RV1 is the pot to adjust to lower the output level, below is an extract from the user manual regarding it:
3.4 Output Level Adjustment
The output timecode amplitude can be adjusted by means of trimmer RV1 [...]. The output is factory set to the recommended output level 3Vpp. If a different output level is required, monitor the output at the module BNC connector on an oscilloscope. Use a small screwdriver to trim RV1. Take care not to adjust the carrier phase trim potentiometer RV2, a multi-turn trimmer, by mistake.