Thank you Roger and Albert for the excellent advice.
I have the rackable 7603 and side-probing on the live system is not possible. But if the middle bay is left empty then signals can be brought outside easily, it just needs some planning since you connect test leads when the unit is on the desk and then carefully plug it in.
It turns out Q424 was bad. Socketed transistors are suddenly cool! The original part is 2N4122 which I replaced with a BC557 which has lower f_max but the same pinout. Other transistors mentioned by Roger are OK.
Now I will move to the calibration part of the 7B80 manual because the sweep is compressed: who knows what has been done to that 7B80 between 1974 and 2007 before I got it.