Commodore 64: how to repair it, step by step

This is not going to be a comprehensive guide on Commodore 64 repair. Still, this article will focus on the main steps to follow in order to get your Commodore 64 back again.


ATTENTION: SOLDER SKILLS REQUIRED. Be sure you have the required solder skills before proceeding. Also consider that I am just telling about my experience on C64 repairing, and if you do damage, this is just your fault.



There are different board revisions of the C64, and the procedure to get them working may vary slightly. There is some way of figuring out what board is inside your Commodore 64 even without opening it. It’s not a safe match, but in most cases there is a good chance of guessing.

Older Commodore 64 have the brown “breadbox” case. In those models, the case is higher than on later revisions, and there are no labels on the rear ports. Keys are dark brown with white letters on them. The case is made up of two shells, hold together by three screws on the front and by six little joints (three couples) on the rear. Some very old models have a silver “Commodore 64” label, those are very rare and sought after among collectors.

Inside these old cases, you will usually find a 250407 board. This is the most common on older C64. Some models (usually those with the silver label) may have an original board (326298) or a KU board. Those were early boards and are quite rare.

Board 250407 part 1
Board 250407 part 2
Original board – Thanks to Marco

Newer breadbox C64s will usually have a 250425 board. It’s not that different from a 250407 board, but PLA and SID have been swapped in place (but the PLA is always U17, not a lucky number given its failure rate I have to say). Also, this later board features less glue logic chips. Newer breadbox have a slightly smaller case, plastic is usually a little different in colour and roughness, and there are now three joints for the shells, stronger than the old ones. Rear ports are usually labeled.

Board 250425 part 1
Board 250425 part 2

The 250466 board is the last revision of the 250425. It is quite similar to it but it has only two 41464 RAM chips (older boards have eight 4164 RAM chips, one per bit). This board is usually found on C64Cs with “old style” keyboards (those with letters on top of the keys and graphics on the front of the key). On some C64Cs the 250425 board may be found as well.

Early C64Cs were soon replaced by later C64s with the Cost Reduced board (250469). It has less glue logic, featuring a new PLA chip that actually integrates other chips, like the memory multiplexors (known as 74LS257s on previous board revisions). Chips were produced with a new and cheaper process, but the Commodore 64 stayed more or less the same, with a slightly different sound chip (the SID 8580, replacing the old 6581) and improved reliability. Those boards are more tough but if they get damaged, they are a bit tricker to fix, due to better solder joints and the way the board was etched (some pins are soldered on big copper areas, making the unsoldering process really difficult at times).

Board 250469 part 1
Board 250469 part 2



For Commodore 64 repair, we will focus on the 250407 board, but most of the concepts apply to the other revisions too. The most important thing to notice is that the “Super PLA” on the Cost Reduced 250469 board runs cool and it’s quite reliable. It may get damaged but it’s rare.

Instead, if you get a black screen on a 250407 board, the first thing to look for may be the PLA (U17). PLAs replace many logic chips, allowing for smaller and cheaper boards. Sadly, the PLA on the C64 gets very hot during normal operation and this heat, given the small dimension of the chip, makes it really fragile. The PLA is not a memory management unit, still, it has a key role in memory control, and decides what chip gets access to memory on a given time.

BUT, before desoldering anything, you will have to do some checks. It’s a good idea to always check the easy things first. Preliminary checks are as follows:

  • the power supply is good? Please check that the power supply has both 5V DC and 9V AC. A damaged power supply may give you a black screen on a perfectly working machine.
  • do 5V DC and 9V AC reach the board? On a 250407 board, if the 9V AC are not present, you will get no picture as it is used to drive the VIC-II chip (the 6569 VIC-II chip requires both 5 V and 12 V, the latter derived from the 9 V AC inside the board).
  • if the power supply is good but you get no voltages on the board, maybe the power connector on the board is damaged (look for cracks on the solder joints of the connector. These are very likely on the old original boards, as they have weaker solder joints due to smaller soldering pads).
  • socket chips may suffer from oxidation of the pins. Just lift them a little bit, place them back in place and do it for a few times. This may eliminate oxidations and reestablish the contacts.
  • try to replace the VIC-II chip. They are always socketed so it’s easy. 6569s are the ones to use on original boards, KU boards, 250407, 250425 and 250466 boards. On 250469 boards, you must use the 8565, and this chip cannot be used on earlier boards (as it requires only 5V, the 12V used for the 6569 will damage it). OK, there is a way to use a 8565 on older boards, but it’s better to find a good known 6569.
  • some boards (most likely those found on made in England C64s) may have bad solder joints. So have a careful look at the bottom of the board, if there are cracks, apply solder to them. If you have to redo a solder on a socket, remove the socketed chip first. If the chip on the socket happens to have some solder on a pin, if you resolder the socket the solder on that pin will melt and you will never be able to get that chip out of the socket).



Like the diagnostic mode on a PC, it is possible to make a C64 “boot” in a simpler mode. This mode is not useful to operate the computer, but it is good for diagnostic purposes.

Without any cartridge, you will still be able to get a normal (or nearly) boot screen on a working machine with the following chips removed:

  • SID chip. You will have no sound, no support for analog input devices, no support for programs using the SID as a random numbers generator, but all other functions of the Commodore 64 will be OK. So, if the black screen goes away after removing the SID, that faulty SID chip was just preventing the machine to boot. Replace it.
  • CIA chips. Both CIAs can be removed and you will get a normal boot screen. You will have no cursor, but if the rest of the machine is healthy, you will get the usual message with the right number of BASIC bytes free (38911) and the ready message. Again, one or both faulty CIAs may prevent the machine to boot and freeze it to black screen.
  • CD4066 (U28). Again, removing this chip on a working machine will let you have a normal boot screen. The computer will operate, but there is no support for analog input devices.
  • CD4066 (U16). You will have incorrect colors, but the machine will still boot and operate (at least, that happened on the machines I tried it).

Game cartridges may help us pushing this process further. If only the KERNAL ROM is damaged (901227-3 or 2 chip), the machine may boot with a black screen. The machine will work again if some game cartridges are inserted (like Commodore’s Jupiter Lander for instance). This is because such a cartridge just skips the KERNAL.

Game cartridges will usually work on a machine with a damaged BASIC ROM (901226 chip). A damaged BASIC ROM is usually easy to diagnose as it will give a blank screen with border (with the usual colours).

The CHARACTER ROM just provides the shapes of the characters to be shown. It is just a built-in font inside the computer. Cartridge games usually have a special character font, so they will usually operate properly even with a damaged character ROM. Again, this is another tip for diagnosing.



Everything gets easier if you have a DEAD TEST CARTRIDGE. With such a cartridge, you will get a test screen after a few seconds on a working machine with the following chips removed:


It should work even with the CD4066 chips removed, but I never tried it.

So, if a non working machine gives you the test screen after removing the above chips, one of those is obviously damaged. With little effort you will find the guilty chip.

But the dead test cartridge goes much beyond this. You may not get a test screen, but you may get a flashing screen. That is not what everyone would like, but it is actually quite good because it is detecting some sort of problems. And that means at least the PLA, the CPU, the VIC-II and at least most glue logic chips are working. That is not bad indeed.

Usually, a flashing screen with a Dead Test Cartridge indicates a problem with RAM memory. Please be sure ROMs are removed, as some times they may cause that flashing screen as well. Still, if those are soldered, you may try the following steps before unsoldering them anyway.

You will have a number of flashing, following by a pause and then the flashing again. That means: the machine is broken, but some parts of it are still functioning and let the machine speak to you with a “flash code”. The number of flashing tells you the RAM bit which is not working. Have a look at the Dead Test Cartridge Manual (easily available online) to find out what RAM chip may be defective.

PLEASE BE CAREFUL: a damaged 74LS257 chip may cause that flashing as well. That is important to note, because on a 250407 or 250425 board there are 8 RAM chips, and only 2 74LS257 chips. Those are called “multiplexor” and play a key role on letting the CPU see the memory (well, I’ve put it simple). So, if one or both of them are damaged, they may produce the same problems as damaged RAM memory.

You may have a look at the boot screen (if any) without the Dead Test Cartridge: if you get random garbled character, it may be likely a RAM memory issue, so you can proceed by replacing the RAM chip reported by the flash code. If you get a pattern of characters repeated again and again, it has to do with logic and one or both 74LS257 multiplexors are likely to be faulty.

BAD 74LS257 multiplexor chip.
BAD 74LS257 multiplexor chip.

I tell you what. I usually replace the multiplexors first (socketing them, of course), then I proceed replacing the RAM chips, one by one following the flash codes, till the machine gives a test screen with the cartridge. I replace the multiplexors first because I don’t want to end up replacing all the 8 ram chips and eventually realize that they were just good.

I don’t use the piggyback trick, I just found the Dead Test Cartridge to be awesome.

If the Dead Test Cartridge doesn’t give you a test screen after removing all of the above mentioned chips, well, the trouble is somewhere else, and the PLA is the first to suspect. But still, you may try to replace the 6510 CPU before the PLA, especially if the 6510 is socketed.

The above steps may require unsoldering, but some C64 boards have plenty of socketed chips, so you may be able to get your machine working without any soldering skills.



Original PLAs are quite expensive. Those are custom chips made by Commodore but of course they are no longer produced. Still, it’s just some sort of glue logics chips integrated on a single chip, so there must be a way to replace it. There is a way actually, and you can find several replacement PLAs on the web for the Commodore 64. Some are good, some are not.

The “Plankton” is the best PLA replacement I know. It works just fine with all C64 boards.

There’s also a way of building your own PLA, starting from an EPROM and doing a special adaptor. But this is really time consuming if done properly, I’ve done it some times just for fun and got a replacement PLA working even on an original board, the most reluctanct with non original PLAs.

A bad PLA replacement may get your machine going, but it may be incompatible with some software and it may get some chips too hot during operation. So select a replacement PLA with care.

In a nutshell, a PLA is different from an EPROM, but it can be replaced by it because today EPROMs are just as fast or faster than old PLAs. The equates of the original PLA are written on the EPROM, and you get it going on your machine with an adapter. But to get the right responses, you require on your adapter some capacitors and resistors to build RC filters. Not very easy to build, especially without a custom little board. So, my advice is: just buy it.

A bad PLA may produce the following problems:

  • black screen;
  • flashing colours on characters;
  • crashes with some programs or games;
  • the machine may work for a few seconds after being turned on, then crash.

If you get a test screen after replacing the PLA, you are almost done. If you get a flashing screen, it means that the PLA was not the only component faulty, so you must go on with the above steps. You may also consider replacing the 6510 CPU if you have not done it yet. It may cause a blank screen, but it can be the reason of program crashes as well. Still, I’ve found a damaged 6510 CPU rare on my personal experience.

One thing to notice: if your PLA is soldered on the board, please do all the pleriminary checks before unsoldering it. Those PLAs are really prone to get damaged when unsoldered, so you risk to damage a good PLA by unsoldering it. Still, when unsoldering a PLA, be sure to take your time and not to get it too much hot. I advice you NOT to socket a good known PLA. Just leave it on its place if it works.



A faulty CIA chip may prevent a Commodore 64 to boot, but still, a machine may boot but suffer from some partial failures.

U1 CIA: this chip handles the keyboard, the joystick and the tape. So, if your keyboard is good but some keys don’t work, or you get random characters appear on the screen like a ghost was typing something for you, then CIA U1 chip is very likely to be faulty. Still, joysticks may not work properly and you may not be able to access the tape. Once I had a strange partial failure of this chip: most games would load, but the Cyberload on the videogame Turtles would crash on the “cyberload now loading…” screen. After replacing the CIA U1, I got this game loading just fine. It was a C64C with a 250466 board.

U2 CIA: it may give problems related to disk access and VIC-II memory management. If you cannot access to disk, try to replace this chip. A 7406 chip is very likely to be the problem as well. This chip may also prevent the computer to boot and give you a blank screen.

U2 CIA may also cause problems with the VIC-II chip bank switching scheme. With some cartridges, instead of the normal screen you may get a garbled screen with some I, J, and Ks on screen. I fixed that by replacing the U2 CIA on a 250407 board once.

Finally, U2 CIA may also cause a non working RESTORE key, as it triggers the NMI interrupt required for it to work. It may not work also due to a bad 556 timer chip (this chip is used to reset the machine, so it may also prevent to boot the machine, causing a blank screen).



A machine may show a problem that seems to be caused by a CIA, but it is not. Sometimes, joystick doesn’t work properly because of a bad solder joint on the connectors. So, before unsoldering a CIA, be sure to check those solders first.

Likewise, a tape unit may not work due to a bad tape-computer connection. The tape connector on the C64 board can easily get dirty-oxidized. A simple eraser is all that is required to get shiny contacts on that connector and get it going again.

If luck is not with you, check traces near the CIAs and the connectors: a broken trace may cause the issue (rare).

I will never stop saying that it is always a good idea to check solder joints on connectors. When the machine is hot and you plug/unplug something, you just stress those solder joints very much and they will finally get cracked. This may cause intermittent operation which may get you very frustrated.



OK, I didn’t say this at the beginning because you would have just stopped reading this article. But this is true on the Commodore 64. Due to its architecture, many chips share the same lines, so a line may get stuck due to several chips.

If the above procedures fail, instead of replacing everything I still use the following hints:

  • if you ever get a strange boot screen with things like “Bommodore 64 bacic V2” then the 74LS258 chip may be at fault.
  • an apparent blank screen machine may show you something after some seconds or even some minutes (it happened to me once). So it’s a good idea to leave an apparently dead C64 on for some minutes.
  • failure statistics: generic logic chips are usually good, but MOS technology counterparts were not as good. So, if you have MOS logic chips on a non working machine, if everything else fails try to replace them. Also, 7406, 556s and others with dark brown packages (with EL code) are easy to fail: replace them. I once had a C64C with such EL 7406 chip that couldn’t actually access the disk drive. After replacing it with a good one, it worked fine.
  • if you have replaced the VIC-II, you have both voltages, but you still get a blank screen (a really dark one with no brighter vertical lines as usual), then you may have a broken crystal, or a broken 7701/8701 MOS chip (on 250425/66 boards) or logic chips counterparts on a 250407 or older boards (74LS629N, 74LS193, 74LS74, 4044). I once had to replace a 7701 chip to get a Commodore 64 going.
  • A damaged 4044 chip may show diagonal lines on screen.
  • A damaged 74LS373 chip may cause scrambled characters.
  • Try to replace the remaining logic chips, look carefully for broken traces and again for cracks on solder joints on all the board, better if by using a magnifier.

Most C64s are repairable, but some of them may not just be worth the effort. Consider using a C64 for spares if it gets you mad and buying a “new” one.

But, with the above hints I’m sure you’ll be able to repair quite a number of C64s.



I did my best to summarize the procedures I use to fix a Commodore 64, still I’m sure many things are left, so you may want to have further readings.

  • Ray Carlsen’s site is the most complete source for Commodore 8 bit repair. Most of my knowledge comes from his site and his kindness. He never refused to give me help when I required to, and I will always owe very much to this great person. Just read the articles he wrote: they are not just on Commodore but they are a good guide to logic approach on everything as well.
  • Commodore 64 troubleshooting and Repair Guide: by Robert C. Brenner and published by SAMs.
  • Troubleshooting and Repairing for your Commodore 64, by Art Margolis. Maybe the most known C64 repair book.
  • Commodore 64 Dead Test Cartridge Manual.
  • Commodore 64 Programmer’s Reference Guide (it provides a basic understanding on C64 programming which is really helpful for repairs in my opinion).


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4 thoughts on “Commodore 64: how to repair it, step by step

    • retro64 il said:

      No, I am not willing nor going to make a good deal from my writing. Ads are used to gain “Altercents” for more web space. This blog exists to give help to others, not to make money. Thanks for visiting.

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