C64 Diagnose: The Commodore Diagnostician by Ian Perry

The Project 64 etext for Commodore diagnostics, converted by The
Basic Bombardier. This etext was created by concatenating several
documents into one. Please see the header of the particular document
for original sources.

The Commodore Diagnostician, by Ian Perry.
Ray’s C-64 Problems Solved, by Ray Carlson.
Various usenet articles about 1541 repair.

C-DIAG10.TXT, June 1996, etext #45.

The Project 64 etext of The Commodore Diagnostician. Tables were
converted to list format.


A symptomatic chart for diagnosing faulty IC
components on Commodore Computers and peripherals


   PO Box 48
   Victoria 3444

COMMODORE C-64 Reference Aid #4


  • Cursor jumps back to home position. 6510
  • Abnormal colours appear in the letters. PLA
  • Different characters are displayed and cursor is locked when turned

on and off. 6510, 6567

  • System does not reset and the ‚RESTORE‘ key does not work. 6510,

PLA, ROM 227

  • Cursor disappears after the system warms up. 6526
  • SYNTAX ERROR displayed after system warms up. ROM 226
  • The system resets when it warms up and long programs do not load.

6510, ROM 226

  • Keyboard does not operate correctly when the system warms up.6526,

ROM 226

  • Cassette motor keeps running. 6510
  • The cursor disappears when the cassette is plugged in. 6510
  • Cassette runs extremely slow. The program seems to load but will

not run. 6510

  • Blank screen on power up. 6510, 6567, PLA, 8701 [, RAM ]
  • No cursor displayed. Intermittent blank screen. Intermittent

graphics. 6510

  • Powers up with ‚PRESS PLAY ON TAPE‘ message and the display blanks.


  • On power up the cursor locks up. 6510
  • When ‚RETURN‘ is pressed after a run command, the cursor goes back

to home position. ROM 226

  • Poke command does not work. ROM 226
  • Joystick does not operate correctly. 6526
  • No character lettering is displayed on the screen. 6526, 6567,

PLA, ROM 225

  • Graphic characters instead of letters displayed. 6526, 6567
  • Power up message appears but no cursor. 6526, ROM 226
  • DEVICE NOT PRESENT ERROR is displayed when disk is used. 6510,


  • Disk drives continue to search when trying to load. 6526
  • Incorrect screen colours or no colour at all. 6567, PLA
  • Keyboard does not function. 6526
  • Cassette recorder problem. 6510, 6526
  • Flashing colour or blocks. PLA
  • Game Cartridge does not function. PLA, ROM 227
  • Unit dead (30-40% possibility PLA). PLA
  • User port does not function (eg. modem). 6526
  • No sound. 6581 [, Bad RF Modulator ]
  • Missing notes. 6581
  • Game paddles do not function. 6526, 6581
  • White band scrolls down the screen (60 HZ HUM). Check power supply

and VR2 regulator.

  • Wavy screen after the system warms up. Check external power supply

and 6567 chip.

  • Black band scrolls through screen when system warms up. Check

external power supply and VR2 regulator.

  • Out of memory error on power up. Check Rams 4146 U1-U12,

U21-U24. [ May be Hot! ]

  • Powers up with graphic display and blinking cursor. Check U14.
  • Powers up with all the characters displayed as blocks. Check U26.
  • Unit completely dead. Possible power supply failure.
  • After a few minutes, characters are all over the screen, then the

unit locks up. Possible power supply problem.

Note-PLA chip is an 82S100, ROM chips are 901226-01 (Basic),
901227-01 (Kernal), 901225-01 (Character)



  • Error LED stays on all the time. 6501, ROM 32530201, ROM 90122905
  • Drive motor runs continuously and red LED stays on. 6522 [, LED

Flashes: UC7 ]

  • Drive motor runs continuously and red LED stays off. 6502, ROM


  • After the drive warms up the motor runs continuously. 6522
  • Does not load when hot or LED flashed 3 times. ROM 90122905, LM311
  • Searches with LED flashing continuously. ROM 90122905
  • Searching with no red LED. ROM 3253021
  • Drive fails to read. LM311, 592, 9602
  • Fails to format disk. 6522, ROM 32557201
  • Stepper Motor does not step forward. 6502, 6522
  • Will not save when the drive heats up. 6502
  • Lights stay on, motor runs continuously. Is Power cord plugged into

wall outlet correctly? Is Power cord plugged into disk drive
correctly? Check line fuse and power switch. Check +5/12 volt

  • No LED’s on power up. Check all RAM and ROM locations.
  • Error LED flashes on power up.
  • Drive motor runs continuously with no red or green LED’S. Check

VR2. Check Power Transformer. [ Hint: No Reg. +5V ]

  • Loads programs with red LED flashing. Check drive speed. Check

stepper motor.

  • Loading is intermittent. Check ROM’s and drive alignment.
  • Message of ‚FILE NOT FOUND‘ is displayed. Clean drive head

w/alcohol. Check 0 stop adjustment. Check alignment. [ Check for
sticky slide rails. 1571: Check head lifter release mech. ]

  • Drive speed will not stabilize. Check DC motor. [ Check belt (old


  • Locks-up when loading. Check serial port components. Check


  • Fails the performance test and displays a 21 read error. Check

test diskette. Check Drive Motor.

  • Fails the performance test and displays a 27 read error. Check

stop adjust.

  • Passes performance test to track 18 then displays 21 read errors.

Check read/write head.

  • Passes the performance test but will not load certain programs.

Check stepper motor.

  • Flashing light on and off during loading. An alignment problem or

bad diskette.


(*) U.S. version of the 6569 [ PAL ] is 6567 [ NTSC ].


1) Over 50% of faulty computers are caused by the external power
supply delivering excessive voltages.

2) Check power supply to see if your unit has a fuse which could be

3) Check all interconnecting cables.

Replacement Chips (IC’s) can be obtained reasonably from:

Europe --
HRS Electronics Ltd.
Birmingham, England
Tel. # (02)17712525
USA --
(Commodore) National Repair Ctn.
Toll Free 800-248-2983
(800-BIT BYTE)
Asia --
Saysun Electronics Ltd.
Hong Kong
Tel.# (05)767757

Disclaimer: I hereby disclaim any responsibility for the accuracy of
the information on this chart and disclaim all liability for the use
thereof. This chart is supplied as an adjunct to Commodore computer
users. It is intended for assistance in isolating a faulty chip and
is to be used by persons experienced in the repair of computers and
computer related devices.

Printed in Australia – 1986. No path of the Commodore Diagnostician
may be reproduced nor copied for any purpose without the author’s
approval. (c) Copyright.

End of the Project 64 etext of The Commodore Diagnostician.

The Project 64 etext of the Ray’s C-64 Problems Solved. By Ray
Carlson, CARLSEN ELECTRONICS, <rrcc@u.washington.edu>.

Schreibe einen Kommentar

Deine E-Mail-Adresse wird nicht veröffentlicht. Erforderliche Felder sind mit * markiert