Amplify sine-pulses with K8081

we bought a Amplifier Kit (K8081). We would like use it in our laboratory to amplify sine-pulses. The input-signal comes from a function-generator with a range of 1kHz to 15kHz and level of 50mV to 600mV (rms). I think your product is perfect for our application. After the assembling everything functioned very well, also the self check worked. Now I have put a signal at the input. The signal was amplified clearly but only the negative pulsform, whereby the positive pulsform was limited at 2,2V(peak). With interest I can also send a pic from the oszi-waveform. I hope that you can help me with our problem.

Hard to tell what happened, as we have no experience with this problem.
Is the DC protection enabled when the amp is turned on ?