C64 common problems and how to fix it

On this page you can find all the gathered information I found on the web to fix your Commodore 64. There are pictures with the common problems, but also descriptions of failing parts of the C64. Hope this helps to fix your Commodore.

To some of the original author(s):
I’m just preserving your valuable information. Just in case it would ever go offline.
As always I would like link and point out to the original sources:

Over time I’m collecting more tips and screenshots from various sites and videos. Good luck finding the solutions you need.

C64 Board reference to all U-chips

C64 Board reference to all U-chips

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Amiga problems?

On this new page you can find all the gathered information I found on the web to fix your Amiga.

VIC-20 problems?

Check this page with a comprehensive guide about the Commodore VIC-20 computer.

CRT monitor problems?

Check this handy Troubleshooting flowchart:

Non-fault #1 – Red/Green Bars

Whenever grey raster (or anything with rapidly alternating pixels) is drawn, the grey color instead appears as bars or red and green. This is not due to a fault in the C64 itself. It happens when composite video or RF is used and the high frequency pattern is misrepresented as color information. Using S-Video should fix or lessen this problem, as it keeps the brightness and color components separate.

Color bleeding of Composite/RF

Non-fault #2 – Graphical errors in Games

In some parts of Giana Sisters, graphical errors blink for a fraction of a second. This is just a bug in the software, and can even be reproduced with emulators. In fact, 99% of the time when you see simple graphics anomalities like this in games, it is simply caused by unpolished software. So before suspecting your C64 hardware, make sure it’s not actually normal behavior for the game/program in question. NOTE: the video is slowed down to make it easier to see the errors.

Unpolished software

Non-fault #3 – Video out of sync

Computer operated normally, except video output was glitched, as if it were out of sync. Both RF and Composite displayed the same problem. Swapping in another VIC chip and re-capping did not fix it either. It turned out to be a compatibility problem with the LCD TV. The television worked fine in other use, but would not work properly with a C64. NOTE: ‘Device not present error’ not related to this problem.

Image(s) by novoiperkele @ com64.net

Related web page

Incompatibility between C64 and LCD TV

Non-fault #4 – fuzzy colors

Fuzzy colors on video output. This happens if you connect a C64 to a TV with a s-video cable, but the TV either doesn’t support it or has it switched off. The fuzzy colors are caused by luminance/chrominance leakage.

Using s-video cable with a TV that doesn’t support s-video or s-video mode not switched on

Also, it’s best to use an original Commodore 1701 or 1702 monitor. It has separate Luma and Chroma inputs. it would be best to use a L/C/A cable.

CIA1 Faults U1

image 1:
Relatively normal startup screen, but frozen with a rapidly flashing cursor and ‘LOAD’ command run automatically. No response to keyboard commands. A cartridge game loaded up properly but joystick and keyboard did not work. Picture re-created with VICE C64 Emulator.

Image(s) by Ozsoft @ lemon64.com

images 2,3,4:
Caused various abnormal startup screens, including a simple black screen on a couple of boots. The most common outcome was an empty blue screen with borders (first picture). After about 10 seconds of waiting, most of the normal startup text appeared along with some garbage characters, and in some cases automatically typed load/run commands (second picture). Sometimes the characters kept switching between upper and lower case. On one boot, screen was filled with white garbage characters (third picture). The computer seemed unresponsive on every boot.

Though in this case the problems were simply caused by a bad connection between the CIA1 chip and its socket, similar symptoms can likely be caused by a failing CIA chip.

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.

– Bad 906108-02 (6526) CIA
– Bad socket contact on U1 (CIA) – bottom pin(s)

CIA2 Faults U2

Normal borders, but garbage in the middle. Sometimes after a while or when you try to load something from disk (the drive won’t start to load).

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.

– Bad U2 906108-02 (6526) CIA

Basic ROM Fault U3

Startup screen with normal blue colors and borders, but content area completely empty. A cartridge game worked normally. It turned out to be a faulty Basic ROM chip. A blank blue screen like this is the most common symptom of a bad Basic ROM. Cartridges bypass the Basic ROM so they are good for diagnosing problems with that chip.

Image(s) by danko @ lemon64.com

Blank screen w/ border. Cartridge works.

– Bad U3 (Basic ROM) 901226-01

Basic KERNAL Fault U4


Normally booted to a black screen. With Jupiter Lander game cartridge inserted, produced the garbage screen shown above. Removing the Kernal ROM allowed Jupiter Lander to boot normally, and replacing that chip resolved the issue. Note that Jupiter Lander bypasses the Kernal ROM and is usually not affected by a fault in that chip. But as this fault proves, it’s still possible.

Image(s) by mrr19121970 @ lemon64.com


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.


Bad U4 (Kernal ROM)

Character ROM Fault U5

Normal operation of the commodore 64, everything works, but you’ll see artifacts in characters like
– stripes through characters
– random moving / changing characters

Image(s) by craftsman1234 @ lemon64.com

Blank screen with border or screen full of shimmering lines or characters. Partial failure: “garbage” characters or blocks where startup page info should be. Cartridge works.

Bad U5 (Character ROM) 901225-01

Color RAM fault U6

Startup screen with some missing characters. Certain columns are missing. Games also had missing characters. The computer was mostly functional, commands worked normally. Diagnostic Cart Rev 586220 froze when it reached the Color RAM test. Swapping in a new Color RAM chip fixed the problems. Image(s) by jts-78 @ com64.net

other screenshot
Startup screen was normal and the computer was operational, but changing the cursor color to white revealed a problem: another character also turned white. Which character was drawn white depended on the position of the cursor. In addition, games had abnormal graphics. Replacing the Color RAM fixed the symptoms. Image(s) by lolhead @ lemon64.com

3rd screenshot
System worked fine except that colors were incorrectly displayed. Problem went away after U6 color ram was replaced. Image(s) by zxspectrum_16k @ lemon64.com

Shimmering colors on characters.

Bad U6 (Color RAM)

CPU fault U7

Garbage, hanging system, problems with larger programs.

Blank screen, no border. Cartridge doesn’t work.

Bad U7 (CPU) MOS 6510

Image(s) by Fastah @ lemon64.com
Image(s) by jts-78 @ com64.net
Image(s) by josephdewes @ lemon64.com

Logic fault U8

Startup screen filled with vertical bars & blank space. NOTE: weird colors due to display/camera, not related to this problem.

Machine was working normally, except that it hung when trying to load via a disk drive. There was no reaction from the drive. The screenshot depicts the point where it hung. Swapping the CIA chips U1 and U2 did not make a difference. Replacing the 7406 logic chip at U8 finally resolved the problem.

Blank screen. No drive reset when computer switched on.

other 2:
Blank screen. Drive spindle motor runs continuously with computer on. RUN/STOP-RESTORE doesn’t work.

Bad U8 (Logic) 7406
ad. other 2: 74LS14 LOGIC

Image(s) by thasti @ forum64.de

RAM Problems U9 – U12, U21 – U24

Startup screen problems with:

  • out of memory error,
  • wrong number of memory available,
  • colored random characters.

Blank screen, no border. Shorted chips may get warmer (sometimes very hot) than the other RAM chips. Partial failure: will sometimes produce “garbage” screen, abnormal number of bytes free (lower than 38911) or “OUT OF MEMORY IN 0” error on startup screen.


Bad RAM chips or bad socket contacts of the RAM
(M41464) 2 RAM CHIPS (64K X 4 DRAM)

Please see description of the images for the cause


Logic problem – U13

A startup screen full of random characters or strange images, similar to one caused by bad RAM. In most cases the C64 still works. Also in many cases the C64 works with a (deadtest) cartridge.


Bad Logic Chip U13

Logic problem – U14

A startup screen with incorrect characters, you’ll see most of the time that there’s incorrect placement of text. In all cases the Commodore does function correct, but incorrect display of characters.


Bad Logic Chip U14

PLA problem – U17

A start up screen with demo scene like problems like colored characters, flashing colors, stripes etc. looks like a Bad PLA U17 chip.


Bad PLA Chip U17

Replacement PLA chips

PLA problem / Source: Jameson on Lemon64 (this video above)

SID problems – U18

A defective SID can (next to no synthesizer sound or no digi, which means unable to play samples) produce a black screen. You can debug to remove the SID and turn the C64 on. A C64 will work without a SID chip installed.

Blank or “garbage” screen if shorted (remove to test), otherwise normal screen. No sound or garbled sound. Mouse or graphics tablet pointer stuck or jitters.

NOTE: again, computer will work without a SID plugged in.
NOTE: 8580 and 6581 are pin compatible and somewhat interchangeable, but only with board component changes: pin 28 supply voltage either +9 or +12VDC, and two capacitors values must be changed for correct voicing. No sound: also check capacitor C77 (open).

Never ever interchange a 6581 with a 8580 (without proper modifications), it can and will kill your C64!


Bad SID Chip U18

VIC problems – U19

A defective VIC presents all kinds of video problems. For example black vertical stripes, strange colors, a shadow etc.


Bad VIC Chip U19

Logic problem – U25

Initially had a black screen on startup, which was fixed by a PLA swap. After that, random black/blue characters (shown above) displayed on startup. Dead test cartridge worked and the memory test completed successfully. Swapping in new RAM, U13 and PLA did not cure this symptom. Replacing U25 finally did the trick and the machine booted up normally. The failed U25 was a MOS version of the chip, which are known to fail more often than their non-MOS counterparts.


Bad Logic Chip U25

Logic problem – U26

Seeing distortion smaller than the size of characters, then it could be the Logic Chip U26


Bad Logic Chip U26

Oscillator problem – U31

Typical out-of-sync video problems? Then it’s the oscillator.


Bad U31 (Oscillator)

MOS 8701 Clock generator

Black screen? If you turn on the c64 and you don’t see that common flash before the black screen. Chances are that the MOS 8701 Clock generator is faulty. Or blank white screen and no border.


8701 or 7701 MASTER CLOCK OSC.

Thanks to MindFlareRetro


Wrong colors on characters.




CT1 Trimmer capacitor near VIC

Black and white screen on analog screens and on modern tv’s semi black and white with colorful flash (see video)


This capacitor is broken. CT1 is the trimmer capacitor to fine tune all the clock frequencies.
It nudges the xtal to alter its frequency. All clocks are derived from this:
– The dot clock at about 8MHz
– the VIC generates the CPU clock at 1/8 of this
– The colour subcarrier clock at 3.57 (NTSC) or 4.33 (PAL) Just get any 40pF trimmer and replace the one that’s there.


Bad trace

A bad trace on the board (broken or causing short circuit) can be the cause of all sorts of problems.

Picture 1:
The display shown in this picture is with the 586220 diagnostic cartridge installed; without cartridges produced only a blank black screen at power up. The symptoms with a cartridge were vertical bars with colors and patterns varying depending on the cartridge installed, and sometimes included patches of random pixels around the screen, and the computer was otherwise frozen up. The cause of the problem was a worn or broken trace in the RAS line between U26 pin 11 (74LS373) and the VIC pin 18. U26 pin 11 also connects to the RAS pins on the RAM chips (pin 4 for older boards with 8 chips, pin 5 for newer boards with 2 chips), so a similar problem might appear if the same break was between the RAM and U26.

Picture 2:
Normal boot-up produced a black screen. In this case it turned out to be a short between pins 7 (A1) and 8 (Vcc) below RAM chip U10. After that was fixed, both dead test and normal boot worked fine again.

Tips & info

Piggy back an IC

The piggy back method gives you an educated guess identifying the faulty IC. Just push a known good memory chip over the chip to be tested. If the fault changes/improves, you have a good chance of pin pointing the faulty chip without soldering. This method has been done many times in the Arcade scene / repairs.

note: do this at your own risk btw 🙂

Black screen?

    1. Do you see a flash? No, replace the clock generator 8701 (small chip next to the VIC chip).
    2. If you see a flash on power-on, first try to remove the SID; still doesn’t work?
    3. Replace the VIC chip; still doesn’t work?
    4. Replace the PLA (906114-01); still doesn’t work?
    5. check this video:

another problem can be your power supply, here’s how you can test your power supply:

Another video with a black screen repair:

More blackscreen repairs

2 cases of black screen repairs:

Cassette problems (datarecorder)

Cassette motor will not turn when FF/REW or PLAY is pressed.


  • check fuse inside computer or

Power supply (PSU) issues

POWER PACK: +5VDC at 1.5Amps and 9VAC at 1 Amp
Can produce many problems like blank screen (red power LED on, dim or off), program lock-up, “garbage” screen, hum bars moving on screen, hum in audio, damaged RAM chips, intermittant operation after warmup, etc. As common a failure as it is, the supply should be checked (by substitution) first.

No cursor?

You can still type and it seems to work, but no cursor is shown!
The problem can be Ram(1) and Character ROM. See the whole thread here.

Looking at faulty chips

Future proofing your Commodore 64


from R. Carlsen, link
(just preservering your information mate)

Commodore Diagnostician II

This information was present on Commodore.ca, but it’s gone (at least at the link I had), therefore I preserved this on my site from the archive.org. All credits of course go to the original authors (see yellow cell below)

Commodore Diagnostician II
By Ian Perry, Kyneton, Victoria 3444, Australia
Sept 1989

Symptomatic Chart Diagnosing
Faulty IC Components On
Commodore 64 &
Commodore 128

6510 Proc 6526 Complex Interface ADP 6581 Sound Interface Device 6567 VIC NTSC (6569 PAL) 82S100 Prog Logic Array 901226 Basic ROM 901227 Kernal ROM’ 901225 Char ROM 8701 IC Clock Gen 2114 Color RAM 4164 Memory RAM Power Supply
Cursor jumps back to home position x x
Abnormal colors appear in the letters x x
Different characters are displayed, cursor is locked when turned on & off x x x
The ‘RESTORE’ key does not work x x
Cursor stops blinking when system warms up x
Keyboard does not operate correctly when system warms up x x x
Syntax error x x
Cursor disappears when cassette is plugged in x
Cassette runs extremely slow, the program see,ms to load but will not run x
Blank screen on power up x x x x x x x
Intermittent blank screen or graphics x x x x x
Powers up when ‘PRESS PLAY ON TAPE’ message and the display blinks x x
When ‘RETURN’ is pressed after a run command, cursor returns to home position x x x x x
Poke command does not work x
Joystick does not operate correctly x
Unreadable letters appear on screen x x
Keyboard is unresponsive x
Graphic characters instead of letters are displayed x x x
‘DEVICE NOT PRESENT’ error is displayed, when disk is used x x
Disk drive continues to search when trying to load x x
Inconect screen color or no color at all x x x
Cassette does not save x
Flashing colors or blocks x x
Game cartridge does not function x x
User port doeS not function (eg. modem) x
No sound x
Missing notes x
Game paddles do not function x
Long programs do not lOad x x x
Unit completely dead (30 – 40% possibility PLA) x x x x x x
Wavy screen after system warms up x
‘OUT OF MEMORY’ error on power up x
After a few minutes, characters all over screen x x x x

3 replies
  1. Erix jaison
    Erix jaison says:

    I have a commodore 128. And when I try to load a game by typing “Load”*”,8,1″ it always says “device not present”. I tried it on both 128 and 64 modes. But it shows the same message. I tried to readjust the serial port connector but I still get the same response that the drive is not present. I have a commodore 64c also and the disk drive works fine on that.
    Could u please let me know what the problem is?

    • Patrick Tighe
      Patrick Tighe says:

      If the drive works on one machine but not the other, it sounds like there might be a problem with the 128’s CIA chip (Complex Interface Adapter – MOS 6526). That controls things like floppy and cassette drive access, keyboard, joysticks, etc. I’d start troubleshooting there by replacing that chip.

      – PT

  2. LikelyYou.com
    LikelyYou.com says:

    Now the error that caused these to be recalled seems to be what was known as the sparkle bug .  Apparently, the video chip, the VIC, caused light blue sparkles on a dark blue background when it heated up. Furthermore, customers were complaining that the color scheme of the C64 seemed rather garish.  This was caused by assembly line workers adjusting the color saturation all the way up, resulting in overpowering colors. Add the fact that Commodore needed to ship the C out in time for the Christmas season and you ll find that many early models had defective VIC chips, simply because when production lines encountered a shortage of these chips, they d go to the bin with the defective chips, remove the label and put them in anyway. Nonetheless, Commodore overcame these problems and the rest is, as they say, history.

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