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I can't see what the problem is here... "When TR2 is on causing the right pin of D2 to be grounded", C2 is fully charged ("-" at the left plate and "+" at the right) plate. The diode brings down its right plate and fixes it to +7 V. The capacitor begins to recharge and its left plate gradually increases until reaches about +0.6 V when T1 begins turning on... and the self reinforcing positive feedback begins acting. The circuit rapidly switches... T2 cuts off ... D2 cuts off as well... and disconnects C2 from the T2 collector thus improving the rising edge...