Commodore 64

I picked up this C64 with a C2N Datasette for nothing due to the fact it wasn't working. An evening's tinkering soon got her running again.

First things first - check the power supply. I was told it still worked, but didn't find any voltage between any of its pins. Not too surprising, considering the fuse had blown! Maybe I managed to short something whilst probing with the volt meter but it was easy enough to replace.

When I started the computer, it produced nothing but a black screen on the TV, which could've been caused by pretty much anything. Common causes are failure of either the PLA or one of the RAM chips. A bad PLA can give itself away by flashing colours on the screen if the machine is power cycled a few times. This didn't happen, so I left the machine on for a few minutes to see if any of the RAM was getting overly warm, indicating failure. Even after ten minutes, all the RAM was barely warm to the touch.

Next my attention moved to a replacement ROM which had been shoehorned in using a couple of IC sockets and a few wires. I reseated this and a couple of smaller 74LS257 chips which were also socketed. This was enough to get a familiar-looking blue screen with slightly non-standard header text. The replacement ROM is actually the 64'er System kernal, which features a fast loader and a few handy keyboard shortcuts.

The after-market ROM needed reseating.

Success! But what's that on the screen?

Even though the machine was now running, only about 10% of the keys worked, and some of those were a bit iffy at that. After opening the thing up and probing resistances between different points, the culprit was identified. As the machine had apparently been stored with its keyboard disconnected, the header on the motherboard was quite badly oxidised. This meant that a lot of the connections were either intermittent or completely non-existent. A bit of fine-grained sand paper, followed by a quick wipe with alochol, removed most of the oxide and the keyboard was revived. A few of the keys were quite sticky but most loosened off after some vigorous pressing. Some day I'll clean and lubricate the ones that still stick a little.

Cleaned up keyboard header.