What usually happens with these is the contacts fall off the switch rotor.
See below...
The switch contacts are held on by posts molded into the plastic. They are hot staked in place by heating the post and pressing it down to form a clamp for the contacts.
First if you opened up the analyzer be very observant and find the loose contact.
You will have to glue it back on in the proper antiabortion.
In one respect you are lucky. the resolution bandwidth switch rotor is the rear most one and is affixed to the inner shaft.
Get a copy of the manual; therein you will find disassembly instructions for those controls.
You will need some special tools...
Spline wrenches to remove the collets holding the plastic rotors in place.
You will need a good camera to take pictures of the switch assembly as you take it apart.
The reason for this has to do with the positioning of the collets, they don't explain that in the manual......
You will need allen wrenches (hex wrenches) to fit the knob set screws.
Take lots of pictures and don't overlook the position of the collets. As I said above THEY HAVE TO BE INDEXED PROPERLY otherwise the switch will not rotate through its complete range.
Be warned. The knob set screws are mild steel in a brass hub, pressing against a stainless steel shaft. They can become seized. Pennitrating oil can help...
In stubern cases you will have to purchse a left hand drill (actually two one small enough to fit in the hex head set screw, and one ether just at or just under the drill tap size for that set screw. (they are #4 set screws. ) ) I had the sweep time knob o the second 8569 that needed to have the set screw removed with a left hand drill. I had to chase the threads with a 4-40 tap... Don't use a starting or thread forming tap...
I'll be starting on the second 8569 front panel this January... There has been a number of delays in getting that project started. I will repost links to my pictures from my Google photos account. in the HP-8569 thread.
This is not a project for the faint at heart, good mechanical skills are a must, along with attention to detail. I had to rebuild the Reference Level / Input Atten knob assembly three times before getting it right on the first 8569, so be patient.
In the end it is worth it, that is a damned good analyzer.