I am having an issue with controlling a Sony XBR-75X850D via RS-232 (which is a 3.5mm jack on this model).

Previously I have used an Extron MLC 62 RS D which sent a RS-232 command to a Sony CEC converter (used a classic DB9 connector for this one). The CEC converter converts the RS-232 to CEC and sends it along an HDMI cable to the TV. This was working for a few months but has suddenly stopped. Now I am trying to go directly to the TV with the RS-232, though I have to use a 3.5mm jack to do so. I am having trouble getting the TV to recognize any RS-232 command from that 3.5mm jack. I triple-checked the wiring and the settings on the TV to make sure everything is set correctly.

Does anyone have any experience/success stories with controlling these XBR's via serial? Or is it a lost cause that I should abandon?