Over on [Ken Shirriff]’s blog is a tricky Commodore PET repair: tracking down 6 1/2 bad chips. WARNING: contains 8-bit assembly code.

The Trinity of 1977 which started the personal computer revolution were the Apple II, the Commodore PET, and the TRS-80. In this project it’s a failing Commodore PET which is being restored.

In the video below the break you can see [Ken Shirriff] and [CuriousMarc] team up to crack this tough nut. Resolving the various issues required a whole heap of software and equipment. Most notably a Keysight DSOX3104T oscilloscope, a Retro Chip Tester Pro, an old Agilent 1670G logic analyzer (this thing is rocking a 3.5″ floppy disk drive!), an old Agilent 54622A oscilloscope (also rocking a floppy drive!), a Data I/O 29B Universal Programmer With UniPak 2 insert, and the disassembly software Ghidra.

In the end there were 6 (and a half) bad chips which needed to be discovered and then replaced. This project is a reminder that it’s nice to have the right tools for the job!

If you’re interested in the Commodore PET you might like to read A Tricky Commodore PET Repair And A Lesson About Assumptions or Tracking Satellites With A Commodore PET.

The Commodore 1541 was built to do one job—to save and load data from 5.25″ diskettes. [Commodore History] decided to see whether the drive could be put to other purposes, though. Namely, operating as a standalone computer in its own right!

It might sound silly, but there’s a very obvious inspiration behind this hack. It’s all because the Commodore 1541 disk drive contains a MOS 6502 CPU, along with some RAM, ROM, and other necessary supporting hardware. As you might remember, that’s the very same CPU that powers the Commodore 64 itself, along with a wide range of other 1980s machines. With a bit of work, that CPU can indeed be made to act like a general purpose computer instead of a single-purpose disk controller.

[Commodore History] compares the 1541 to the Commodore VIC-20, noting that the disk drive has a very similar configuration, but less than half the RAM. The video then explains how the drive can be reconfigured to run like the even-simpler MOS Technology KIM-1 — a very primitive but well-known 8-bit machine. What’s wild is that this can be achieved with no hardware modifications. It’s not just a thought exercise, either. We get a full “Hello World!” example running in both BASIC and machine code to demonstrate that it really works.

Code is on GitHub for the curious. We’ve featured hacks with the chunky Commodore 1541 before, too.

Thanks to [Bruce] and [Stephen] for the tip!