eharmon

2023-07-09 — updated 2024-01-31

Overclocking 68040 Macintoshes was a well-known endeavour in the early to mid-90s as many systems came stock with a ton of headroom. With small cooling upgrades the Motorola chips could hit fast speeds and the boards themselves were generally able to handle major clock speed increases.

For instance, the Wombat board used in the Centris 650, Quadra 650, Quadra 800 was designed to scale from 25MHz to 40MHz with minimal changes (though for curious reasons the C650 was particularly crippled1). However, the 40MHz machines never shipped, and the only true 40MHz 68040 from Apple was the Quadra 840AV.

AV machines are quite rare and expensive, and many other models of ‘040 Macs have interesting features: the Q650/Q800 is the fastest A/UX capable Macs, and a Mystic2 offers the best performance a classic form-factor Mac can offer. As a result, pushing the boundaries of these slower Macs is quite accessible and can provide a ton of retro computing fun.

Table of Contents

Clock Chipping

On most boards this could be accomplished by replacing the oscillator providing the processor clock. Marc Schrier published a thorough guide to this overclocking method on Usenet in 1994, here replicated from an FTP archive on the Higher Intellect wiki. Some excerpts:

There has been a great deal of interest expressed over the net about these simple and inexpensive Macintosh modifications that yield 20- 40% speed increases. Over the last year or so I have been doing a fair amount of crystal oscillator swapping/acceleration on Mac’s, and gathering information from others.

This crystal oscillator swapping has been done for years, and some early computers even had jumpers that made it really easy to disable one oscillator and enable another higher frequency one.

While two methods of modification are fairly standard and straightfoward, involving physical modification to the logic board to replace and/or disable the onboard oscillator, the third is quite clever:

The basic idea of modification #3 is building a clip that disables the onboard oscillator, and feeds in a new, faster signal. The beauty of this modification over the others is that you do not have to do any soldering on the motherboard itself, just on the part you clip onto the surface mount crystal oscillator in your Mac.

In essence, a spring-loaded clip was placed over the oscillator on the board, making contact with the pins of the oscillator itself and tapping into the power and signals passing through them. The clip then provided, or connected to, it’s own oscillator which provided a new, faster clock signal.

This method is sufficiently simple and non-invasive that companies started looking at ways to market it.

You can find significantly more information on Marc’s website. This also forms the basis of my Spicy O’ Clip technique, so tremendous credit goes to these original investigators.

Mac Oscillators

Macs of these era tended to use surface mount, SOJ oscillators in a “wide” package. My Quadra 650 has a Seiko Epson SG-615P.

Accelerators

Newer Technology MacClip Jr.

One popular product was the Newer Technology MacClip Junior. This essentially productized the clip-on overclocking method, combining it with a clever set of DIP switches to adjust the clock divider on the fly to provide configurable clock speeds. This took the original methods even further, allowing a user to find the sweet spot for their hardware and use cases. While professionally built, they still feel like a bit of a hack, consisting of two circuit boards sandwiched together with a spring to provide the clip and upgraded oscillator in a single, relatively easy to install package.

You can find pictures on Guillaume Telo’s website.

If you’ve got a MacClip Jr. and want to configure the clock speed, you can follow this guide:

A few other similar products for ‘040s shipped from other producers, if you’d like to go hunting for one:

There’s a great rundown of the state of things in the March 1994 issue of MacWorld, page 96.

MicroMac Speedy

The Speedy takes this idea one step further, moving from a series of DIP switches to an adjustable dial stepping the clock speed by 0.25MHz increments. This allows you to precisely fine-tune an overclock, finding the sweet spot for a specific logic board and processor.

Newer Technologies Variable Speed Overdrive

Unlike the simple clips above, the Newer Technologies Variable Speed Overdrive took things a step further by inserting into the ROM socket to allow for online configurability. This enabled the user to dynamically adjust the overclock through a control panel, providing even more convenience than the MicroMac Speedy. While this still requires the use of two wire clips, installation is nearly as simple as the other options.

This only supports the Quadra 700, 900, and 950, likely because later models did not ship with a ROM SIMM socket. It’s possible adding a socket to a different ‘040 Mac or hand-wiring the lines would provide compatibility.

The PowerPump extended this acceleration for additional flexibility with the PowerPC upgrade cards for these machines.

Swappable Sockets

Alternatively, if you’re interested in an improved version of Methods 1 and 2 from Marc’s guide, cy384 offers a guide for an oscillator socket mod. This allows you to easily swap standard DIP-14 4 lead oscillators into your board.

Spicy O’ Clock

Recently Kero’s Mac Mods and Stephen Arsenault created a modern version of the Speedy: Spicy O’ Clock, a very clever little board which provides a variable clock signal from 18.8MHz to 25.9MHz (internally doubled by an ‘040). This functions as a drop-in replacement for a normal oscillator, but with the benefits of easy tuning and, if you’re brave, online adjustments. It goes beyond the original Speedy, providing 0.01MHz increments for incredible overclock precision.

Typically, this unit is attached to a Mac using Method 1, desoldering the original oscillator and attaching wire leads to the Spicy O’ Clock instead. As a result, the Spicy and it’s adjustable oscillator can take over allowing for highly tweakable overclocking.

However, if you want a more easily reversable and optionally no-solder installation like a MacClip Jr, you can follow my Spicy O’ Clip installation guide

Going Further

Folks continue to push these machines further. This thread on 68klma digs into how far clock speeds can be pushed, particularly on Wombat boards.

  1. C650 Serial Port Modification ↩︎

  2. Mystic upgrades for the Color Classic ↩︎