HP 5342A power supply issues

Many years ago, I was donated this 5342A microwave frequency counter. Opposite to many other items, this one was working like a charm and had no issues at all. It is equipped with the amplitude measurement option and is designed to count up to 20 GHz. It also featured the 10544A OCXO high stability timebase which I removed to start a GPS disciplined oscillator project. It was replaced by a DIY 0.5ppm TCXO replacement described here.

Time went on and I have to admit, owning a spectrum analyzer with built-in counter feature, made me use it quite rarely during the last years. But one of the few times when I actually intended to use it, I switched it on and after a couple minutes there was a lound BANG. The fuse was blown and the magic smoke was escaping the case, what a hell! The rather optimistic approach of just replacing the fuse and hoping for another instance of "RIFA madness" didn't prove right, the short was still present and the unit still dead. So I took the rather tedious route of opening the instrument and removing the (feeling like) one million screws holding it together.
Overall construction is quite nice, all printed circuit boards are in separate chambers made from diecast aluminium. On the bottom a large backplane connects all the various PCBs together. For convenience, the reference oscillator is located outside of the covered diecast pcb space and is rather easy to reach without the need to remove all the cover screws. Nice detail from the HP engineers back then!

I had the unit already recapped in 2013 (see the note on the lid, I usually add remarks inside an instrument for future reference) so the issue must be something different... First thing I checked was the main switching transistors and both were blown. Luckily they are driven by a transformer so I didn't have to mess with checking and replacing the driver section. I didn't even bother checking for the cross reference to these HP parts and just replaced them with two BU508DF transistors. That's a 700V, 8A NPN transistor featuring an integrated snubber diode and an isolated tab which makes it a perfect and durable replacement part for the vintage and discontinued TO-66 part:

And sure enough, the unit works again!