Dear support team,
I have N5181A with problems on A3 unit. Once I have faced with Open loop and VCO test error. I spent hours in troubleshooting and checking, drawing on circuit diagram of VCO A3 unit (attached). I have found that if I remove resistor 4221 the VCO syntezer test successfully passed. But phase control loop test (next one) of corse not.
To be honest I do not completely understand how this part works and, as a result I can not find the root case of fault.
Could you help me with understanding or give me some advices about repair?