Right, so the trainee who was working on this PA was out running errands today so I thought I'd take a look at the faulty PA as I didn't have much else to do.
I found the reason (at least my interpretation) for why the power was dropping at the higher frequency range. Within the diagnostic board there is a BIT test that monitors the collector voltage of the RF transistors on the 4 amplifier boards. It cycles through every ~1.5ms and checks that Vce is not >= 110v. If it is, then it backs off the power/reducing Vce, so come the next cycle (1.5ms) things appear fine so it tries to operate like normal, then the
next cycle Vce is back to being > 100v and the whole process is in a continuous loop giving a ringing effect.
I monitored the collector voltage on a scope and sure enough, when you key the exciter you can clearly see the ringing which peaks above 110v.
I suspected some degrading 47nF polycarbonate caps to be the cause (I can't recall their function without the schematic in front of me sorry) so I stuck 1000pF (all I had on hand at the time) in parallel across 1 of 8 suspected caps and sure enough, I noticed some improvement in the ringing.
It was at this time the trainee returned so I left him to replace the caps and go from there. Hopefully this should fix the problem.
Here is 1 of the 4 amplifier boards with the suspected bad caps. Image of the board taken from eBay, so my apologies for the quality.