I'm particularly curious about the coherent transponder mode where the outgoing transmitted signal is phase-locked to the incoming received signal (prompted by @uhoh's recent question). This is a mode that the Voyagers are capable of and can provide exquisitely accurate velocity and ranging information. This capability has proven invaluable for the Voyagers' studies of the plasma environment and the planetary atmospheres and gravitational fields.
This coherent mode sounds difficult to do with a traditional heterodyne down-conversion receiver because the exact phase/frequency of the local oscillator(s) is unknown. This can presumably be solved if the local oscillators are themselves locked to the signal as recovered at the intermediate frequency (IF). But the signal frequency seen at IF will depend on the frequency of the local oscillator. So I'm puzzled as to how exactly this can work.
But just generally, I would be really fascinated to see these circuit diagrams. They are presumably early 70's technology which is at a time when I was working on microwave links with British Telecoms Research.
[Edit: including some info from @kgutwin who pointed me to an excellent and very relevant Apollo video by CuriousMarc]
The exact same frequency ratios are used in Voyager and apparently even in ISS. So it's likely Voyager uses exactly the same approach and very similar or even identical electronics to Apollo.