Commodore 64 – Basic ROM, CIA replacement and Recap

This Commodore 64 is turning on and outputting a display but is not booting as expected. Instead of the usual 38911 basic bytes free message, it’s simply showing a blank screen with no cursor.

Inside the board was partially socketed but the Basic and Kernal ROMs were not so I started by socketing those as its helpful for whoever may own this board next and I suspect the Basic ROM may be gone here as that would cause this issue.

With the board socketed I start by switching out the Basic ROM and this somewhat fixes the problem. I now have it booting but there is still no cursor. That points to the CIA 6526 chips, they are responsible for handling keyboard, joystick, cursor, cassette and all manner of input/output functionality. Turns out its U1 and I don’t even need to test U2. I have the proper boot screen complete with flashing cursor. Just for good measure I load up Flimbo’s quest to test the joystick, keyboard, cartridge slot and sound. Everything works perfectly. Job done! Well, not quite…

This is a 250407 board and they have a lot (and I mean A LOT!) of electrolytic capacitors. I always like to replace the capcitors if they havent been done already, and this board definitely has not. A short while later and the board is completely recapped:

I’m going to clean up this board a little with some isopropyl alcohol and add heatsinks to the SID, CPU and PLA before using it this weekend to play some Sam’s Journey to ensure its running fine.

3x Commodore 64 repair

Today three Commodore 64 breadbins arrived and all need a little repair work. Turns out not that much.

Computer #1


This 64 is in pretty good shape and booted perfectly although the LED did not come on. I like to do a quick initial test by loading a demo from my Ultimate II cartridge, it tests the serial interface and the cartridge slot as well as a few keyboard and joystick functions in the menus. The demo tests the VIC and SID chip quite extensively and will usually show issues pretty quickly. My demo of choice is Comaland by Censor Design and Oxyron.

This 64 has no sound, potentially a SID chip is required, annoying as its the most expensive part but hey ho. Time to open it up and take a look.

So the LED not working is obvious, this machine has been dismantled before (to be fair after 30 years in the wild it’s hard to find one that hasn’t!) and the LED connector not put back correctly. Easy fix!

The insides dont look too bad and it looks all original judging by the date codes on the chips.  I can’t be sure but the caps look like they might have been replaced as they don’t match. I’ll know for sure when I take the board out to clean, which I’ll probably do tomorrow as I want to get all three of these machines tested today.

SID chips are almost always socketed, so I decided to test the chip from Computer #2 to see if it works. And it does, I’ll close this up until cleaning time. One down!


Computer #2

This breadbin is in pretty dirty outside and in, just look at that dust! It’s also now missing a SID chip thanks to computer #1. Turning this machine on produces no picture at all, not even a black screen. First place that I  check and hey presto there’s the problem, the internal fuse is blown, after replacing it the machine springs into life.

After quickly “borrowing” the original SID from computer #1 and booting the Comaland demo, I manage to verify the computer seems to be running great now.  The keyboard seems a little unresponsive and probably needs cleaning, trying a known working keyboard verifies that its not the board

I’ll have to source another 6581 SID at some point to get this fully running. For now it’s time to close up until cleaning time. Two down!

Computer #3


This is in the best condition of the three breadbins. The case is very clean and the both it and the keyboard have almost no visible yellowing. It’s nice to see well looked after machines like this! Turning the machine on produces only a black screen. Time to open it up:

Wow. The inside is incredibly clean too, this is machine has had some love. All the big chips have been socketed too, which is great because I want to try replacing the PLA. With a black screen I’d normally try the VIC chip first as its socketed and the rest usually aren’t, however with the PLA being socketed I may as well try that first. The PLA on the breadbin models is notorious for breaking and black screen on boot-up is one symptom.

It’s really my lucky day. The new PLA works a treat and the machine boots up perfectly. In fact the picture quality is amazing, probably the clearest picture I’ve seen yet without a Lumafix or Reloaded board. Testing Comaland once again the VIC and SID chip behave as expected and I close the machine up until cleaning time. Three down!

All in all its been a great night for repairs, three broken Commodore 64s and they all were fixed incredibly easily. In fact I didnt even need to turn on the soldering iron. Next step is cleaning them properly, replacing the capacitors, finding an extra 6581 SID for computer #2 and adding internal voltage savers. But that’s one for another post, another time!