This baby was on eBay for a long time. Advertised as being in the notorious "Powers up, not tested" sate. The picture there showed indeed nonsensical display, with conflicting lights and missing segments. But, as this was the military version, with GPIB and oven oscillator it was bugging me to get it. So finally took a chance, as it was cheap.
First try, it did exactly what was shown on the picture, garbled display, no reaction to the buttons. I checked voltages, the -5.2V supply was 0.5 instead. Strange. Pull the GPIB board first. Nothing. Pull the Channel C prescaler board, it works! Check the obvious for shorted caps etc. on the prescaler. Nothing. Put it back, it still works! Very strange.
Put everything together, still works and has been ticking on my desk now for a couple of hours.
I have no idea what was causing this, could have either been some short, like a solder blob or something touching something else, which fixed the issue. Only thing to fix is the small fuse inside the Channel C BNC, it's blown, needs replacing, but bypassing it showed that Channel C works.
Extra bonus: all the buttons work fine, even though this model is known for the buttons failing.
So this repair was quite easy, take it apart and put it back together.