tell this by just glancing at the original Intel486 processor while it's still in the socket on the motherboard. If the 486 chip is installed in the same socket that the OverDrive is to use and there is an extra row of pinholes around the processor, then the socket has 237- or 238-pinhole. This means that he PENTIUM OverDrive will fit.

Is there an empty special socket for an Intel OverDrive processor? To do the PENTIUM upgrade, the OverDrive processor goes into the spare socket.

For a PENTIUM upgrade, particularly one with MMX, a BIOS upgrade will also be on the agenda. Intel maintains a searchable database of BIOS upgrades . If you don't change the BIOS, the system might not work at all with the PENTIUM processor. The computer might chug along until you try changing a card and then fail. In either of these two case, returning the original CPU should bring the machine out of its coma.

How do you remove the CPU chip from its socket? Very carefully! (Of course, as always, make sure that the power cord is removed from the system unit. Before handling the processor chips, first touch a large piece of metal to discharge any static electricity that you may be carrying.) If you are dealing with a ZIF socket simply gently release and lift up the little handle. You will now, if necessary, be able to lift out the original CPU and just drop in the upgrade. Closing and catching the handle completes the installation

Older CPU Socket. Note The Square With The One Edge Angled Off That is Silk-Screened Onto The Motherboard. The Dot On The CPU must Point Towards This Slanted Edge!

The original CPU sockets are a little trickier. Upgrade kits often come with a small tool like a miniature rake for prying the old processor out. I just use a tiny screwdriver. Whatever you use, work a little bit at a time. Start at one end and slowly work your way around. After about half way, the pressure is basically off the pins. You're not trying to catapult the chip out like a tiddlywink or something! The real pain is putting in the replacement CPU. A certain amount of ooomph is needed to seat the thing. Definitely, first make sure that all the pins are properly lined up in holes. If you try to push it in crooked, pins will bend or break. This can kill the chip. Abuse is very rarely covered under the warranty. Also take care as you push the chip into the socket that the motherboard itself does not get stressed and crack. I suggest taking the motherboard out of the case. Remove the plastic standoffs. Place the board on a flat, secure, solid surface before pushing down on the upgrade processor. The level plane will evenly support the board. Hope for a ZIF socket!

486 CPU. The Dot On The CPU (Just Visible In The Bottom Left Corner) Indicates The Orinetation Of The Chip.

Processors must be installed in the proper direction. There is a dot or some other pointer on the chip itself. This must line up with a slant in the square that is silk- screened around the CPU socket. Sometimes a little arrow on the board shows where the dot of the CPU is to go.

In my experience putting a CPU in backwards does not hurt, but it will not work. Getting the little bugger in and out is such a pain, you want to insert it right the first time.

Is any of this worth it? Often it isn't! If you've got a 486SX25, it's essentially de rigeur to upgrade before your friends give you the nickname of Packard Bell!

With almost any of the 486DX chips, the situation becomes less clear. I just don't see the benefit of the PENTIUM processor upgrades for a 486 board. For less than a $100 more, you can install a brand new PENTIUM motherboard with PCI, DIMM, EIDE, auto hard drive identification, and all the other state-of-the-art goodies. If your motherboard uses 30 pin SIMMs and you also need to increase memory, get a new motherboard!

If you're still gung-ho on a Pentium chip upgrade, Intel has OverDrive CPUs for many systems.

For PS2 computers, chip upgrades make more sense. Industry-standard motherboards will not fit in a PS2 case and can not use the PS2 power supply. PS2 RAM, hard drives, floppies, etc will also not work with a "normal" motherboard.

And what really are the bottlenecks in your system? If WINDOWS is choking on paltry RAM, a tired, eggbeater hard drive and/or an impotent video card, a processor or a motherboard upgrade will probably be of no benefit.

Older 5 volt CPUs
Intel 66 MHz 80486-DX2 OverDrive
Cyrix 66 or 80 MHz Cx486DX2
Intel 100 MHz 80486-DX4 OverDrive
Intel 83 MHz Pentium OverDrive

3.3 and 3.45 volt CPUs
AMD 66 or 80 MHz Am486DX2-66 or -80
AMD 75, 100 or 120 MHz Am486DX4-75, -100 or -120
AMD 133 MHz Am5x86-133
Cyrix 100 or 120 MHz 5x86-100 or -120
A voltage regulator, a tiny circuit board installed in the motherboard socket, is required for these chips to operate in a 5 volt motherboard. The 3 volt processor is then plugged into the voltage regulator.

For reliable operation, motherboards that don't support a write-back cache, the AMD 133 MHZ Am5x86-133 must be used with a voltage regulator that disables write-back cache.

To attain full operating speed the Am486DX2-80, the Am486DX4-120, the Cyrix 80 MHZ Cx486DX2, and the Cyrix 5x86-120 must have an external bus clock running at 40 MHZ. VLB (Vesa Local Bus) motherboards were not speced to run over 33 MHz. Running VLB motherboards at 40 MHz is over-clocking. This is a risky gamble.