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Introduction
I purchased a lot of six non-working Commodore 64s from a seller on eBay. I liked the description of the lot because it included the symptoms that each was experiencing. This gave me some clues when I made the purchase as to what the problems might be. I was particularly encouraged by the fact that each of the six seemed to provide some sort of video output resulting in a back screen or otherwise indicating that the VIC-II video chip and the rest of the video circuitry was likely intact. Replacing a VIC-II with an original chip or Kawari can be expensive at $100 or more (e.g. over $100 on eBay).
Case Study
I first confirmed that the unit did indeed boot to a black screen. I then tried the Dead Test to see if I could get some clues. The Dead Test would not boot.
I then checked the voltages as I demonstrated in a previous case study. The 5V and 12V lines looked ok. Further, none of the chips on the board were hot to the touch which would suggest a short. I also checked the front and back of the board for any damage or broken traces. Nothing. It was nice to see all of the main IC were socketed. This makes repair much easier if one or more is faulty. A few keys are missing.
On further inspection of the board, I noticed that one of the two 74LS257 multiplexers (U13) was socketed. This caught my eye because it could have been a previous repair by an owner and a broken trace under the socket would be hard to repair. While taking a closer look I noticed one of the pins looked weird. So, I pulled the chip out of socket and discovered one of the pins was bent under and not making contact with the socket. I straightened it out, re-inserted it in the socket, and retested the computer thinking this was going to fix it. My last case study (#1) has non-working MOS-branded multiplexers. This did not fix the computer but likely contributed to the black screen. A good lesson in checking pins and sockets!
I next broke out the oscilloscope (I bought a Siglent) and started probing the 6510. I always start with the CPU to make sure its signals look good. It is also a prime suspect, along with the PLA, if there is a black screen and the Dead Test doesn’t work. This is because the Dead Test bypasses most of the main ICs and only makes use of the CPU, PLA, and VIC-II to test the RAM and other components. The video below shows normal activity on the data bus. Further probing all the pins of the 6510 CPU revealed that address lines 14 (pin 22) and 15 (pin 23) were stuck high and not fluctuating.
Next, I moved on to probing the PLA (pinout below) and confirmed that address lines 14 and 15 were also stuck high there. Interestingly, the chip select line (pin 19) was also stuck high which is not good. This led me to believe the PLA (U17) was the problem. I replaced it with a new GAL PLA (available on eBay for around $15) and the computer booted right up and passed all the diagnostic tests! Fixed!
Lastly, I tested the 6581 SID chip by playing some chiptunes with a SID player on my Kung Fu Flash (available on eBay for $30 to $40). As you can hear in the video below, the SID sounds really good! Phew!
Addition improvement to the board could include adding an RF replacement for S-video out. This would improve the quality of the video along with the stock Revision 8 VIC-II that is already installed. I went ahead and clipped capacitors C1 and C2 in the RF module. As I previously described, this improves the sharpness of the video signal (see below and compare to one above). I would also remove the metal bracket around the video circuit as it is not needed. I will also add heatsinks (I buy these for the ARM) onto the VIC-II when I am finished. This will prolong its life.
Finally, I finished up this board by fixing three broken white plungers and replaced three missing keys. I ordered these parts for about $10 from Retroleum in the UK.
Comments
This black screen repair of a Commodore 64 250407 Revision B board involved straightening a bent pin on 74LS257 multiplexer (U13) and replacing a faulty PLA with some bad signals likely resulting from a short. The PLA is a nerve center of the c64 and a common point of failure for a wide variety of symptoms including the back screen seen here. The GAL PLA is an inexpensive replacement (available on eBay for around $15).