Commodore 64C General Maintenance
The Commodore 64C was built on two mainboards. Commodore's 1992 service manual identifies them as the B-3 (PCB assembly 250466, schematic 252278) and the E (PCB assembly 250469, schematic 252312).[1] The E board is the short board: two DRAMs, one combined BASIC and KERNAL ROM, a memory controller in place of the PLA and most of the glue logic, and no on-board regulator. The B-3 keeps the separate ROMs, the 82S100 PLA and the 7812 and 7805 regulators of the long boards, so its routine care follows Commodore 64 General Maintenance.[2]
For fault-finding see Commodore 64C Troubleshooting Guide; for the machine itself see Commodore 64C.
Confirm you have a short board
[edit | edit source]Commodore's identifying factors for the E board are the KERNAL and BASIC ROMs combined into U4 and a new memory controller at U8. The layout page records PCB assy no. 250469, PCB no. 252311 Rev. 3.[1] The B-3 also has only two RAM chips (U9 and U10), but it has separate BASIC (U3) and KERNAL (U4) ROMs and the PLA at U17.[2] On a B-3, use Commodore 64 General Maintenance.
Power
[edit | edit source]The short board takes the same 7-pin DIN supply as the earlier C64: regulated +5 V DC and 9 V AC.
| Pin | Function |
|---|---|
| 1, 2, 3 | Ground |
| 4, 5 | +5 V DC, regulated in the external supply; switched by SW1 and filtered by C58 (100 µF 16 V) |
| 6, 7 | 9 V AC, through fuse F1 (1.5 A) and line filter L1 to bridge rectifier CR2 |
The E board generates no +12 V rail. CR2 rectifies the 9 V AC into C63 to make the 9 V unregulated rail, which feeds the cassette motor drive (Q1). C65, CR3 and CR4 build a higher supply from the 9 V AC on top of that rail and store it on C66 (330 µF 35 V on the schematic, 470 µF 35 V in the parts list). Q5 regulates it to the "+9/+12" supply for the SID and audio transistor Q3.[1]
SID supply
[edit | edit source]On the E board the SID's VDD (pin 28) comes from the higher supply on C66 through pass transistor Q5, with zener CR7 setting the voltage. Commodore's notes on schematic 252312 give the parts that change with the SID fitted:[1]
| SID fitted | CR7 | C70, C71 | C80, C81 | R8 |
|---|---|---|---|---|
| 6581 | 12.6 V | 470 pF | 1800 pF | Fitted |
| 8580 | 9.6 V | 22000 pF | 2200 pF | Removed |
Check CR7 before swapping a SID. A board built for a 6581 supplies about 12 V, and the 8580 is a 9 V part. Commodore's spare parts list for the C64GS version of the board gives the 8580 zener as an RD10ESB2 with Vz 9.48 to 9.90 V at 5 mA.[3]
Supply checks
[edit | edit source]Measure the external supply's +5 V output before connecting it. Nothing on the board regulates that rail. See Commodore 64 Power-Supply Protector (C64 Saver) for a protector that sits between the two.
| Signal | Where | Expected |
|---|---|---|
| +5 V | 8500 CPU (U6) pin 6 (VCC); pin 21 is ground | 4.95 to 5.10 V; a supply reading above 5.2 V is failing[4] |
| RESET | 8500 pin 40 | Low at power-up, then +5 V |
| Φ0 in | 8500 pin 1 | 1.023 MHz NTSC, 0.985 MHz PAL (the 8.18 / 7.88 MHz dot clock divided by eight) |
| Φ2 out | 8500 pin 39 | Same frequency |
| VIC-II supply | VIC-II (U7) pin 40 (VCC) and pin 13 (VDD) | +5 V on both; the HMOS VIC-II needs no 12 V |
| SID supply | SID (U9) pin 28 (VDD) | About 9 V with an 8580, about 12 V with a 6581 |
| SID audio | SID pin 27 out, pin 26 external in | Signal present when a voice is playing |
| +5 V at the user port | CN2 pin 2; ground on pins 1 and 12 | As at U6 pin 6 |
| 9 V AC | CN7 pins 6 and 7, or user port CN2 pins 10 and 11 | 9 to 12 V AC[5] |
Pin 40 of the 8500 is RESET and pin 6 is VCC. Pin 3 is IRQ.[1]
Chip positions
[edit | edit source]| Position | Part | Commodore part no. |
|---|---|---|
| U6 | 8500 CPU | 318012-01 |
| U7 | VIC-II, 8562 (NTSC) or 8565 (PAL)[6] | 318027-01, 318014-01; the 1992 manual and the C64GS list disagree on which is which[3] |
| U9 | 8580 SID, or 6581 | 318013-01 / 906112-01 |
| U1, U2 | 6526 CIA | 906108-01 |
| U8 | Memory controller | 251715-01 |
| U4 | Combined BASIC and KERNAL ROM (23128B) | 251913-01 |
| U5 | Character ROM (2332C) | 901225-01 |
| U10, U11 | DRAM, 64K × 4 (50464-150) | 390083-07 |
| U19 | Colour RAM, 2114 | 901453-07 / -09 |
| U20 | 8701 clock generator | 251527-08 / -09 |
| U3, U22, U23, U18, U21 | 74LS08, 7406 or 7416, 74LS14, 4066 |
The colour RAM is a separate 2114 at U19, not part of the memory controller. The character ROM is U5; U3 is a 74LS08.[1]
Capacitors
[edit | edit source]Every capacitor in Commodore's 250469-01 parts list is a through-hole part: aluminium electrolytics, ceramics and one trimmer (CT-1). There are no tantalums.[1]
The reservoir is C63 (1000 µF 16 V, or 2200 µF on the French build, which also fits a 20 Ω resistor at R2). The other electrolytics are C66, C43, C65, C58, C36, C44, C77 and C59. On this board C88 and C90 are ceramics (220 pF and 470 pF). On the long boards the same designators are the 1000 µF and 470 µF electrolytics, so a recap list written for a long board does not fit here.[1]
Commodore 64C Capacitor Replacement Guide has the full list with values and Commodore part numbers. See Capacitor Failure Symptoms for recognising a failed part.
Regular cleaning
[edit | edit source]Case and keyboard
[edit | edit source]- Wipe the case with a damp microfibre cloth and a drop of mild detergent. Avoid solvents.
- Key caps pull straight up. Clean the stems with isopropyl alcohol.
- For yellowed plastics see Retrobrite, and weigh the cosmetic gain against the risk of making the plastic brittle.
Internal
[edit | edit source]- Disconnect the supply before opening the case.
- Blow dust out with dry air and brush with a soft anti-static brush.
- Wipe stubborn grime with isopropyl alcohol on a lint-free cloth without flooding the board, and let it dry fully before reassembly.
Edge connectors and sockets
[edit | edit source]- Clean the cartridge port, user port and cassette edge connectors with isopropyl alcohol. Avoid abrasive erasers, which wear through thin gold plating.
- Commodore's C64GS spare parts list for the same board (250469-09/-10, Rev. B) puts the sockets on U20 (16-pin), U4 and U9 (28-pin) and U7 (40-pin). Reseating a socketed chip wipes its contacts.[3]
- Look for bent contacts in the cartridge port.
Common faults
[edit | edit source]The supply is the first suspect for almost any fault, from a dead machine to random crashes, because the board has no regulation of its own on the +5 V rail. Measure it on its own before suspecting a chip. The CR7 zener explains most SID supply problems after a SID swap. Symptoms and checks for the E board's chips are on Commodore 64C Troubleshooting Guide, and the diagnostic cartridges are listed on Commodore Test Cartridge Index.
Preventive checklist
[edit | edit source]- Measure the supply's +5 V output before connecting it to the machine, and replace or protect a supply that is out of specification.
- Inspect the solder joints at the power socket, the power switch and F1.
- Reseat the socketed chips.
- Clean the cartridge and user-port connectors.
- Check the keyboard ribbon for cracked traces or corrosion.
- Replace electrolytics only after the supply has been cleared.
Tools
[edit | edit source]- Anti-static strap and mat; digital multimeter
- Fine-tipped soldering iron, flux, braid, desoldering pump
- Isopropyl alcohol (99 %), lint-free cloths, compressed air, soft brush
- Contact cleaner suitable for edge connectors
- A diagnostic cartridge; see Commodore Test Cartridge Index
See also Recommended Tools.
Related pages
[edit | edit source]- Commodore 64C · Commodore 64C Troubleshooting Guide · Commodore 64C Capacitor Replacement Guide
- Commodore 64 · Commodore 64 General Maintenance · Commodore 64 Capacitor Replacement Guide
- Commodore 64 / 64C Service Manual
- Commodore 64 Power-Supply Protector (C64 Saver) · Capacitor Failure Symptoms · Recommended Tools · Retrobrite
References
[edit | edit source]- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 Commodore Business Machines, C64 / C64C Service Manual, March 1992, PN-314001-03, hosted at Commodore 64 / 64C Service Manual. C64C board identification (p. 17), parts list for PCB assembly 250469-01 C64-E (p. 40), E board layout (p. 41) and schematic 252312 (p. 42).
- ↑ 2.0 2.1 Commodore, C64 / C64C Service Manual (PN-314001-03), parts list for PCB assembly 250466-01 C64-B3, p. 34: U3 BASIC ROM, U4 KERNAL ROM, U17 PLA 82S100, U9 and U10 64K × 4 DRAM, VR1 MC7812CT, VR2 MC7805CT.
- ↑ 3.0 3.1 3.2 Commodore Electronics Limited, C64GS Tech Data, PN-400401-01, November 1990, pp. 4-5, Commodore International Spare Parts List for PCB assemblies 250469-09 and -10, Rev. B. Hosted at Commodore 64GS Technical Data (400401-01, 1990 Nov).
- ↑ Power Supply—link(accessed 2026-10-01) Section "Failure": 4.95-5.1 V normal, 5.2-5.5 V failing, above 6 V the level at which most supply-killed C64s died, usually with RAM damaged first.
- ↑ Commodore, C64 Diagnostic Instruction and Troubleshooting Manual, assembly 326070-01, 1992, section 2, "System Power Supply", p. 2-3: AC in on CN7 pins 6 and 7, "9 to 12 VAC". Written for the long boards; the CN7 pinout is the same on the E board. Hosted as File:C64_Diagnostic_Instruction_and_Troubleshooting_Manual_(326070-01).pdf.
- ↑ VIC—link(accessed 2026-10-01) Lists the 8562 (NTSC) and 8565 (PAL-B) as the C64's HMOS VIC-II parts.