They all should self oscillate with in a small range as seen in the video.
Try setting the unit as in the video and unplug your power supply and then back in again.
The frequency knob will change the pitch but only from 7 to about 10 o’clock on the knob position.
If you feel your unit is defective send it into the service department.
They are great guys and do the work FAST!

Now if you want really crazy feedback use the technique I used in the Effectology Halloween show.
I used a Y adapter with a Tube EQ.
The same adaptor with the Tube Zipper turns the Zippert into a CRAZY mini synthesizer.
Screams through out the entire range and the trill works great.
Its a big fun!!! With an echo its insane.

Here is the link how it was done with the Tube Eq its the same with the Zipper.

https://www.ehx.com/forums/viewthread/2029/

Bill Ruppert