Troubleshooting and Repairing an SX64

So today I am diving into repairing one of my SX64s. Most of you know that these machines can be in need of repair from time-to-time. The issue I am working on today relates to the screen prompt that comes up when I turn the machine on. Sometimes the screen goes completely blank, and other times I turn it on all the letters are garbled up. I have a breadbox 64 that I am using to pull spare chips from to try. According to Ray Carlsen's notes this can be caused for a variety of reasons, and gives a list of suspect chips to replace. I've replaced the UB2 906108-02 (6526) CIA on I/O board, and still the same issue. I've replaced the 901225-01 CHARACTER ROM on CPU board and still no change. The next chip I am trying to replace is the UD3 251104-01 KERNAL ROM on CPU board, but there doesn't seem to be a c64 replacement in the breadbox for this. I am researching to see if there is a duplicate for this chip on the C64. I have a feeling that the Kernal Rom is in fact the culprit. I will keep everyone updated on how this turns out. Last time I had an issue with the Ready cursor not being present and all I had to do was replace the 6526 and all was well in the world.  I was able to replace the SX64 Kernal Rom with the breadbox 64 Kernal Rom which is the 901227-03 in the breadbox. It just gives you the standard blue screen C64 instead of the white and light blue SX64 screen with the 251104-01 chip, but they are interchangeable. After I replaced them there was still the garbled screen. So the next culprit was the VIC chip 6567.
After replacing the VIC chip it turns out it was the UF4 906109-04 (6567) VIC (has heat sink attached) on CPU board.
     "Blank white screen, no border. Sometimes will produce garbage or
"checkerboard" screen, or screen that lacks contrast. If screen is
blank or garbled from bad VIC, "blind" disk commands from keyboard
-may- still work."
http://personalpages.tds.net/~rcarlsen/cbm/sx64.txt


Ray Carlsen is the man.