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C64 CHIPS and COMMON SYMPTOMS
latest additions and corrections: 12-18-2012
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 on startup screen.
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.
U6 2114 SRAM (COLOR RAM)
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.
U8 7406 LOGIC CHIP
Blank screen. Partial failure: drive access problems: drive resets
but "device not present" error.
U9 THRU U12 & U22 THRU U24 8 RAM CHIPS (4164)
note: only 2 4464 RAM chips in C64C
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
U13 74LS257APC or A or AN but not PC (alt MOS7708) LOGIC
Blank screen, less than 38911 bytes free at startup or "garbage"
screen. Don't use NS or TI chip as replacement... it will not work.
U14 74LS258 LOGIC
U15 74LS139 LOGIC
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 and it should run no hotter than the SID.
U18 906112-01 (6581) SID
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 is 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.
U20 556 LOGIC, TIMER
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.
Don't use NS or TI chip as replacement... it will not work.
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.
U29 74LS74 LOGIC, DIVIDER
Startup screen shows normal characters in multicolored "rainbow".
U31 8701 MASTER OSC. (Some C64s use 74LS629N w/ support logic)
Blank white screen, no border. Normal "whine" in nearby AM radio
will be missing if the oscillator is not running. Check also VIC U19
U32 4044 CMOS LOGIC
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