These are my nots on trying to repair my Commodore computers and there various issues.

• https://plus4world.powweb.com/forum/25481
  • MIK
    • Q: Unlucky Nightmare! I've had it happen and I'm sure many people have. A second hand Plus/4 gives up the ghost after being powered on for a short while, from maybe not being used for many years! Something I can not explain but it's happened to me once before now with a second hand P4. Everything was running sweet and then BOOM, black screen. Never fixed it as I'm running out of dead machines to fix working ones.
    • A: Sounds like the CPU.
  • MIK
    • If you can see anything on the screen right now such as messed up text and stuff then I would of gone for the TED also but a totally black screen is normally the CPU.
  • SVS
    • Unit 1551 #9 can work only if a unit 1551 #8 is connected and turned on. This means that a 1551 #9 cannot work even if a 1541 #8 is connected to Plus4;
    • The drives (any type) have to be turned on and initialized *before* to turn on the Plus4;
  • crock
    • Right to the problem, the CPU is just as prone as the TED to a premature death, and a likely cause of your black screen but it is not the only thing. The kernel, PLA, *any* ram chip, any component in the reset circuit or the crystal will also cause the same effect, so don't lose hope yet.
• Commodore Plus/4 Specifications
  • Modulator - Analogue RF Channel 36
  • Processor - MOS 7501/8501 @0,885/1.76 MHz
  • Memory - 64 KB RAM (upgradeable) and 64 KB ROM
  • OS - BASIC 3.5
• Dead/Non-Responsive Serial Port
  • Scenario:
    • I was using my commodore plus4 for a couple of days to capture tapes using TTAPSERV.PRG and an XE cable which worked well and then all of a sudden it stopped working.
    • I can see communication using VC1541 and a XE cable, but it is inconsistent and cannot be used.
    • The inconsistent transfer error is what I got when I used the wrong parallel port mode or a parallel port that was not compatible check the motherboard of the PC
    • It is either the serial port has failed or the parallel port
    • NB: If your adapter never worked from the beginning, the most likely cause is the printer port is not compatible with the X range of adapters.
  • Checklist
    • Check the parallel port with something to make sure it is not faulty
      • Port Tester
      • Parallel Printer
      • Try all different port modes available in the BIOS
    • XE Adapter
      • check for dry joints
      • could be the diodes on my XE adapter as they are
    • Commodore Drive
      
      • Check the serial port with a known good piece of commodore hardware (+4/C16/C64)
      • could be the IC controlling the serial bus in the commodore drive
      • could be the diodes near the serial port which apparently is common fault
      • check the physical connection of the serial port as sometimes the pins become dry-jointed etc.
• Commodore 8 Bit Repair - VCF 2012 | YouTube | bilHerd - Part 1 of 2: From VCF East 2012 - Workshop on diagnosis and repair of vintage 8 bit computers; PET through C128. Presenters Rob Clarke and Bil Herd (both ex Commodore employees) demonstrate existing tools and methods for troubleshooting yesterdays computers.
• Diag264 - Diagnostic tool for Commodore 8-Bit Machines
  • This document describes operation of the Diag264 diagnostic tool and harness. It can be used to assist in the identification of faults with RAM, ROM, I/O & CPU ports, and TED registers. It has been tested on a wide range of 264 series (aka TED) machines, including some non-standard configurations and PAL and NTSC machines. This includes the C116, Commodore 16, C232 and Plus/4, and even a V364!
  • The design and operation of Diag264 is closely modelled on the operation of the cartridges available for the 64/128 and VIC-20. One of the aspects that I was most keen to copy was the functionality of the Dead Test Cartridge of the 64. The primary use of this cartridge was to find RAM issues in a machine that otherwise appeared dead.
• Classic computer Commodore plus 4 repair and testing | YouTube | Phil Kruman - Repairing a classic computer - The Commodore Plus/4 computer is more difficult than most of the 1980's micro's due to its short production run and Commodore only processors. As an alternative, I try an Italian made CPU daughterboard with surprising results for this ailing classic computer.
• Commodore Plus/4 repair and demonstration | YouTube | Artic retro - I finally got my hands on a couple of Commodore Plus/4 machines. One of them not working. In this video I do a repair job, some restoration and then a demonstration of the working plus/4 machine with a few games.
• Commodore 16 and 'Device Not present' error - Forum (#35118) - Plus/4 World
  • I have picked a Commodore 16 a few days ago from a local sale. The machine looks OK and will boot up normally. Problem is when I want to use my SD2IEC connected to the serial port, I'll just get a 'Device Not Present' error message while trying to load the SD2IEC browser. I have tried also with a 1571 Disk Drive but same message . What this can be? Faulty chip?
  • Diagnosis of this hardware issue
• Commodore Plus/4 Repair & Restoration – Adam's Vintage Computer Restorations
  • A while back I got my hands on my first Commodore Plus/4, yet another 1980s 8-bit computer to add to my collection. The machine was in good condition and came with several accessories and its original box, however it was sold as "untested".
  • After checking that the PSU was working OK, I did a quick power-on test – the computer seemed to output video, but only displayed a “garbage” screen with flashing blocks and lines, and various artefacts which changed on reset. The unit required repair.
• C64 "Searching for $" repair (serial bus issue) | YouTube | bwack
  • Two days ago I picked up a breadbin style Commodore 64. It worked, but the disk drive loading was intermittent. Next day it was almost impossible to get directory listing and later it was impossible.
    I swapped CIA chips, no change. I swapped C64's, bingo. Problem is within the new C64 and not the drives.
  • According to the service manual, the serial bus should be checked. On the serial bus, in both ends, there are 7604 buffers. They are like the amplifiers of the VIA (in the 1541 drive or VIC20) and CIA chips (C64). This had ofcourse failed and desperatly i borrowed one from another drive. I promise i promise i promise to replace with new one soona and clean up the rework. :)
• Commodore Plus/4 and temperatures - Davide Bucci
  • Is temperature a big deal? Spoiler: Nope.
  • An in-depth teardown to look at this issue.
• Repair of an Commodore Plus/4 (264) ! - Stone Oakvalley Studios - One Crazy Multi Talent - A loving restoration of an unwanted Plus4
• IC Chip Removal
• How to remove chips without damaging the circuit board | Yourube | Adrian's digital Basement
  • It can be tough to remove through hole ICs from boards without causing any damage. I'm using a desoldering iron, so if you don't have one, you will need something to remove solder. (Braid, manual pump, etc.) These usually leave some solder behind on double sided boards making it very hard to get chips out without causing damage.
  • I use a combination of removing as much solder as I can from the bottom and hot air on the top to get the chips out without causing any damage.
  • Makes it look easy.