Commodore 64 Chips and Common Symptoms
I've been repairing computers for a few years now. Here is a list of the more common chip failures I have seen:
U1 906108-02 (6526) VIA
Screen normal, but no cursor. No keyboard or joyport access. Partial failure: some keys or joystick positions don't work. Cartridge works.
U2 906108-02 (6526) VIA
Screen normal. No drive or user port access. Cartridge works. Characters sometimes will 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. Cartridge works.
U5 901225-01 CHAR ROM
Blank screen w/ border. Garbage where startup page info should be. Cartridge works.
U7 906107-01 (6510) MPU
Blank screen, no border.
U8 SN7406N LOGIC CHIP
Drive access problems: drive resets, but "device not present" error.
U9 THRU U12 & U22 THRU U24 8 RAM CHIPS (4164)
Blank screen, no border. Shorted chips will get warmer than the other RAM chips. Will sometimes produce garbage screen, abnormal number of bytes free or "out of memory" error on startup screen.
U17 906114-01 (82S100PLA) PLA
Blank screen, no border. Sometimes will produce colored screen or flashing garbage on screen... sometimes after warmup. This chip normally runs hot! Most common chip to fail.
U18 906112-01 (6581) SID
Normal screen. No sound or garbled sound. Mouse or graphics tablet pointer stuck or jitters. If shorted, can cause garbage screen. NOTE: computer will work without a SID plugged in. This chip normally runs hot!
U19 906109-04 (6567) VIC
Blank white screen, 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.
U31 8701 MASTER OSC.
Some C64s use 74LS629N logic chip. Blank white screen, no border.
POWER PACK: can produce many problems like blank screen (red LED on or off), program lock-up, garbage screen, hum bars moving on screen, hum in audio, damaged RAM chips, intermittant operation after warmup, etc.
Questions or comments are welcome!
Ray Carlsen
Carlsen Electronics
email: rrcc@u.washington edu or rcarlsen@i-link-2.net
