[Amps] Ameritron ALS-500M repair

[Martin Sole]

I am about to start replacement of the transistors in an ALS-500M. One
transistor in each of the two pairs appears to be open circuit base to
emitter. Base to collector on the bad devices and both junctions, B-E and
B-C on the good ones show the expected 0.6v diode drop.

I have the four devices but am wondering if I need to replace all four or
are they likely to be close enough so as not to matter?

Any idea as to what might have caused these devices, one in each pair, to
fail?
Any way to prevent it in the future?

73's
Martin, HS0ZED