Hi, my guess would be that the high frequency loop is not locking. Instead it sweeps over its range 490 to 990 MHz over and over again. There are three phase-lock loops in this generator. Try to locate the respective tuning voltage for each loop and you may find if any loop is not locked. It is far easier to measure the tuning voltage using a scope compared to measuring a 500 MHz signal. And from there you can continue searching for what is failing.
G?ran
G?ran Krusell