Ask Hackaday: Calling All 68k Experts

This is a tale of old CPUs, intensive SMD rework, and things that should work but don’t.

Released in 1994, Apple’s Powerbook 500 series of laptop computers were the top of the line. They had built-in Ethernet, a trackpad instead of a trackball, stereo sound, and a full-size keyboard. This was one of the first laptops that looked like a modern laptop.

The CPU inside these laptops — save for the high-end Japan-only Powerbook 550c — was the 68LC040. The ‘LC‘ designation inside the part name says this CPU doesn’t have a floating point unit. A few months ago, [quarterturn] was looking for a project and decided replacing the CPU would be a valuable learning experience. He pulled the CPU card from the laptop, got out some ChipQuick, and reworked a 180-pin QFP package. This did not go well. The replacement CPU was sourced from China, and even though the number lasered onto the new CPU read 68040 and not 68LC040, this laptop was still without a floating point unit. Still, it’s an impressive display of rework ability, and generated a factlet for the marginalia of the history of consumer electronics.

Faced with a laptop that was effectively unchanged after an immense amount of very, very fine soldering, [quarterturn] had two choices. He could put the Powerbook back in the parts bin, or he could source a 68040 CPU with an FPU. He chose the latter. The new chip is a Freescale MC68040FE33A. Assured by an NXP support rep this CPU did in fact have a floating point unit, [quarterturn] checked the Mac’s System Information. No FPU was listed. He installed NetBSD. There was no FPU installed. This is weird, shouldn’t happen, and now [quarterturn] is at the limits of knowledge concerning the Powerbook 500 architecture. Thus, Ask Hackaday: why doesn’t this FPU work?

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Upgrading And Desoldering A Fake CPU

[quarterturn] had an old Apple Powerbook 520c sitting around in his junk bin. For the time, it was a great computer but in a more modern light, it could use an upgrade. It can’t run BSD, either: you need an FPU for that, and the 520 used the low-cost, FPU-less version of the 68040 as its main processor. You can buy versions of the 68040 with FPUs direct from China, which means turning this old Powerbook into a BSD powerhouse is just a matter of desoldering and upgrading the CPU. That’s exactly what [quarterturn] did, with an unexpected but not surprising setback.

The motherboard for the Powerbook 500 series was cleverly designed, with daughter cards for the CPU itself and RAM upgrades. After pulling the CPU daughter card from his laptop, [quarterturn] faced his nemesis: a 180-pin QFP 68LC040. Removing the CPU was handled relatively easily by liberal application of ChipQuik. A few quick hits with solder braid and some flux cleaned everything up, and the daughter card was ready for a new CPU.

The new FPU-equipped CPU arrived from China, and after some very careful inspection, soldering, and testing, [quarterturn] had a new CPU for his Powerbook. Once the Powerbook was back up and running, there was a slight problem. The chip was fake. Even though the new CPU was labeled as a 68040, it didn’t have an FPU. People will counterfeit anything, including processors from the early 90s. This means no FPU, no BSD, and [quarterturn] is effectively back to square one.

That doesn’t mean this exercise was a complete loss. [quarterturn] did learn a few things from this experience. You can, in fact, desolder a dense QFP with ChipQuik, and you can solder the same chip with a regular soldering iron. Networking across 20 years of the Macintosh operating system is a mess, and caveat emptor doesn’t translate into Mandarin.