Right part has lower peak voltage ...
I think 3V pp is the minimum as the signal is filtered by a capacitor and so the gate will switch at TTL levels which mean signal has to grow up about 1.4V minimum and there maybe some more necessary for a schmitt trigger input. Maybe just switch at 1.6V up which means you need 3.2V pp. So the signal seems to be at the border.
Gozzo, you are right about the connectors. I picked up a cable I had mistakenly packed up with the TS1000. It had left and right audio terminating with red and white phono plugs on each end. It did plug into the TS1000 but was the wrong plug! When I used a cable with 3.5mm plugs on each end it loaded the tape perfectly.