Transformer behavior

[Shock]

Can anyone describe whats causing the flattening of the peaks on the top waveform at about 17:10?

[Monadnock]

Current only flows at the peak of the AC sine wave when the caps are charged. The impedance of the transformers is high enough that the voltage drop across this impedance during the charging of the caps causes the top of the AC sine wave to get cut off.

[Circlotron]

Yep. The transformer has no load on it until it's voltage gets as high as the capacitor voltage. Then it has to work! The transformer voltage still rises a little after this, and during this time it is pulling the capacitor voltage up a little too.

[basinstreetdesign]

Yes, but only when the load resistor is drawing current.  If the load were removed then the sine wave peaks would spring back up and the ripple voltage on the load would disappear.  Since no current is drawn by the load then none (or almost none) is delivered by the transformer/rectifier combination to the cap.