486SocketBlaster

486SocketBlaster

486SocketBlaster is a simple voltage adapter for 486 processors. It allows you to use a 3-volt 486 CPU on an older 5-volt-only motherboard. For example you can use a DX4 or an Am5x86 on an old 486 motherboard that would normally only work with 5-volt CPUs. This basically converts any 3-volt CPU to an "Overdrive".

How does it work ?

It is really simple: All CPU pins are passed through to the motherboard socket, except for the Vcc. Those are connected to the output of a mini buck converter that can be soldered to the 486SocketBlaster board.

Soldering/Assembly Instructions

This board is not the easiest thing to solder, but can be done with a standard soldering iron and some patience. The best way to solder the headers is to start with the innermost one and work outwards in a spiral manner alternating between the top and bottom side as needed (see image below). This way you only work on the outside and can easily reach all the pins.

A desoldering gun may be useful when you accidentally clog up other holes. But it is probably best to be very careful and avoid clogging them in the first place. Also using flux makes it easier.

Please make sure you check the solder joints (also check for continuity) before you add another row of headers on the same side, because the new row will permanently block your access to the solder joints! Desoldering a header is almost impossible even with a desoldering gun due to the very tight hole tolerances. So it has to be soldered correctly from the first try.

Voltage regulator

The 486SocketBlaster can be powered by a cheap of-the-shelf buck converter, or by an external one. The circuit board accepts two common types of small voltage buck converters, usually found using keywords like "mini buck converter". These are commonly rated at up to 3A of current. Given that 3-volt 486 CPUs are usually very efficient (the DX4-100 is rated at max 3.55/5.22W typical/max) such a regulator should be adequate in most cases. For best stability try to source a good quality part.

If you are planning to overclock or to use a more power hungry CPU, please use an external regulation circuit plugged in to the ExtPwr header, or use a very good quality mini buck converter.

WARNING!!!

Please make sure you know what you are doing! Improper assembly/settings/use can damage both your precious motherboard and your precious CPU!

Features

• External Power Header. This can be used either for monitoring the CPU voltage, or for providing power externally, either with an external power supply or an external voltage regulator.
• CLKMUL Header for selecting/overriding the CPU multiplier. This controls CPU pin R17. 1-2 connects it to the motherboard socket pin and 2-3 connects it to ground. This reduces the multiplier of a DX4 to 2x (66MHz) and sets the multiplier of an Am5x86 to 4x (133MHz).
• WB/WT Header for selecting/overriding Write-Back/Write-Through cache. This controls processor CPU pin B13 (which is the CLKMUL pin in earlier 486 CPUs). 1-2 connects it to the motherboard socket pin, and 2-3 connects it to Vcc, which enables Write-Back.

Bill of materials

Gerber files can be found in the releases: https://github.com/scrapcomputing/486SocketBlaster/releases

Please note that the headers are a very tight fit. So please make sure that you get headers of the correct pin diameter.

With the limited testing I have done so far, it seems that you need at least one 10uF through-hole ceramic capacitor, otherwise the voltage ripple is too high and the system can become unstable with a DX4 @ 100MHz, using a cheap MP1584-based buck converter.

Videos featuring the 486 Socket Blaster

• Original video with rev0.1 PCB (part 1): https://www.youtube.com/watch?v=n5qlx4qKbSE
• Rev0.2 PCB and trying out smoothing caps (part 2): https://www.youtube.com/watch?v=7anzRWXtba0
• Choosing capacitor values for a 5x86 (part 3): https://www.youtube.com/watch?v=CLPSJ0jOQsU
• Detailed assembly instructions by Epictronics (rev 0.4 PCB): https://www.youtube.com/watch?v=vEpAoVE4KjU

Changelist

• Rev 0.5 : Adds WB/WT header and fixes CLKMUL table silkscreen for am5x86.
• Rev 0.4 : Larger holes for the female headers (0.62mm instead of 0.57mm). These should fit headers with 0.55mm pin diameter.
• Rev 0.3 : Updates silk screen for larger capacitor values and fixes CLKMUL table.
• Rev 0.2 : Fixes initial revision bugs.

List of other similar 486 adapter projects

• With an onboard voltage regulator: https://github.com/LimeProgramming/486SocketBlaster/tree/4-layer-onboard-regulator