Commodore 64 – Blank Screen of Death & PLA upgrade
It had the ‘Blank screen of death’. This can be caused by several things, U4 (Kernal ROM), U17 (PLA), U7 (6510 MPU), U3 (ROM), U8 (7406 IC), U19 (VIC II), U9-U12 (4164 RAM), U21-U24 (4164 RAM), VR2, CR4, VR1.
Turned on the computer and let it warm up for 5 minutes and felt the top of all the chips -> if your lucky one or more of the 4164 RAM chips are super hot indicating that its bad. In this case all the chips felt fine and so it made sense to test the next highest probability being the PLA chip. These are known to cause the Blank screen and run hot so they fail often. Unfortunately it was soldered to the board on this one, but on one of the other C64’s I bought the chips were socketed. So I decided to unsolder the suspected PLA and solder on a socket, make life easier.
When I put the socket in and tried the other PLA chip the system worked! So I knew what the problem is…now to figure out the cheapest/best solution.
The PLA was originally a MOS chip based on the 82s100 which is impossible to find. So whilest surfing the net I discovered Ray Carlsen’s post regarding using a more common and re-writable 27C512. I happen to have a bunch laying around from pulls from bad arcade boards…so here was the attempt.
First erase the 27C512.
Then program the chip with the modified PLA data (this chips pins are in a slightly different configuration so the software must match the hardware…)
Next up – wire the chip to a socket modifying a few of the pins around to more match the original PLA. And plug it into the socket. Details are here if you can read German: http://vic20.de/html/eprom_pla_8296_und_c64.html
Not the prettiest wire job, but hey I didn’t even know if this would work and its hidden under the keyboard anyhow…
Flipped the power and got this screen:
So, after a bit more research it turns out that the chip that I used was rated at 120ns, seems fast enough but it has been suggested that the replacement needs to be quite fast under 70ns to pull of the trick. I dug around a bit more and found some 45ns’s lying around and decided to give it another go.
Thanks Ray! Sweet, the new chip runs cooler than the original and I can replace them indefinately for free!!