VIC 20 repair

Missing color on screen

Looking at the video signal (yellow), the luma output of the 6561 (pin2) and the chroma output (pin 3) we see:

Video on yellow, luma on cyan and chroma on magenta. The chroma line carries its 4.4MHz burst, but none of it reaches the composite output.

The chroma signal is there but is completely missing from the output.

It turned out the previous owner had made a modification that went bad, disconnecting the capacitor that connected chroma to the luma signal. Adding the capacitor fixed it:

With the capacitor put back the chroma rides on the composite signal again, at 15.6kHz line rate.

Capturing a line in the middle of the screen easily shows the blue border on the left and right sides:

One line captured with video trigger. The wide blocks at each end are the blue border; the quieter stretch between them is the screen content.