My HP 5342a acquisition

[GRFixedGear]

Important discovery... be very very careful using NanoVNA's as a signal source

Did some more reading on this and now understand the square wave output strategy the nanovna uses. I learn something new every day.

[shakalnokturn]

Sorry didn't re-read the topic so may have missed the information if it was there.
As an attempt to solve your hot Zener troubles, if oven consumption is closer to 5W than 10 you could try running your oven transformer from a single primary rather than have them paralleled it will increase the internal resistance. (I doubt it's much of a problem, that was almost the case for 100V wiring with the original transformer.)
That and maybe a couple of 15V zeners may feel comfortable enough.

[GRFixedGear]

could try running your oven transformer from a single primary

The potential downside to that is that the mid-point of that output (nominally 13.5Vdc) is what feeds into the transistors which are used to drive the primary switching transistors on A19 through some isolation transformers (also on A19). I can't predict where the crossover point is between those being happy and not.

There is also some involvement of the case fan with the primary windings of that transformer. I haven't completely digested how that works.

So long story short, its more complicated than I would want to start messing with if I can just increase the power handling tolerance of that zener function.

Thanks!
Jason