latest updates or corrections 1-31-06.

U1        906108-02 (6526) VIA (interchanges with 8521)
          # unknown (8521) VIA later version C128 
     C64 startup screen normal, but no cursor. May not start up in 128
mode. No keyboard, joyport, or cassette access. Partial failure: some keys
or joystick positions don't work. Cartridge works. Swap U1 with U4 to
check. See KEYBOARD. 

U4        901108-02 (6526) VIA 
     Startup screens normal (64 & 128), but no serial or user port access. 
Drive resets on power up but "File not found" error when drive accessed.
Cartridge works. Swap U4 with U1 to check. Check also buffer IC U30.

U5        906112-02 (6581) early 128 SID: 12 volt VDD
          318013-01 (8580) late 128 SID: 9 volt VDD 
     No sound (check also transistor Q2), weak or bad sound of one or more
voices. Proportional mouse or graphics tablet doesn't work, or mouse
pointer frozen or jitters. If chip is shorted, will produce blank screen.
Remove chip to check. 

U6        315020-01 (8502) MPU (Microprocessor) 
     No bootup in any mode. Blank screen with no border. Cartridge don't
work. If intermittant, screen freezes after warmup or running programs
lock up. 

U7        310389-01 (8722) MMU (Memory Management Unit) 
     No bootup in any mode. Blank screen with no border. Partial failure: 
CP/M inoperative. 
U10       906150-02 (Z80B) CPU (Microprocessor)
     No bootup in any mode. Blank screen with no border. Partial failure:
CP/M inoperative.

U11       315012-01 (8721) PLA (Programable Logic Array) 
     No bootup in any mode. In 40 columns: blank screen, white screen,
light blue screen (no border), random checkerboard pattern or flashing
"garbage" characters. 

U14 & 15  74LS257  logic, RAM interface
     Can cause blank screen. Partial failure can cause abnormal bytes 
free at startup.

U18       390059-01 CHARACTER ROM
     No characters. Startup has border, but "garbage" where characters
belong. Entire area inside border just lines or blocks. 

U19       generic SRAM (6116 - LC3517A) COLOR RAM 2K X 8
     128 startup screen and colors normal. C64 screen characters & colors
flash rapidly. 

U21       315009-03 (8564) VIC (40 column screen) 
     Dead chip: Blank white screen in 40 columns and black screen in 80
columns. Partial failure: 80 column OK but weak or garbled 40 column
video, no video or no color. Check also U28 and crystal Y2.

U22       315014-09 (8563) VDC for C128 80 column screen.
          315092-01 (8568) VDC for C128D 80 column screen. 
     40 column screen OK. 80 column screen: Blank screen. Entire screen
shifts horizontally. May not switch from 128 to 64 mode with reset switch.
Check also crystal oscillator Y1. If one or more colors missing, check

U23 & U25    generic 4416 Video RAM (16K)
             generic 4464 Video RAM (64K VRAM upgrade) 
     In 80 column mode only: blank screen or some characters garbled.
Shorted chips may get hot. 

U24       generic 74LS244 Buffer 
One color (such as red) missing from 80 column screen. Check also U22.

U28       251527-09 (8701) Clock gen. (Master oscillator) 
     Dead chip: no bootup in any mode. Partial failure: diagonal lines on
monitor or TV (horizontal sync bad), no color in 40 column mode, or
"barber pole" colors. Check also crystal Y2. 

U32       251913-01  C64 Kernal/Basic ROM.
     Blank screen at startup in C64 mode only. Cartridge works.

U33       318018-02  C128 BASIC LO ROM
     No BASIC startup screen in 128 mode only. Computer goes into
"monitor" mode. 

U34       318019-02  C128 BASIC HI ROM
     Blank screen with border at 128 mode startup. Lines or "garbage"
inside border. 

U35       318020-03  C128 Kernal ROM
     Comes up in C64 mode, but cartridge doesn't work. Partial failure:
blank screen in 128 mode only. 

U36       socket empty from factory... for "upgrades".

U38 THRU U53   16 RAM chips (4164)
                4 RAM chips (4464) in later versions. 
     Blank screen, no border. Shorted chips may get hot. Will sometimes
produce "garbage" screen, abnormal number of bytes free (check also U14 
and U15) or "out of memory" error on startup screen, and program crashes 
when loading (depends on what part of memory is bad).

Y1       16 MHz Crystal Oscillator
     Blank screen in 80 column only.

Y2       14.31818 MHz crystal (Master osc. reference)
     See U21 VIC.

M1       251917-02 RF Modulator (tin can "module"). 
     TV output only: no picture, no sound, snowy picture, no color, weak
video or sound, interference, etc. Note: direct A/V outputs still work

     Stuck (shorted) keys will show up when another key is pressed... it
either will not work, or both appear on screen. Character may repeat,
depending on which key is bad. If keyboard is suspected, unplug it...
the computer will start up normally without it. Check also U1 CIA chip.

     Most common problem is "dead" computer, but can produce many symptoms
like blank screen (power LED on or off), screen freeze-up, "garbage
screen", hum bars moving up screen, intermittant operation, etc. 

     For intermittant operation or "glitches" on screen, random
characters, etc. check solder of power supply connector and power switch
on motherboard. Also, reseat socketed chips: lift up slightly at each end
(but do not remove) and press back down. Reseat suspected chips first for
any problem, especially intermittants. 

     For in-depth troubleshooting, Sams Photofact (schematic and parts
list) has flow charts and step by step diagnostics. It requires an
oscilloscope and the necessary skills to interpret the readings. The Sams
book for the C128 is CC18 and CSCS25 for the 128DCR. They are available at
electronic parts distributors. Copies may be viewed at the main branch of
the Public Library. 

Ray Carlsen CET 

Your questions or comments are always welcome, especially if you spot any 
errors here. Thanks!