575 step amplifier


I'm a new member of this very interesting discussion group. I'm living in France and I have some old Tektronix devices. The last I got is a 575 curve tracer.

It's in very good cosmetic condition but it can't give the family curves for a transistor under test.
Only one curve is displayed. It seems that the problem is in the step amplifier.

The step generator is ok because the step waveform is correct on the input of the step amplifier but there is no step on its output.

I thought that the Ge transistors 2N2148 and 2N277 were probably out of work.
So, I have changed these transistors with 2 other Ge power transistors but they instantaneouly burned.

The voltages given by the power supply are ok.

Has someone already encountered this problem ?

Many thanks.