Tricky Repair of Commodore PET Computer and a Lesson Learned About Unfounded Judgments
Hey there! Let's chat about an interesting repair job undertaken by a guy named Ken Shirriff on an old-school home computer – a Commodore PET 2001. The computer, much like many vintage systems, started displaying weird symbols on its screen, making its ICs seem pretty cranky, though it did manage to power up.
Upon assessing the situation, two faulty MOS MPS6540 ROM chips were identified first, which was solved by replacing them with 2716 EPROMs using an adapter. But the random symbols issue persisted, so out came the logic analyzer. The analyzer helped identify 3 bad RAM ICs, fixing the display problem for the most part, albeit leaving a 2x2 checkerboard pattern and some peculiar behavior when running BASIC programs.
The 6502 MPU was later found writing to the wrong addresses, causing the mess on the screen. Here's the kicker – this blunder was due to a misplaced byte in one of the replacement EPROMs because the programmer used couldn't quite reach the required voltage! Using a better programmer sorted that out, but soon enough, another RAM IC bit the dust, bringing the total of kicked-the-bucket silicon to four RAM and two ROM ICs.
So, the moral of the story? Always double-check your replacement ROMs before sticking them into a system, y'all! Vintage computer repair ain't just about replacing a few ailing parts – often, it takes some patience, dedication, and a good understanding of those quirky old systems.
Now, if we were to delve deeper: Early PETs like the 2001/3000 models struggled with issues like the Video RAM Access Problem (Killer Poke), slow SRAM, and display and keyboard concerns due to their age and design. Potential fixes include replacing the SRAM, modifying the video circuit, and volging (slang for maintenance of the power supply). Faulty components, such as capacitors or ICs, were common culprits behind these problems. Keep in mind that these tips are generally applicable to old-school computers like the Commodore PET and not necessarily related to Ken Shirriff's specific repair job.
Programming errors can also cause issues in old-school systems, such as the Commodore PET 2001, just like the one that was encountered due to the programmer's inability to reach the required voltage in the replacement EPROMs. Technology advancements have made it easier to identify and fix such programming mistakes in modern systems.
