Hello, i'm using a lock in amplifier (model 5210 from signal recovery) for some experiments with photodiodes. Firstly, there is an optical chopper that "feeds" the lock in (reference signal). My input is a photodiode (in current mode). The problem is,the unlock (unlk) light is always on in the lock in amplifier, but, in the display section, i can see the reference frequency from the chopper. However, it was working well in previous experiments, as my professor stated. I tested the exiting signal from the chopper with an oscilloscope and it is the right one. Also, everything else in the lab is tested and working well. So, my question is, is it possible that something in the lock in isn't working?(i'd guess it's the mixer) I'm sorry if the question is dumb or something, I don't have any previous experience with the lock in and i'm desperate. Any other ideas would be appreciated.