The 80pf capacitor is charged up during the positive part of a square wave through the 470 ohm resistor and diode. Then when Tr3 conducts the positive plate of the capacitor is pulled to ground, and the negative plate is pulled below ground. For the base of Tr4 then ground is positive compared to its emitter and it conducts for a fixed time until the 80 pf capacitor discharges. The higher the input frequency the more time Tr4 spends conducting.
It seems like there is a lot of amplifier circuity before all that though that could be replaced by some sort of positive feedback Schmitt trigger.