Commodore 64C General Maintenance

Commodore 64C short board, assembly 250469.

The Commodore 64C short board is the simplest C64 mainboard Commodore built: two DRAMs, one combined ROM, a memory controller in place of most of the glue logic, and no 12 V regulator. It is also the revision most often described wrongly, because maintenance notes written for the long board get copied onto it. This page covers routine care specific to the 250469 E board.

The electrical figures here are taken from Commodore's own C64 / C64C Service Manual (March 1992, PN-314001-03) — the parts list for PCB assembly 250469-01 and schematic 252312.

For fault-finding see Commodore 64C Troubleshooting Guide; for the machine itself see Commodore 64C; for the long boards see Commodore 64 General Maintenance.

Confirm you have a short board

Commodore's identifying factor for the E board is that the KERNAL and BASIC ROMs are combined into U4 and there is a new IC memory controller at U8. The layout page records PCB assy no. 250469, PCB no. 252311 Rev. 3.

If your board has eight RAM chips and separate BASIC and KERNAL ROMs, it is a long board and the wrong page is open.

Power

The short board takes the same 7-pin DIN supply as the bread bin: regulated +5 V DC and 9 V AC.

Power input CN7 on the E board

Pin	Function
1, 2, 3	Ground
4, 5	+5 V DC, regulated, from the brick — switched by SW1, filtered by C58 (100 µF 16 V)
6, 7	9 V AC — through fuse F1 (1.5 A) and line filter L1 to bridge CR2

The E board generates no +12 V rail. The 9 V AC is rectified by CR2 into C63 to make the 9 V unregulated rail, which feeds the SID supply, the cassette motor drive and the audio amplifier Q3.

The SID supply is a zener, not a regulator

On the E board the SID's Vdd comes from the 9 V unregulated rail through pass transistor Q5 and zener CR7. Commodore's note on schematic 252312 gives the zener by SID type:

SID fitted	CR7	Filter capacitors C70, C71	Filter capacitors C80, C81	R8
6581	12.6 V	470 pF	1800 pF	Fitted
8580	9.6 V	22000 pF	2200 pF	Removed

This is the thing to get right before swapping a SID. Fitting an 8580 to a board built for a 6581 puts 12.6 V on a 9 V part.

Supply checks

Measure the brick's +5 V output before plugging it into the machine. There is no regulator on the board between it and every chip. The original supplies are potted and fail without warning. See Commodore 64 Power-Supply Protector (C64 Saver).

Go/no-go checks

Signal	Where	Expected
+5 V	8500 CPU (U6) pin 6 (VCC). Pin 21 is GND	4.9 – 5.1 V
RESET	8500 pin 40	Low at power-up, then +5 V
Φ0 in	8500 pin 1	1.023 MHz NTSC / 0.985 MHz PAL
Φ2 out	8500 pin 39	Same frequency
VIC-II supply	VIC-II (U7) pin 40 (VCC) and pin 13 (VDD)	+5 V; the HMOS VIC-II needs no 12 V
SID supply	SID (U9) pin 28 (VDD)	9 V with an 8580, 12 V with a 6581
SID audio out	SID pin 27; audio in is pin 26	Signal present when a voice is running
+5 V (user port)	CN2 pin 2; ground on pins 1 and 12	4.9 – 5.1 V
9 V AC	CN2 pins 10 to 11	9 – 11 V rms

Pin 40 of the 8500 is RESET and pin 6 is Vcc. Earlier versions of this page told readers to measure +5 V at pin 40 and the clock at pin 6; both are wrong, and pin 3 is IRQ, not reset.

Chip positions

250469-01 integrated circuits

Position	Part	Commodore part no.
U6	8500 CPU	318012-01
U7	VIC-II, 8567 or 8562	318014-01 / 318027-01
U9	8580 SID (or 6581 on some builds)	318013-01 / 906112-01
U1, U2	6526 CIA	906108-01
U8	Memory controller, the "Super PLA"	251715-01
U4	Combined BASIC + 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, 7406/7416, 74LS14, 4066

Two corrections worth making explicitly, because both appeared on this page before:

• The colour RAM is a separate 2114 at U19. It is not inside the memory controller.
• The character ROM is U5. U3 is a 74LS08.

Capacitors

There are no SMD tantalums on the 250469 board. Commodore's parts list is entirely through-hole: axial and radial aluminium electrolytics, ceramics and one trimmer.

The reservoir is C63 (1000 µF 16 V, or 2200 µF on the France build), and the other electrolytics are C66, C43, C65, C58, C36, C44, C77 and C59. C88 and C90 on this board are ceramics, not electrolytics — 220 pF and 470 pF respectively — so a note telling you to replace "C88 in the modulator" is describing a long board.

See Commodore 64C Capacitor Replacement Guide for the full list with Commodore part numbers. It is not repeated here, so that there is only one place on this wiki to correct.

See Capacitor Failure Symptoms for recognising a failed part.

Regular cleaning

Case and keyboard

• Wipe the lighter C64C plastics with a damp microfibre cloth and a drop of mild detergent. Avoid solvents; the C64C's finish marks more easily than the bread bin's.
• Key caps pull vertically. Clean the plungers and stems with isopropyl alcohol.
• For yellowing see Retrobrite, and weigh the cosmetic gain against the embrittlement.
• Lubricate the space-bar stabiliser wire with a plastic-safe grease if it sticks.

Internal

• Disconnect the supply and let the board sit before opening it.
• Blow dust out with dry air; brush with a soft anti-static brush.
• Wipe stubborn grime with isopropyl alcohol on a lint-free cloth, without flooding, and let it dry fully before reassembly.

Edge connectors and sockets

• Clean the cartridge, user and expansion connectors with isopropyl alcohol. Do not use an abrasive eraser — the gold plating is thin.
• Reseat the socketed ICs annually to wipe the contacts. On the E board the sockets Commodore fits are 16, 28 and 40 pin.
• Inspect for bent contacts on the cartridge and cassette ports.

Common faults on the short board

Component	Symptom	First check
Power supply	Anything from a dead machine to random crashes to dead chips	Measure the brick's +5 V output on its own, before connecting it
Memory controller (U8)	Black screen, no boot, random crashes	Feel for heat; substitute a known-good part
SID (U9)	No sound, distortion	Check Vdd at pin 28 against the CR7 table above, then the signal at pin 27
VIC-II (U7)	No video, wrong colours, vertical bars	Check +5 V at pins 40 and 13; feel for heat
CIA (U1, U2)	Dead keyboard, joystick or IEC	Swap U1 and U2; if the fault follows the chip you have found it
DRAM (U10, U11)	Random characters, memory errors	Dead-test cartridge; see Commodore Test Cartridge Index
Colour RAM (U19)	Correct characters, wrong colours
Character ROM (U5)	Correct layout, corrupted glyphs	Check the socket before the chip
8701 clock generator (U20)	No clock, no display	Check for activity at U20 and at 8500 pin 1

Cooling and ventilation

The short board runs cooler than the long boards — there is no 12 V regulator dissipating on it — but the VIC-II, the SID and the memory controller still get warm. Small self-adhesive heatsinks are cheap insurance. Keep the top vents clear.

Preventive checklist

1. Test the power supply output before every session with an original brick; replace or protect it if it is out of spec.
2. Inspect and reflow the solder joints around the power input and the switch.
3. Reseat the socketed chips annually.
4. Clean the cartridge and user-port connectors.
5. Check the keyboard ribbon and membrane for cracked traces or corrosion; conductive ink repairs broken traces.
6. Replace the electrolytics if the machine shows video noise or instability once warm — after clearing the supply.

Tools

• 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
• Conductive trace pen for keyboard repairs
• A diagnostic cartridge — see Commodore Test Cartridge Index

See also Recommended Tools.

Related pages

• 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 — the source for the figures on this page
• Commodore 64 Power-Supply Protector (C64 Saver) · Capacitor Failure Symptoms · Recommended Tools · Retrobrite