Probably easy for the experts here, but i am scratching my head too long now about this topic.
I am troubleshooting the power supply of a Fluke 5440A and have a burnt zener diode.
Before i just replace the zener with a spare i want to fully understand the function of this part of the circuit.
Here is the part of the schematic:

I have drawn a red line to show the relevant path.
As i see it there is no series resistance for the diode and zener combination.
So the only resistane would be the combined impedance of the zener and diode.
At 22 volts across them how can you keep these parts from glowing?
From the "List of replacable Parts":
CR 5 DIODE, SI, BV= 75.0V, IO= 150MA, 500MW 1N4448
VR 13 ZENER, UNCOMP, 5.1V, 5%, 20.0MA, 0.4W 1N751A
Thanks for helping
Joerg