In ALT mode both base and emitter of Q260 are tied to -12.2 V through resistors.
For Q 260 to enter in conduction it is required a voltage swing (on emitter) of 0.2/0.3 V (germanium) minimum.
Using a silicon bjt, the minimum voltage swing must be 0.6/07 V.
Check the amplitude of the signal on the emitter of Q260 (junction of C260 and C261) and verify the effective amplitude of the incoming pulse.
