Re: Purpose of Q32 in S-52


Albert Otten
 

You are right Ed. I overlooked that the negative supply at R72 would also be missing.

I don't remember that I have seen such a protection with an active component (so not counting clamping diodes and fuse-like resistors) elsewhere in Tek equipment, except perhaps in SMPS.

Albert

On Wed, Jul 8, 2020 at 11:26 PM, Ed Breya wrote:


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

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