C64 Chips and Common Symptoms

Mirrored from Ray Carlsen’s Archive


latest additions and corrections: 1-07-2020

There were at least five different motherboards made for the C64.

This composite list includes chips used in all board versions except the
latest C64C 250469 short board. See c64c-ic.txt for that information.

Not all chips are found in all board versions.

U1 906108-02 (6526) CIA
Startup screen normal, but no cursor. No keyboard or control port
access but cartridge works. Partial failure: some keys or joystick
positions don’t work, one character appears ahead of startup cursor or
random characters appear at startup. Blank screen if chip is shorted
(remove to check) and chip may get hot to the touch.

U2 906108-02 (6526) CIA
Startup screen normal. No serial or user port access. “File not
found” error when drive accessed. Cartridge works. Characters may show
as blocks or garbage screen is produced. Shorted chip will show blank

U3 901226-01 BASIC ROM
Blank screen w/ border. Cartridge works.

U4 901227-03 (early -02) KERNAL ROM
Blank screen, no border. Most cartridges don’t work but a few game
carts (example: CBM Kickman and Jupiter Lander) will work with a normal
screen because they bypass the Kernal ROM.

U5 901225-01 CHARACTER ROM
Normal startup border, but “garbage” characters where startup page
characters should be. Cartridge works.

Startup screen shows flickering characters with shimmering colors.
Check also PLA U17.

U7 906107-01 (6510) MPU
Blank screen, no border. Cartridge doesn’t work.

Blank screen. Partial failure: drive access problems: drive resets
but “device not present” error.

U9 THRU U12 & U21 THRU U24 8 RAM CHIPS (generic 4164 DRAM)
note: only 2 4464 RAM chips in later C64C boards
Blank screen, no border. Shorted chips will get warmer than the
other RAM chips. Partial failure: will sometimes produce “garbage”
screen, abnormal number of bytes free or “out of memory” error on
startup screen.

U13 74LS257APC or A or AN but not PC (alt MOS7708) LOGIC
Blank screen, less than 38911 bytes free at startup or “garbage”
screen. See U25 Note: MOS77xx series chips have a high failure rate.

U14 74LS258 LOGIC
Blank screen.

U15 74LS139 LOGIC
Blank screen

U16 CD4066 (generic CMOS quad bilateral switch)
Color problems such as random color “checkerboard” pattern on screen
or no color.

U17 906114-01 (82S100PLA) PLA
Blank screen, no border. It can produce colored screen or flashing
color garbage instead of startup screen. It can cause intermittant loss of
cursor, screen freeze and/or program crashes after warmup and it can put
random characters on screen. This chip normally runs hot. It is the most
common chip to fail in the C64 because it runs hot normally, like the SID.

U18 906112-01 (6581) SID Note: 8580 SID used in latest C64C
Normal screen. No sound or garbled sound. Mouse or graphics tablet
pointer stuck or jitters. If shorted, can cause blank or “garbage” screen.
NOTE: computer will work without a SID plugged in (unplug to check). This
chip normally runs hot.

U19 906109-04 (6567) VIC (PAL version: 6569)
Blank screen (may be light or dark) with no border. Sometimes will
produce garbage or “checkerboard” screen, or screen that lacks contrast.
If screen is blank or garbled from bad VIC, “blind” disk commands from
keyboard may still work.

Blank screen. Computer will not reset, or RESTORE doesn’t work.

U25 74LS257APC or AN or A but not PC (alt MOS7708) LOGIC
Blank screen, “garbage” screen or less than 38911 bytes at startup.
See U13 Note: MOS77xx series chips have a high failure rate.

U26 74LS373 LOGIC
Startup screen has normal border, but characters are scrambled.

U27 74LS08 (or MOS7712) LOGIC
Normal startup border, but screen full (or nearly) of “garbage”

U28 CD4066 (generic CMOS quad bilateral switch)
Normal startup screen. Proportional mouse or graphics tablet
doesn’t work in one or both control (joystick) ports.

Startup screen shows normal characters in multicolored “rainbow”.


U31 74LS629N (w/ support logic) or 7701/8701 MASTER OSC.
Blank white screen, no border. Normal “whine” in nearby AM radio
will be missing if the oscillator is not running. Check also VIC U19

No monitor picture, just diagonal lines on screen.

Can produce many problems like blank screen (red LED on, dim, or off),
program lock-up, “garbage” screen, hum bars moving on screen, hum in audio,
several bad RAM or other chips, intermittant operation after warmup, etc.

Ray Carlsen CET

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