Adjust R473 until the transponder replies to the interrogation. Tweak R473 until you can change the 8.0 microseconds as low as you can high before the transponder fails to reply on each side. Typical window will be 1.6 microseconds, so tweak the range for 7.2 to 8.8 microseconds. Put the scope on U420 pin 1, and look see the pulse length. Put the scope on U420 pin 2, and adjust R469 for the same pulse length.
Nobody even knows how to check.