Altering deleting control characters on tcp stream import

[confused]

Hi I have a logstash.conf input to import data:
// { tcp{ codec => "json" mode => "client" }}
This is importing some data, but my data has a ctrl 03 character at the end of each line rather than newline.
I have tried using
//codec => json_lines {delimiter => "\03"}
//codec => json_lines {delimiter => "\u0003"} etc.
My understanding is that the json_lines entry with delimiter should replace the control char with a newline and process properly.
Can anyone help?
thanks