# Restoring a Commodore 64: Part 1


After [rescuing and restoring a lovely Amiga 500](https://sinrega.org/2025-09-23-adventures-amiga-500-part-1/), I decided to look for another old computer to restore. Commodore 64 machines are still relative cheap in the second hand market, specially when announced in unknown condition.

So it's time to test and fix this lovely _breadbin_ Commodore 64, which is almost as old as I am!

{{< image src="breadbin.jpeg" caption="Photograph of a breadbin Commodore 64" width="600px" >}}

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### First symptoms

The machine has an intermittent boot issue. Sometimes it boots normally, sometimes it shows a black screen, and sometimes it produces **random characters on the display**.

{{< image src="random_characters.jpeg" caption="A screen showing random characters, produced by a logic issue in a Commodore 64" width="600px" >}}

This kind of behavior points to **an almost dead PLA**, which is a very common failure on the C64. Fortunately, the community has **reverse engineered this logic array** and created open hardware replacements.

### Replacing the PLA

First step, remove the original PLA from the board.

{{< image src="pla_original.jpeg" caption="The original PLA to be replaced" width="600px" >}}

Rather than soldering the replacement directly to the board, I installed a socket so it can be easily swapped in the future.

{{< image src="pla_socket.jpeg" caption="PLA evolves into socket!" width="600px" >}}

And here's the new PLA, based on [Daniël Mantione's design](https://www.freepascal.org/~daniel/c64pla/), sitting in its new socket.

{{< image src="pla_new.jpeg" caption="A GAL-based PLA replacement" width="600px" >}}

### The real culprit

With the new PLA in place, I powered on the machine and... **it's still failing, with the same symptoms**. The PLA wasn't the problem after all.

This time, instead of looking for match for the symptoms online, I decided to use my own eyes to take a closer visual inspection of the board. I noticed some **ugly looking solder joints on the first CIA**. Applying some pressure on the chip got the machine working, and releasing it made the cursor stop blinking (lost interrupts?). In any case, **I was definitely on to something!**

{{< image src="cia_solder.jpeg" caption="Ugly looking solder joints on the first CIA" width="600px" >}}

While removing the old solder from the CIA to redo the joints, I discovered something unexpected: **one of the CIA's legs was laying on the wrong side of the PCB**, instead of going through the hole. WTH???

{{< image src="cia_wrong_leg.jpeg" caption="One of the CIA's legs on the wrong side of the PCB" width="600px" >}}

I believe this defect is **original from the factory**. The solder looks old with no signs of rework. Enough solder must have flowed through to make a good-enough connection, but **after ~40 years it degraded** to the point of becoming intermittent.

### Fixing it

After **some serious convincing from my part on to the PCB**, I managed to get the replacement leg through the pin hole, finally getting **a happy C64 that boots properly every time!**

{{< image src="c64_booting.jpeg" caption="A happy C64 that boots properly every time" width="600px" >}}

The PLA replacement turned out to be unnecessary, but since it's a component that's expected to fail eventually, I'm calling it **future-proofing**.

### What works and what doesn't

Everything seems to work fine, **including the SID**, except for **two columns of keys** that don't register any input. I suspect a broken trace or a bad solder joint somewhere, but that's a problem for another day.

{{< image src="anykey.jpeg" caption="AnyKey program running on a Commodore 64, showing two columns of keys unpressed" width="600px" >}}

To be continued...