No audio signal when "MON" is turned on

Slawomir

I have a problem to solve. Since I have two transceivers, so I don't have to switch headphones or speakers between them, I decided to use a small mixer. I use headphones connected to the Flex 8600M's headphone jack, primarily to monitor the Flex's outgoing signal while the monitor is on. Today I connected my new Behringer FLOW 8 mixer. I unplugged the headphones from the Flex's headphone jack and connected them to the mixer's input. I connected the headphones to the mixer's output. I adjusted the levels and heard the reception on the band. I made a few QSOs, and everything worked fine. I wanted to listen to my modulation on the DLEX output. I turned on the monitor, and surprise! No signal. I don't understand why there's silence from the same headphone jack where I monitored the signal on the headphones, but now when it goes through the mixer? This only applies when the monitor is on, because I can hear the correspondents normally. The levels are good. Any suggestions?