Commodore 128 General Maintenance
Routine care for the flat Commodore 128: the external power supply, the on-board supply, sockets, heat and the keyboard. For fault-finding see the Commodore 128 Troubleshooting Guide; for the capacitor lists see the Commodore 128 Capacitor Replacement Guide.
Power supply
[edit | edit source]The external supply
[edit | edit source]The C128 takes regulated +5 V DC and 9 V AC from an external supply. Ray Carlsen describes it as a switching supply that runs cooler and lasts longer than the Commodore 64's linear brick.[1] Tynemouth Software gives its rating as 2.5 A at 5 V and 1 A at 9 V AC.[2] One owner measured a C128 drawing 1.8 A from the 5 V line and 0.2 A from the 9 V line.[3]
Part numbers seen on C128 supplies include 310416-01 to -03 (Ismet, PAL)[4] and 310416-05, one of which had a Mitsumi 252449-01 board inside.[5] Commodore's service manual prints the circuits of the Mitsumi 252449-01 and the Dee-Van 252449-03 "for reference only".[6] The Mitsumi circuit has a 600 mA primary fuse F1, a 1.5 A fuse F2 in the 9 V AC winding, a 200 Ω adjustment (R11) on the 5 V regulator, and a crowbar on the 5 V output made of a 6.2 V zener (V9) and a TIC106D SCR (V10).[6]
The supply plug has the same shape as the Amiga 500, 600 and 1200 supplies', but they are not compatible.[7] Check the label before plugging anything in.
Faults in the supply
[edit | edit source]Warning: the supply contains mains wiring and a transformer primary. Unplug it before opening it.
- Factory glue. The glue that holds parts in place turns brown and then black near hot parts, and black glue absorbs water. Carlsen finds it corrodes tracks and component leads, and scrapes it off with a dental pick.[1]
- Cracked solder joints on the power transistors and the large filter capacitors. Carlsen resolders them, with new solder, whenever he opens one.[1]
- Output drift. Tynemouth found a supply delivering about 4.5 V and trimmed it back to 5 V with its adjustment.[2]
- Overvoltage. World of Jani reports that on the Ismet PAL supplies the 5 V output can rise to between 12 and 15 V if the 4700 µF capacitor C9 fails and then the crowbar fails.[4]
- Fuses. Carlsen describes a fuse in the computer's case bottom that opens on an overload of the 9 V AC line, which a shorted SID blew in one machine, and another fuse on the supply board for the 5 V output.[1] World of Jani found no fuse on the +5 V rail of the Ismet PAL supplies.[4] Check which kind you have.
Carlsen notes that outright component failures in these supplies are rare.[1] Measure the 5 V output on a load before connecting the computer to any supply of unknown history. The supply's capacitor values are in the capacitor guide.
On the main board
[edit | edit source]The +5 V comes in on CN11 and passes the power switch and line filter L5. The 9 V AC is rectified by bridge CR13 to about +18 V and regulated to +12 V by U59, a 7812.[8][6] Commodore's checks, to ±10 %:[8]
| Rail | Measure at | Expect |
|---|---|---|
| +5 V | + leg of C107, or SID (U5) pin 25 | +5 V |
| +12 V | + leg of C111, or SID pin 28 | +12 V |
| Unregulated | Anode of CR10 | About +18 V |
| 9 V AC | AC inputs of bridge CR13 | 9 V AC on both |
Carlsen recommends checking the solder on the power connector and the power switch for intermittent faults.[9]
Sockets
[edit | edit source]Most of the large chips are socketed. Carlsen's first step for any fault, especially an intermittent one, is to reseat them: lift each slightly at each end without removing it, and press it back down.[9] Adam's Vintage Computer Restorations found one board fitted with "very poor-quality sockets at the factory" and corroded, and replaced the sockets for the CIA, the ROMs, the MMU and the SID.[10] On another machine the 8563 VDC was found riding higher in its socket than the VIC beside it.[11]
Heat
[edit | edit source]Wikipedia's article on the 8563 says the chip "tended to self-destruct from overheating".[12] Owners deal with the heat in different ways: one restorer left the RF shields off and fitted heatsinks to the ICs that run warm,[13] and another renewed the thermal grease between the chips and the RF shield.[11]
Keyboard
[edit | edit source]Shift Lock, Caps Lock and the 40/80 Display key are mechanical latching switches.[14] A 40/80 key left down makes the machine start in 80-column mode, which leaves a 40-column monitor blank; Tynemouth wonders how many C128s are thought broken for that reason.[15] The key's switch can also fail itself.[16] The machine starts normally with the keyboard unplugged, which is a quick way to rule out a stuck key.[9]
Diagnostics
[edit | edit source]Commodore's C128 diagnostic kit (assembly 314060) is a cartridge with a keyboard connector board, a serial port connector, and the user port adapter and harness shared with the C64 kit; its manual is hosted at Commodore 128 Diagnostic Instruction and Troubleshooting Manual (314060).[8] World of Jani lists later C128 diagnostic part numbers, including the 785260 cartridge with keyboard adapters 314061-07 for the C128 and 314987-01 for the C128D, and notes that the C64 Dead Test cartridge finds faulty low memory on a C128.[17]
Related pages
[edit | edit source]- Commodore 128
- Commodore 128 Troubleshooting Guide
- Commodore 128 Capacitor Replacement Guide
- Commodore 128 128D Service Manual (314001-08)
- Commodore Test Cartridge Index
- Recommended Tools
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 1.4 Ray Carlsen, C128 power supply repairs, updated 7 April 2012—link(accessed 2026-10-01)
- ↑ 2.0 2.1 Tynemouth Software, Commodore 128 repair, part 1: PSU, October 2014—link(accessed 2026-10-01)
- ↑ Commodore 128—link(accessed 2026-10-01)
- ↑ 4.0 4.1 4.2 C128 PSU schematic (PAL) P/N 310416-01, -02, -03—link(accessed 2026-10-01)
- ↑ Muldrf's Hobbytronic, Commodore 128 power supply recap, 23 February 2019—link(accessed 2026-10-01)
- ↑ 6.0 6.1 6.2 Commodore Business Machines, Service Manual C-128/C128D Computer, November 1987, PN-314001-08, hosted at Commodore 128 128D Service Manual (314001-08). Power supply schematics, printed pp. 78-79; parts lists for 310379 rev. 6 and rev. 7.
- ↑ Individual Computers, Powersupply FAQ—link(accessed 2026-10-01)
- ↑ 8.0 8.1 8.2 Commodore Business Machines, C128 Diagnostic Instruction and Troubleshooting Manual, Assembly #314060-01/02 (1986), section 2.2, hosted at Commodore 128 Diagnostic Instruction and Troubleshooting Manual (314060).
- ↑ 9.0 9.1 9.2 Ray Carlsen, C128 chips and common symptoms, updated 8 August 2013—link(accessed 2026-10-01)
- ↑ Adam's Vintage Computer Restorations, Commodore C128 board repairs, 18 May 2021—link(accessed 2026-10-01)
- ↑ 11.0 11.1 Rubenerd, Troubleshooting my Commodore 128's 80-column mode, part one—link(accessed 2026-10-01)
- ↑ MOS Technology 8563—link(accessed 2026-10-01)
- ↑ Muldrf's Hobbytronic, Flat Commodore 128 maintenance and 64k VDC RAM upgrade, 29 March 2023—link(accessed 2026-10-01)
- ↑ Tynemouth Software, Commodore 128 repair, part 2: keyboard, October 2014—link(accessed 2026-10-01)
- ↑ Tynemouth Software, Commodore 128 keyboard repairs, December 2018—link(accessed 2026-10-01)
- ↑ Commodore 128 80 column not working—link(accessed 2026-10-01)
- ↑ Diagnostic carts and manuals—link(accessed 2026-10-01)