Commodore 64 Diagnostics and Repair
Note: Some of the content found on this page was replicated and edited from other sites
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.
Content credits:
- https://portcommodore.com/rcarlsen (Pict-o-guide and other content)
- https://retrocomputerverzamelaar.nl/commodore-64-problems/
- julrod at lemon64 forums (mainboard layout images)
- https://www.c64copyprotection.com/2019/09/ (Commodore Diagnostician ref I and II)
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.
You can also use the interactive Commodore Repair Toolbox from Loki762 at Lemon64 forums. This was being updated as recently as last week (at the time of this writing, 7/7/2025), and I’m looking into improving some of the imagery if I can, as well.
* Local archive of the project as of 7/7/2025 is backed-up here as well.
Motherboard Revisions
- 326298 Rev A
- 250407 Rev. A
- 250425
- 250466
- 250469
[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.
- Common Faults Reference
- Pict-O-Guide
- Commodore Diagnostician
- C64 Chips and Issues
- C64-C Chips and Issues
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
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.
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
Pict-o-guide
Mirrored from Ray Carlsen’s Archive, formatted for accessibility.
C64 CHIPS and COMMON SYMPTOMS
latest additions and corrections: 1-07-2020
There were at least five different motherboards made for the C64.
This composite list includes chips used in all board versions except the
latest C64C 250469 short board. See c64c-ic.txt for that information.
Not all chips are found in all board versions.
U1 906108-02 (6526) CIA
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.
U2 906108-02 (6526) CIA
Startup screen normal. No serial or user port access. “File not
found” error when drive accessed. Cartridge works. Characters may show
as blocks or garbage screen is produced. Shorted chip will show blank
screen.
U3 901226-01 BASIC ROM
Blank screen w/ border. Cartridge works.
U4 901227-03 (early -02) KERNAL ROM
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.
U5 901225-01 CHARACTER ROM
Normal startup border, but “garbage” characters where startup page
characters should be. Cartridge works.
U6 2114 SRAM (COLOR RAM)
Startup screen shows flickering characters with shimmering colors.
Check also PLA U17.
U7 906107-01 (6510) MPU
Blank screen, no border. Cartridge doesn’t work.
U8 7406 LOGIC CHIP
Blank screen. Partial failure: drive access problems: drive resets
but “device not present” error.
U9 THRU U12 & U21 THRU U24 8 RAM CHIPS (generic 4164 DRAM)
note: only 2 4464 RAM chips in later C64C boards
Blank screen, no border. Shorted chips will get warmer than the
other RAM chips. Partial failure: will sometimes produce “garbage”
screen, abnormal number of bytes free or “out of memory” error on
startup screen.
U13 74LS257APC or A or AN but not PC (alt MOS7708) LOGIC
Blank screen, less than 38911 bytes free at startup or “garbage”
screen. See U25 Note: MOS77xx series chips have a high failure rate.
U14 74LS258 LOGIC
Blank screen.
U15 74LS139 LOGIC
Blank screen
U16 CD4066 (generic CMOS quad bilateral switch)
Color problems such as random color “checkerboard” pattern on screen
or no color.
U17 906114-01 (82S100PLA) PLA
Blank screen, no border. It can produce colored screen or flashing
color garbage instead of startup screen. It can cause intermittant loss of
cursor, screen freeze and/or program crashes after warmup and it can put
random characters on screen. This chip normally runs hot. It is the most
common chip to fail in the C64 because it runs hot normally, like the SID.
U18 906112-01 (6581) SID Note: 8580 SID used in latest C64C
Normal screen. No sound or garbled sound. Mouse or graphics tablet
pointer stuck or jitters. If shorted, can cause blank or “garbage” screen.
NOTE: computer will work without a SID plugged in (unplug to check). This
chip normally runs hot.
U19 906109-04 (6567) VIC (PAL version: 6569)
Blank screen (may be light or dark) with 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.
U20 556 LOGIC, TIMER
Blank screen. Computer will not reset, or RESTORE doesn’t work.
U25 74LS257APC or AN or A but not PC (alt MOS7708) LOGIC
Blank screen, “garbage” screen or less than 38911 bytes at startup.
See U13 Note: MOS77xx series chips have a high failure rate.
U26 74LS373 LOGIC
Startup screen has normal border, but characters are scrambled.
U27 74LS08 (or MOS7712) LOGIC
Normal startup border, but screen full (or nearly) of “garbage”
characters.
U28 CD4066 (generic CMOS quad bilateral switch)
Normal startup screen. Proportional mouse or graphics tablet
doesn’t work in one or both control (joystick) ports.
U29 74LS74 LOGIC, DIVIDER
Startup screen shows normal characters in multicolored “rainbow”.
U30 74LS193 LOGIC, COUNTER
U31 74LS629N (w/ support logic) or 7701/8701 MASTER OSC.
Blank white screen, no border. Normal “whine” in nearby AM radio
will be missing if the oscillator is not running. Check also VIC U19
U32 4044 CMOS LOGIC
No monitor picture, just diagonal lines on screen.
POWER PACK:
Can produce many problems like blank screen (red LED on, dim, or off),
program lock-up, “garbage” screen, hum bars moving on screen, hum in audio,
several bad RAM or other chips, intermittant operation after warmup, etc.
Ray Carlsen CET
CARLSEN ELECTRONICS
Mirrored from Ray Carlsen’s Archive and formatted for accessibility
C64C (true C short board) – CHIPS and COMMON SYMPTOMS
Latest update and/or revision: 1-06-2020
There were several versions of model C64C that featured a slimline white case, white keys and an internal heat sink shield. The earliest versions used
the original 64 motherboard (250425), the same board as some brown case C64’s.
Interim boards (250466) used two 4464 RAM chips instead of eight 4164, used
either version SID or CIAs, but retained the old MPU, PLA and VIC-II. You
should reference the “c64-ic.txt” file for correct board ID numbers of those
earlier boards.
This latest 250469 “cost reduced” motherboard is narrower and uses the
fewest, most integrated chips. Most of these ICs are not interchangeable with
earlier models, and chip ID numbers are different than earlier boards. For that
reason, this article deals with this latest board only, although basic
troubleshooting is similar for all versions of the C64.
U1 906108-02 (6526/ A or 8521) CIA
Startup screen normal but no cursor, or blank screen if chip is shorted
(remove to test). No keyboard or joyport access. Partial failure: some keys or
joystick positions don’t work, “stuck” line may print a character at startup.
Cartridge works.
U2 906108-02 (6526/ A or 8521) CIA
Lines or blocks instead of startup screen, but blank screen if chip is
shorted (remove to test). Partial failure: marginal or no serial / user port
access, keyboard and cartridges still work.
U3 74LS08 LOGIC
Blank screen, no border.
U4 251913-01 BASIC and KERNAL ROM (same IC as U32 in C128)
Blank screen, no border. Cartridges don’t work except the few that bypass
the Kernal ROM such as CBM Kickman and Jupiter Lander. Those can be used as a
diagnostic for a bad Kernal.
U5 901225-01 CHARACTER ROM
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.
U6 318012-01 (8500) MPU aka CPU (6510 works as sub, see text)
Blank screen, no border. Cartridge doesn’t work.
U7 318014-01 (8562 or 8565) VIC-II
Blank -white- screen, no border. Partial failure may produce garbage or
“checkerboard” screen, or smeary screen that lacks contrast. If screen is blank
or garbled from bad VIC, “blind” disk commands from keyboard may still work.
NOTE: earlier 6567 will -not- sub for this chip.
U8 251715-01 (LH5062B OR YM3535) PLA, MEMORY CONTROL
Blank screen, no border. Runs cool, very rare failure.
U9 318013-01 (8580) or 906112-01 (6581) SID (SOUND INTERFACE DEVICE)
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: for test purposes, 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.
U10 and U11 (M41464) 2 RAM CHIPS (64K X 4 DRAM)
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.
U18 4066 CMOS QUAD SWITCH (POTX/Y BUFFERS)
Startup screen normal, but mouse/graphics tablet don’t work.
U19 LC3514A or MN2114-2 SRAM 1K X 4 (aka COLOR RAM)
Shimmering colors on characters.
U20 8701 or 7701 MASTER CLOCK OSC.
Blank white screen, no border.
U21 4066 CMOS QUAD SWITCH
Wrong colors on characters.
U22 7406 LOGIC
Blank screen. No drive reset when computer switched on.
U23 74LS14 LOGIC
Blank screen. Drive spindle motor runs continuously with computer on.
RUN/STOP-RESTORE doesn’t work.
M1 252405-02 RF MODULATOR
Weak or no RF output (snowy screen) to TV. Internal defect in modulator can
affect A/V outputs (no audio or video) since those signals go -through- modulator.
Q1 2SD313 TRANSISTOR, CASSETTE MOTOR DRIVER
Cassette motor will not turn when FF/REW or PLAY is pressed. Also check
fuse inside computer.
F1 1.5 AMP FB FUSE, 9VAC PROTECTION
If fuse is open, computer appears to work normally, but no cassette
operation, no 9VAC to user port, and no sound from SID.
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.
Troubleshooting any version of the C64 is often difficult because some symptoms can be caused by any one of a dozen chips. The most common, a blank screen with no border, is sometimes frustrating for even a tech to diagnose.
We usually start by process of elimination, keeping in mind the chips most likely to fail in order. Each board is different in how many of the major chips are socketed, but that’s a good place to start. Reseat all socketed chips by lifting up on each end slightly (pry between chip and socket with a small flat-blade screwdriver) and press the chip back down. There is no need to remove the chip just for reseating. You risk bending pins if you remove it. If you suspect a chip is bad based on symptoms, the best way to test it is by installing it in a working computer. If none are available, and/or you have no spare parts or boards, the job is nearly impossible. Substitution of chips or parts is the only practical way for most people to diagnose a fault. Don’t overlook the power supply when you are troubleshooting. It deserves its bad reputation for damaging computer chips under some fault modes. Check by substitution only. The original supply is “potted” with epoxy inside and is not repairable. After-market power supplies are “beefier” and usually repairable. The 85xx series chips used in the C64C are, for the most part, not interchangeable with earlier C64 ICs. They represent a different logic “family” and in some cases have a different pinout. Substitutions should therefore not be made casually.
As indicated above, the computer will work with a few of the major chips removed. This is helpful as a diagnostic. For example, the SID can produce a blank screen if shorted, but the computer will still work without it, except of course without sound, and the POTX/Y inputs will not work (mouse, graphics tablet, etc.) but a joystick will still work. The two CIA chips can be removed to test for shorted chips (they’re bad for sure if they get hot) and the computer will come up if the chip is bad although the startup screen will be affected (see chip list for U1 and U2). A bad keyboard row or column is usually caused by a bad CIA. The two CIAs can be swapped to aid in diagnosis if no other parts are available, and the fault will follow the bad chip, but of course with different symptoms.
Unlike the one in the C64, the PLA in the C64C runs cool to the touch. I’ve never seen a bad one. Of course new and old versions are not interchangeable. The older 6510 MPU seems to work in the C64C as a sub for the 8500. I have tried an 8500 in an old C64 and it worked (diagnostic test only), but I don’t know how well these chips interchange in actual use. All of the 8500 series chips run cooler than the older 6500 series. The later C64C uses some of the same chips as a C128. Note that an open fuse in the older C64 computer will make it non-functional because the converted 9VAC runs the VIC and SID chips. An open fuse or failed 9VAC (bad power supply) with the later C64C may not be immediately apparent… the computer will still work but you will have no sound, the user port will not get the 9VAC, and the cassette motor will not work.
The cabinet screws and the ones inside the computer may be the newer Torx (star-shaped) type rather than standard Philips. If so, you may need to get a T-10 bit to remove those screws. I’ve found that a standard small flat-blade screwdriver fits well enough if a Torx driver is not available. As mentioned before, the “blank screen” symptom is the hardest to troubleshoot because many different problems can produce that symptom. It would therefore be helpful to know what signals should appear on each pin of the microprocessor, the heart of the machine. If, for example, the reset line was being held low for some reason, that could be traced down to the specific problem and fixed. With that in mind, here is the pinout for the 6510/8500 microprocessor used in the C64 and C64C.
Readings are taken with computer on and no external devices connected, keyboard unplugged. You’ll need an oscilloscope or logic probe to make sense of some of
these readings.
1 CLK 1 IN OSC SIGNAL
2 RDY DATA, ANDED BA FROM PLA + /DMA
3 /IRQ DATA (HIGH)
4 /NMI HIGH, FROM /RESTORE CIRCUIT
5 AEC DATA, ANDED AEC FROM VIC + /DMA
6 VCC ALWAYS +5VDC
7 A0 DATA \
8 A1 DATA |
9 A2 DATA |
10 A3 DATA |
11 A4 DATA |
12 A5 DATA |
13 A6 DATA |
14 A7 DATA |
15 A8 DATA | ADDRESS LINES
16 A9 DATA |
17 A10 DATA |
18 A11 DATA |
19 A12 DATA |
20 A13 DATA |
21 GROUND 0 VDC |
22 A14 DATA |
23 A15 DATA /
24 P5 +3V
25 P4 HIGH
26 P3 LOW
27 P2 HIGH
28 P1 HIGH
29 P0 HIGH
30 D7 DATA \
31 D6 DATR |
32 D5 DATA |
33 D4 DATA |
34 D3 DATA | DATA LINES
35 D2 DATA |
36 D1 DATA |
37 D0 DATA |
38 R/W DATA /
39 CLK 2 OUT OSC SIGNAL
40 /RESET HIGH, FROM RESET CIRCUIT
Ray Carlsen CET
CARLSEN ELECTRONICS
If you have any questions or comments, please feel free to write,
especially if you spot an error here. Thanks!
