Why was Canberra able to listen to Voyager 2 but not talk to it?

Question

The New York Times's When Voyager 2 Calls Home, Earth Soon Won’t Be Able to Answer explains that because Voyager 2's trajectory has taken it way below the ecliptic, of the three 70 meter dishes in the DSN only Canberra's can talk to it. So when it undergoes renovation soon, there'll be no more talking.

However, the article points out that reception of Voyager's transmissions will still possible.

Question: Why exactly was this so? Was it only the 70 meter dish's transmitter that's being fixed? Why can't a pair of 34 meter dishes do the task but at a lower data rate? What was the technical explanation for receive-only?

While the team won’t be able to command Voyager 2, they will still be listening to the spacecraft. By combining the power of the other antennas in Canberra, they will be able to collect its scientific observations.

“The Canberra site will still be getting data back from the spacecraft,” Ms. Dodd said. “The science data will still be coming down.”

Being able to only listen could prompt some anxiety. While Voyager 2 will keep collecting and sending back science data, should something go wrong, members of the team will be powerless to help it, and will just have to watch with their hands tied.

“We’ve been planning on this for over a year,” Ms. Dodd said. “I think like any good planning, we’re prepared for it. And we’ve done our best, you know, we’ve done the best that we can.”

note aspherical cows in foreground:

Antennas belonging to the Deep Space Network complex in Canberra, Australia. Credit: Canberra Deep Space Communication Complex, By Shannon Stirone

From here:

below: data for the Sun, planets, Pluto, Voyager 1 and Voyager 2, from January 1, 1969 (a good year to start things) until now. Dots are now. Data is from NASA JPL Horizons. Reposted from this question, where additional details are given.

Answer 1

As per the March 4 JPL News story, only DSS43 has the right S-band transmitter, enough power and is in the right hemisphere to talk to Voyager:

Moreover, a special S-band transmitter is required to send commands to Voyager 2 - one both powerful enough to reach interstellar space and on a frequency that can communicate with Voyager's dated technology. The Canberra 70-meter antenna (called "DSS43") is the only such antenna in the southern hemisphere.

It goes on to say:

As the equipment in the antenna ages, the risk of unplanned outages will increase, which adds more risk to the Voyager mission. The planned upgrades will not only reduce that risk, but will also add state-of-the art technology upgrades that will benefit future missions.

I would also speculate that digging out a low signal-to-noise ratio signal from Voyager received by the 34-meter dishes is easier with the more modern hardware available on the ground than for Voyager's 40+ year old radio and computer to correctly decode the super weak signals transmitted by the same 34-meter dishes. This is somewhat supported by the information in Deep Space Telecommunications Systems Engineering book at JPL's DESCANSO site. Page 350 (368 in the PDF) says:

...spacecraft missions have employed(...) and a single-channel digital command detector (Viking Heritage). A new single-channel digital command detector (NASA standard) has been developed and will probably be used on new missions of the near future.

(note that the book is from 1983). It then says:

The Viking Heritage command detector was originally developed for the Viking Mission, and was later adopted by the Voyager and Galileo Projects.

(emphasis mine). Section 7.3.3.1 on page 387 in the book (405 in the PDF) talks more about the S/X-band transponder (S-only for uplink, S and X-band for downlink) and again discusses the two versions - a Voyager version and the newer NASA standard version. This likely again means that the newer 34-meter dishes don't have transmitters that can talk to the old Voyager-style transponders.