Learning something from exams!!!

Last week, I have electronics lab exams. I went as usual and got a simple experiment to form a wave form as shown in figure using clipping, clamping and rectifier circuits from a sine wave. The frequency of input sine wave was not given so that we can choose a convenient frequency. I drew appropriate circuit which I was studied earlier and got "proceed" command from the external examiner to do it.

I quickly build circuit on bread board and double checked it. Then input is taken from signal generator and output is fed to CRO. I gave all power supplies and swtched on Vcc to circuit, CRO and signal generator. Alas! signal is different. I rechecked my circuit again and again but no use. I got a signal like this.


But the required signal is different.. First half cycle is Ok. But second halfcycle is clipped off. I couldn't understand what's the reason. Wasted a lot of time by rearranging the circuit components and adjusting the knobs of signal generator and CRO.The actual signal required is also given.Chances were going to be worse. Started to troubleshoot it from one corner, I realised that the bridge rectifier circuit using four 1N4007 diodes is the problem centre. I rechecked all the diodes using multimeter, but no use. At last, I replaced some new diodes, no use.. I passed a simple sinewave through the circuit, again second half cycle is clipped off. All diodes are correct. There is only 10 minutes to complete exam. Dresses become wet even under the fan in full speed. Nothing to do more than return the components. But I prayed GOD, what a destiny. Then I got an idea to replace the signal generator by a transformer directly from power supply since the frequency os oscilation was not given. I just replaced it, but got the correct answer. Really- but had some noise, its not a probelm. I comleted exam within 5 minutes and showed correct output.
Actually what may be the reason? May be some evils playing inside the signal generator. But I think some strong reasons are there. In the case of signal generator, there is no negetive voltage exists to forward bias the dioes in the bridge rectifier even when we 'feel' some negetive effect on CRO. No use by adjusting 'offset' knob in the signal generator. That effect only occur in the CRO display, but not in physical.