1702 Monitor had picture but no color.
First thing to check was that the Commodore 64 computer itself wasn’t the culprit, so I swapped it for another video source and the problem continued. I’ve noticed that touching the switch in the back that selects between FRONT and REAR inputs had some effect and the color was present for a brief moment, but then it fade away leaving just a black and white screen. Both REAR and FRONT inputs had the same issue.
I’ve cleaned this switch with no success.
Time to take apart the monitor.
(PLEASE BE VERY CAREFUL WHEN WORKING ON CRT MONITORS, THEY KEEP HIGH VOLTAGES THAN CAN HURT/KILL YOU, EVEN WHEN THE MONITOR IS OFF AND DISCONNECTED).
Rear switch was now clean.
Video inputs were cleaned and tested for continuity, all seems ok.
I removed the chassis for a closer inspection and found a not too good looking trim pot sitting next to the crystal oscillator.
The green thing is the trim pot that tunes the crystal, setting it with a ceramic screwdriver made the color comeback. I did this with the monitor on to see changes on screen.