check the jumper wires on the chroma board, of just feel the power resistors that feed the plates of the diff amps, my guess is the wires are broken, common problem on CTC-12 thru 17 at least
see here at 5:22
https://www.youtube.com/watch?v=YJ1JWdn_kTw
oh and that set had the short vert too, turned out to be the 100k boost resistor.