Problem with the original Clark zapper super de luxe

Hello to the forum

I purchased an original clark zapper de luxe 2006 and following the instructions of management, always gives the message "no load" after a few seconds of "start".
The wristband is moist and well-connected to the zapper.
I note with the oscilloscope wave output that is not always positive (as it should) but centered on zero with symmetrical sides positive and negative, and a range of 2 to 3 V and maximum output peak to peak.
The amplitude output is not between 9 and 15V as shown the instruction manual.
Someone has this zapper that can tell me if this is normal?
Thank you and greetings Tweet