There are several tests that you can run. If it phase locks over any portion, it means the source is likely OK for power amplifiers.
There is no attenuator in the 8753A.
You should run the source pretune service test and then the source phase-lock service test. First you must move a jumper on the 8753A CPU board to allow the service test results to be written to the flash memory.
A likely cause of trouble is drift in the cavity oscillator (3.8 GHz fixed oscillator) that doesn't allow the low frequency phase lock. If it has drifted too far, then the pretune function can't find the right signal. Search this forum for 8753 source and you will find many posts on servicing the source.
pull out the source, and test the cavity oscillator. Should have 3.8 GHz +-10 MHz at about +8 to +12 dBm; if it is off by more than about 30 MHz, it can't lock. You might need to unhook and bias it separately, if you don't have a source extender board.