Commodore 64 common problems reference database

by cmack · Published November 10, 2023 · Updated February 20, 2024

Note: The content found on this page was replicated and edited from https://retrocomputerverzamelaar.nl/commodore-64-problems/
The intent was not to plagiarize or steal anybody’s thunder. The internet being as ephemeral as it is, I wanted to do my part to help ensure this treasure trove of knowledge was not lost to time or changing personal interests. The original site’s author credited individual sources where possible, and I will maintain (and update) links to the sources, keeping the knowledge safe and ensuring all those wonderful minds that play a part in this collection continue to be remembered and honored for years to come.

On this page you can find all the gathered information 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.

Motherboard Revisions

[1982] – C64-326298 Rev A-Schematic #326106

These boards turned up in silver label C-64s, and later versions of it turned up in the first rainbow-label breadbins. These boards are notable for their 5-pin video ports. They are scarce and very collectible. This generation of boards has compatibility issues, except, notably, the CP/M cartridge. Making matters worse, the video quality on these boards was subpar due to the values of some of the resistors in the video circuit.

[1983] – C64-250407-Schematic #251138

The consensus seems to be this is the most common C-64 motherboard, and it was the board Commodore was building at the peak of the 64’s popularity. It has 8-pin video. Even though cost reduction was one of its design goals, it’s much improved over the early 1982 boards. The PLA is nearest the serial port, and the SID is by the VIC-II. It had three revisions, A-C. If you or your friends had C-64s in its heyday, there’s a very good chance your 64 had this board, or someone you know did.

[1984] – C64-250425-Schematic #251469

This board is also very common. Commodore sometimes referred to this as the 64B. This board is more integrated and cost-reduced than previous boards. The most visible difference between this and earlier boards is that the VIC-II isn’t in a metal can anymore. Notably, the PLA switched spots with the SID on this board. The PLA is now by the VIC-II, and the SID by the serial port. So if you’re swapping parts between multiple boards, pay attention to where the chips go so you don’t blow up a SID by plugging it into the PLA socket. This board is desirable because of its excellent video output quality. If you want the classic C-64 experience, this is a good board to look for.

[1986] – C64C-250466-Schematic #252278

This board, known as the 64C long board, turns up in some late breadbins and early 64Cs. It still has the 6581 SID. The fuse is horizontal behind the cartridge port if you want to spot one without opening up the machine. The most visible change on this board from its predecessors is the presence of two RAM chips, down from eight, leaving lots of empty space in the lower left. This is a highly regarded board. It has lots of parts commonality with the older boards but with the simplicity of fewer RAM chips. It also has excellent video output quality, like its immediate predecessor.

[1987] – C64C-250469-Schematic #252312

This board is known as the 64C short board, or the 64E board. There’s no date on the board but it seems to have first appeared in 1987. This is a smaller, further cost-reduced version with lots of changes, including a much higher degree of integration. The dreaded PLA was replaced with a Super PLA, which is much more reliable. It has the lowest parts count of any C-64 board Commodore produced. It used the 8580 SID and 8500 CPU. It had four revisions in total. Rev. 3 and 4 were the earliest. Rev. A and Rev. B integrated the color RAM into the Super PLA. Rev. B is rare in the USA.

Assy 250425

Some words here

Non-fault #1 – Red/Green Bars

Description
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.

Cause
Color bleeding of Composite/RF

Non-fault #2 – Graphical errors in Games

Description
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.

Cause
Unpolished software

Non-fault #3 – Video out of sync

Description
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

Cause
Incompatibility between C64 and LCD TV

Non-fault #4 – fuzzy colors

Description
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.

Cause
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

Description
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.

General
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.

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

CIA2 Faults U2

Description
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).

General
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.

Cause
– Bad U2 906108-02 (6526) CIA

Basic ROM Fault U3

Description
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

General
Blank screen w/ border. Cartridge works.

Cause
– Bad U3 (Basic ROM) 901226-01

Basic KERNAL Fault U4

Description

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

Related web page

General

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.

Cause
Bad U4 (Kernal ROM)

Character ROM Fault U5

Description
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

General
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.

Cause
Bad U5 (Character ROM) 901225-01

Color RAM fault U6

Description
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 depending 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

General
Shimmering colors on characters.

Cause
Bad U6 (Color RAM)

CPU fault U7

Description
Garbage, hanging system, problems with larger programs.

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

Cause
Bad U7 (CPU) MOS 6510

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

Logic fault U8

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

2nd:
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.

other:
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.

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

Credits
Image(s) by thasti @ forum64.de
Ray Carlsen

RAM Problems U9 – U12, U21 – U24

Description
Startup screen problems with:

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

General
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.

Cause
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

Credits
Bad ram ICs

Logic problem – U13

Description
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.

Cause
Bad Logic Chip U13

Logic problem – U14

Description
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 correctly, but incorrect display of characters.

Cause
Bad Logic Chip U14

PLA problem – U17

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

Cause
Bad PLA Chip U17

Replacement PLA chips

Plankton: Lemon64 – Poly.play
SuperPLA v4: icomp
The Ultimate PLA: vintage computers

PLA problem / Source: Jameson on Lemon64 (video has been removed)

SID problems – U18

Description
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.

General
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!

Cause
Bad SID Chip U18

VIC problems – U19

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

Cause
Bad VIC Chip U19

Logic problem – U25

Description
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.

Cause
Bad Logic Chip U25

Logic problem – U26

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

Cause
Bad Logic Chip U26

Oscillator problem – U31

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

Cause
Bad U31 (Oscillator)

MOS 8701 Clock generator

Description
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.

Cause
8701 or 7701 MASTER CLOCK OSC.

Source
Thanks to MindFlareRetro

4066 CMOS QUAD SWITCH

Description
Wrong colors on characters.

Cause
4066 CMOS QUAD SWITCH

Source (original)
Ray Carlsen: https://portcommodore.com/rcarlsen/cbm/c64/c64-ic.txt
(also mirrored here on this site, in case the source goes away)

CT1 Trimmer capacitor near VIC

Description
Black and white screen on analog screens and on modern TVs semi black and white with colorful flash (see video)

Cause

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 color subcarrier clock at 3.57 (NTSC) or 4.33 (PAL) Just get any 40pF trimmer and replace the one that’s there.

Source
https://www.lemon64.com/forum/viewtopic.php?t=49061&start=30

Bad trace

Description
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?

Do you see a flash? No, replace the clock generator 8701 (small chip next to the VIC chip).
If you see a flash on power-on, first try to remove the SID; still doesn’t work?
Replace the VIC chip; still doesn’t work?
Replace the PLA (906114-01); still doesn’t work?
Review the videos to the right for more options, processes and possibilities for black screen repair

Cassette problems (data recorder)

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

Cause:

check fuse inside computer or
2SD313 TRANSISTOR, CASSETTE MOTOR DRIVER

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, intermittent operation after warmup, etc. As common a failure as it is, the supply should be checked (by substitution) first.