Albert,
When the -12.4 V is lost, it's essentially zero V, so the emitters of Q72 and Q74 are effectively grounded through R72, and the long-tail pair cannot provide feedback. Assuming they're Si transistors, the TD voltage can't be high enough to turn Q74 on (unless the actual TD current is very high up its diode curve, and in danger anyway). The -12.4 V emitter feed via R72 makes it able to operate around zero, and without it, it's open-loop.
Ed
